"As voltage drop exists only when current is flowing and varies according to the amount of current it is essential that the circuit is tested 'under load', i.e. whilst passing its normal current. In certain cases this current will be measured using a test ammeter."

A car ammeter displays the current flowing into (charging) or out of (discharging, except cranking current) the battery hence has a centre zero and moves left of that to indicate a battery discharge and to the right of zero to indicate battery charging. Almost all need you to remove the brown wires from the starter solenoid and run two very heavy gauge wires capable of taking at least 45 amps from there to the ammeter. As well as these two new connections which can corrode, those on the back of the gauge can come loose, and either wire can short to earth and being unfused could cause a fire. There are reputed to be 'remote shunt' ammeters around (although I've never seen one) where you make the same interruption down by the solenoid but connect an insulated bar between them to carry the main current, then run two much thinner wires up to the gauge. This does away with potential failures up at the gauge but still leaves those down by the solenoid and the risk of shorting-out. An analogue ammeter needs a scale running from at least -45 amps to +45 amps, and is practice can be even higher, which means the 'normal' range of needle movement is compressed into a tiny section either side of zero. It will almost certainly be a moving-iron meter and be unstabilised i.e. have a trembling needle. Both these factors make it difficult to see whether it is showing a slight charge as it should, or a slight discharge which is bad. Under fault conditions the battery voltage could be reducing but an ammeter stills shows a slight charge, or conversely an overcharging fault could gradually be raising voltage higher and higher but still not be showing an excessive current. If you really want to connect an ammeter then see here.

A voltmeter avoids all these issues and is a much simpler proposition requiring just two light-gauge wires to an ignition switched source and earth. Under normal circumstances it is the charging circuit that is supplying all the electrical loads, even at idle in the case of an alternator, as well as trickle-charging the battery once the cranking losses have been replaced. Ordinarily a voltmeter will show something above 14v with a charged battery and a minimal electrical load, reducing as the current load goes up and gets towards the maximum capacity of the (say) alternator. When the current load exceeds the output of the alternator the voltage will drop below 12.5v and the battery will then be supplying part of the load, and hence discharging. This voltmeter has a coloured scale which is useful for showing at a glance if the voltage is in or out of limits. Up to 15v is shown as 'green' and dynamo-equipped cars can show this although an alternator should not get that high. At the lower end 12v is also shows as green, and between 11v and 12v as 'marginal', but for me if it shows that for any length of time I'd be concerned. Others show up to 16v as green which would be high even for a dynamo, yet other show 10v to 14v as 'good' which is too low for both upper and lower limits.

Voltage can drop for a number of reasons including an owner having added some high-current loads but not uprated the alternator, or the alternator is failing, or it could just be some iffy connections somewhere. In all cases a voltmeter will indicate these problems - also the problem of overcharging - much sooner and clearer than an ammeter will. The only added fault liability of a voltmeter is one of shorting of the 12v connection, but as long as this is fused even this is eliminated. The voltmeter must be connected to an ignition switched source, particularly the analogue type with the expanded scale from about 8 to 18 volts (the coloured scale which not all instruments have is particularly helpful as you can see at a glance whether it is correct or not instead of having to read the numbers and know what they mean) as these are usually thermal (slow-acting) devices and will reduce the charge in the battery over time if permanently registering. Digital instruments take less current but I still wouldn't want to leave them powered all the time.

However, unless you have an ignition relay and connect the voltmeter via an in-line fuse to that, the green circuit (fused ignition, which is probably the most obvious place to connect one) on most MGBs will show a voltage which can be significantly lower (up to 2v lower is classed as 'acceptable' by Lucas) than the alternator and battery voltages, even worse with a digital gauge where owners get paranoid about tenths of a volt. I've also seen one digital gauge that displayed half a volt less than two other test meters connected to the same point! Beware of voltmeter vendor claims that the instrument will tell you the 'strength' of your battery. It doesn't, all it does is tell you is system voltage, which isn't true battery voltage.

Both ammeter and voltmeter will tell you if the battery is being charged or not in their different ways, and a correctly operating warning light will do so as well. But none of them will tell you if the car is going to start next morning! You could say, if you were really desperate to win the argument, that the warning light might fail when you were driving along, and something else might happen to stop charging. But like I say, you would have to be desperate.

January 2015 Well, I said desperate, but Adam Liptrot did experience a situation where the warning light did NOT indicate a problem, where especially a voltmeter and possibly an ammeter would have, as recounted on the MGOC bulletin board. On a wet winter's night on country lanes over the Pennines he drove through a large puddle, and after that became aware that his indicators were slower, his lights were dimming, and about half an hour later he ground to a halt and had to be recovered home. A flat battery was diagnosed, but subsequent testing showed that whilst the system voltage was about 14v with the engine running with minimal electrical load, it dropped to 11.5v with the lights on. Turning them off again it climbed back to 14v. That is a symptom of a very weak alternator i.e. only able to put out a fraction of the current it is supposed to be capable of. A replacement alternator delivered 13.7v at idle with lights, fan and indicators all on, so somehow the water splash had damaged the alternator. Because it was still putting out some current, as indicated by the 14v with minimal electrical load, that was enough to keep the ignition warning light extinguished, even when the system voltage dropped to 11.5v. The reason the warning light didn't come on is because it is comparing the voltage at the alternator with the voltage at the rest of the cars electrical system. When these are both the same the light will not glow, and when both alternator and system voltages are low as in this case it still will not glow. In this case a voltmeter would have immediately and clearly shown the problem, but it has to be said that dimming lights and slowing indicators should also have alerted him. Unlikely to have enabled him to do anything about it at the time, but he would perhaps have been able to stop at a warm pub to ring his recovery organisation, rather than being stranded in the middle of nowhere. I've often wondered, if that happened to me on some of our jaunts around the country and Europe before I had a phone with sat nav, just how I would describe to the AA (in my case) exactly where I was!

Schematics

  Schematics

Types

Mounting

Alternatives May 2016

Ignition Warning Light (aka 'Idiot Light') and Charging Theory Schematics

Dynamo Control Box April 2013

  Alternator operation

Warning light resistor:

Testing Output:

  Trickle-charge current:

Charging Faults:

Alternator Brush Replacement March 2013

Alternator Harness Plug Clip October 2014

Pulleys August 2019

Converting Dynamo to Alternator

Schematic

Probably the main reason for converting is to get a higher charging current as the dynamo is limited to 22 amps. Although this is usually adequate for most normal, and particularly 'classic' use, when stuck in traffic with headlights, heater fan etc. on the charge will almost certainly not be adequate which means you will be discharging the battery and on a daily driver this can rapidly reduce the battery to a point where it will no longer start the car. Bad connections will limit current flow, and will contribute to a drop in system and charging voltage, and I think it is this which people are seeing as much as insufficient output from the alternator. Even 'normal' volt-drops up from the solenoid and particularly with a voltmeter on the green circuit can be enough to reduce the indicated voltage below 12.5v, even though the voltage at the solenoid and hence the battery is above this critical point. A good example of a little knowledge being dangerous. Of course if you are going to significantly increase the electrical load then you may well need to consider an even higher rated alternator from another source.

One common misconception seems to be that fitting a higher rated alternator is automatically going to push more current through the wiring, and people get paranoid about uprating it. The maximum current that will flow depends (to the largest extent) on the electrical load, not the maximum capacity of the alternator fitted. If you only have 40 amps of load then only 40 amps of current will flow, even with a 60 amp, 80 amp or 100 amp alternator fitted. Having said that high-rated alternators like the 80 and 100 amp will be better at maintaining sufficient charge at idle if you have added electrical loads, as well as when running.

About the first thing to say about the process of converting from dynamo to alternator is that unless it has already been done you almost certainly will have to convert from positive earth to negative earth. Positive earth alternators will probably be very difficult if not impossible to find, a negative earth will be tricky to convert to positive, and the availability of used, rebuilt and new negative earth alternators of various types is almost infinite. Also it might be safer to take things one step at a time and do the polarity conversion first, check everything works OK, and only then do the alternator conversion. Getting the polarity wrong with an alternator connected will probably destroy it, and there is only one very simple step which will be 'wasted'. These notes only cover use of a Lucas alternator, there are too many variations in Bosch and GM Delco alternator connections, although the alternators themselves are quite suitable for use.

March 2013: As well as a suitable alternator you will also need an extended bracket to connect the rear mounting point of the shorter alternator to the original mounting points on the block. Unless this is done rigidly vibration can fracture the front mounting ear on the water pump, so extending the existing bracket in some way is not a good idea. Fortunately a number of places sell a suitable bracket - 12G1053, such as Leacy, Brown & Gammons, and Sussex Classic Car.

The usual advice is before starting any of the following work disconnect the battery earth strap first, and only replace it as the final step. However on cars with the remote solenoid it is very easy to remove the brown wire from the spade on the battery cable terminal on the solenoid, and as long as this can't drift back towards its spade while you are doing the work then this is fine.

Rewritten November 2012 (following the opportunity to work on Chris Mottram's car):

The dynamo circuit originally had a standard gauge brown/green (field) wire, and a thicker gauge brown/yellow (output) wire. Because of this it's best to use the thicker brown/yellow wire for the output of the alternator, even though this goes against the convention on cars equipped with an alternator from the factory. Therefore the brown/green is used for the INDicator wire, this is method 1.

There is an alternative method if you are adding a lot of significant loads to the cars electrics, and consequently fitting a high-power alternator, and that is method 2.

Method 1:

You can leave the control box in-situ and hence use it's spades for interconnecting most of the wires. But you must cut off the small spade from the brown/green and fit a large spade so it can be moved from the F terminal to the D terminal in place of the thick brown/yellow. The displaced brown/yellow then has to have its large spade cut off and be connected in some way to the brown circuit. The method used with a conversion kit from a well-known supplier is a ScotchLok connector, but these have a habit of working loose over time and are not a good idea on things like the alternator output cable which will carry quite a high current, and the slightest resistance will cause significant heat to develop as well as reduce the output of your shiny new alternator. You could instead use a ring terminal of the appropriate size and add it to the solenoid terminal that carries the battery cable. That would give a good connection, but you MUST disconnect the battery earth cable before working on this terminal. However in both cases you would need to extend the brown/yellow, which has its own problems as to what method to use. I suggest it's better to bite the bullet and remove the control box altogether, make proper soldered joints, and give better access to other components into the bargain.

Remove all the wires from the control box, and the control box from the panel on the inner wing (easier said than done with those fiddly nuts behind the panel!). The standard gauge brown/yellow from the WL terminal, and the standard gauge brown/green from the F terminal have their spades cut off, a portion of insulation carefully stripped so no strands are cut, twisted together, and soldered. Put a couple of lengths of heat-shrink tubing just over the soldered section, each about 1/4" longer than the soldered joint, heat-shrinking them one at a time. Then fold the 'spare' bit of tubing over, and fit a couple of lengths of larger diameter tubing over that and an inch or so of the insulation on the wires. This circuit supplies the 'priming' voltage to the alternator to start it charging, and indicate charge failure.

The three original browns from the B terminals on the control box, and the thick brown/yellow that came from the D terminal, are dealt with in much the same way. Cut all the spades off, making the four wires the same length, and strip about half an inch of insulation from each, being careful not to cut through any strands. Check the conductors are clean, using fine emery if tarnished. Twisting four, heavy gauge wires together is not really advisable, so I used the method adopted by the harness manufacture for the sealed, multi-way junctions that exist on some of the MGB harnesses. First I put a short piece of heat-shrink over the insulation of all four wires to keep them together. Then I got a strand of tinned copper wire and wrapped it round the four conductors, for the length of the stripped section, to make a good mechanical joint, only then did I solder the joint. As before a couple of lengths of heat-shrink about a 1/4" longer than the soldered joint are fitted over the soldered part, then the 'spare' bit at the end is folded over, and a couple of pieces of larger diameter heat-shrink fitted over that and an inch or so of insulation, as with the previous two wires.

Finally that leaves the earth wire from the control-box. It's best not to leave this floating around, but rather than cut the spade off I folded it over and again used heat-shrink to insulate it. That leaves three heat-shrunk spurs sticking out of the main harness at various angles, but a cable-tie can be used to attach those out of the way against the panel the control-box came from.

Method 2 is very similar but to carry higher currents, if you are adding significantly to the cars electrical load and hence using a high-output alternator, it would be best to use a new heavy gauge (of the appropriate size for your loads) brown wire from the output of the alternator to the battery cable stud on the solenoid at least. You may also need to use similar gauge wire from there to your non-standard, high power loads, the brown from the spade on the solenoid going to the control-box should be enough for all the factory loads. Again, you MUST remove the battery earth connection before starting work on the battery cable stud on the solenoid. After that you can leave the browns on the control-box B terminals, and the brown/yellows on the WL and D terminals, using the heavy gauge brown/yellow at the alternator for its INDicator terminal. The standard gauge brown/green should be removed from the control-box F terminal and its spades taped back and insulated at both ends to prevent them coming into contact with anything else. Alternatively you can discard the control box and join the three browns from the B terminals, and join the two brown/yellows, and insulate the earth wire, using the techniques described above.

If converting the polarity at the same time leave the alternator unplugged when connecting the batteries the new way round for the first time, if you get it wrong and the alternator is connected you will blow its diodes and burn wiring. Confirm the polarity is correct before continuing by connecting a voltmeter between a brown in the alternator plug (meter +ve) and an engine earth (meter -ve).

After confirming that the polarity is correct connect an analogue voltmeter (digital meters may give unpredictable results) on its 12v scale in place of the battery earth strap. There should no voltage registered. If there is, it will probably be a full 12v, and means some circuit on the car is switched on (courtesy lights? Boot light?) which should be found and switched off before proceeding. When no voltage is shown plug in the alternator. You may now see a few volts registered, which will be the normal microscopic leakage current of the diodes and can be ignored. If a full 12v is shown the alternator diodes are faulty. If the reading is correct, replace the battery earth strap. If you don't have an analogue meter use a low-wattage 12v bulb instead, such as one from one of the gauges. If there is any glow at all from the bulb, current is flowing, proceed as above. Only a significant current flow will cause the bulb to glow, connecting the alternator should not be enough.

You now have the alternative of going for broke and reconnecting the battery earth strap, or taking a smaller step. For this connect a high wattage bulb e.g. a headlamp bulb (e.g. an old one with one filament gone - "If you haven't found a use for something yet ...") in place of the earth strap. This will allow a safe amount of current to flow while you turn each thing on in turn. If any circuit is faulty and is full short the bulb will limit the current and prevent damage to wiring and components. Some things (low current items) will work almost normally, higher current items probably not. Low current items might cause a dim glow from the bulb, higher current items a brighter glow. Only turning the key to crank (nothing will happen) should cause the bulb to glow at full brightness, nothing else. When you are happy, reconnect the battery earth strap.

With the ignition off there should be no glow from the ignition warning light. A glow now indicates faulty alternator diodes or voltage regulator, or incorrect connections to it.

With the ignition on the warning light should glow. If no glow remove the plug from the alternator and connect an earth to the brown/yellow terminal (NOT the brown!). If the warning light glows now the alternator is faulty if not then the circuit is broken back towards the warning light, possibly where the brown/yellows are joined where the control box was, or a blown bulb. There should be 12v on the white at the bulb holder and an earth on the brown/yellow to light the bulb.

With the warning light glowing start the car, and with the engine revved above 1000 rpm the light should go out. If the light remains on the alternator is faulty. Only if the revs drop below about 600rpm should the light come back on, stay off till about 1000 rpm, then go out again as before. Early cars had an idle speed of 500 rpm and if the light comes on at idle, particularly with lots of load switched on, then you would be advised to increase the idle speed to, say, 700 rpm to keep the light out at all times. While the light is on the alternator isn't charging and the battery is discharging, which largely negates the effort of converting!

With the engine at about 1000 rpm, and all loads switched off (and the warning light off), measure the voltage between the brown at the fusebox and earth. You should see about 14.5v, much less or more than this indicates a faulty alternator. Now turn on headlights, brake lights, heater fan etc. The voltage will probably drop, possibly to less than 13v with one of the smaller Lucas alternators. Increase the revs to about 3500 and the voltage should rise above 13v again, indicating the battery is still being charged even with everything switched on. If the voltage doesn't rise above 12.5v check the voltage at the alternator output terminal(s), and if similarly low here it indicates the alternator has a low output current fault, however note that the smaller Lucas alternators will probably not be able to supply anything above the standard factory loads at best. If the voltage is closer to 14v at the alternator then there is a bad connection somewhere between the alternator and the brown at the fusebox, check the voltage on each brown wire and the battery cable at the solenoid.

Converting 16AC Alternator with Separate Regulator to Later Alternator with Integral Regulator Added October 2007

Schematic

First remove the battery earth strap, and don't replace it until you have made all the wiring changes.

The alternator should have the following wires:

Heavy gauge brown (output) going to the solenoid
Black (earth/ground)
Brown/yellow (Indicator) coming from the voltage regulator
Brown/green (field) coming from the voltage regulator

The voltage regulator should have the following wires:

Black (earth/ground)
Brown (12v supply to the voltage regulator) coming from the solenoid
Brown/green (field) going to the alternator
Brown/yellow (Indicator) going the alternator
Brown/black (Indicator) coming from the ignition warning light

These last two (brown/yellow and brown/black) are probably the most important. They are electrically connected together at the voltage regulator, and they must remain connected together and isolated from everything else after the conversion, so that effectively the Ind terminal on the new alternator is connected to the warning light. As well as lighting the warning light, the current flowing through the warning light to the alternator acts as a 'pump primer' and is needed to get the alternator to start charging. If both wires are in the same spade connector then all you have to do is securely insulate that connector so it cannot come into contact with anything else. If they have two separate spade connectors then these should be cut off, the wires twisted and soldered together, and the joint insulated with at least two layers of heat-shrink tubing (slip these on before twisting and soldering!).

Of the other wires the heavy gauge brown goes to the output terminal of the new alternator (See here for Lucas 16/17/18ACR terminals) and remains on the solenoid.

The two black wires and two brown/green wires at the alternator and old voltage regulator are no longer required and should be taped back out of the way of anything else.

Note that the earthing point to the body for the black wires from the regulator and alternator is also the earthing point for four other wires (heater fan, instruments, wipers and headlights), even if the regulator and alternator earth wires are removed the other four wires must still be earthed, you will get some very strange results without it.

That should leave the brown going from the voltage regulator to the solenoid, and this is also no longer required. If the two standard gauge wires at the solenoid (there should only be two apart from the large battery cable and the heavy gauge brown from the alternator) have separate spade connectors it should be easy to determine with an ohmmeter which goes to the voltage regulator and which to the fusebox. The one going to the fusebox must remain connected, but the one going to the old voltage regulator can be taped back both ends. But if these two brown wires terminate in the same spade connector then you have a 50/50 chance of cutting the right wire off. You could cut one wire off and then test, and if you have cut the wrong wire reterminate it on a new spade connector and tape the other one back, or cut both off and test and then reterminate the wire going to the fusebox, taping the other one back. But the best thing to do would be to cut both wires from the existing spade connector, identify the one going to the old voltage regulator and tape that back both ends, then for the other wire that goes to the fusebox reterminate that on a ring connector that will go on the stud with the battery cable. This makes a more secure connection than the spades, as all the current for the cars electrics goes through it. While you are doing that you can do the same with the output cable from the alternator, for the same reason. Later starters did have all the browns terminated with ring connectors on the battery cable stud as standard.

With all the wiring changes done, the new alternator mounted but the brown and brown/yellow not yet connected (make sure they can't short out on anything), and everything in the car switched off including doors etc. closed so the courtesy lights aren't on, check to make sure you haven't shorted any of the brown wires to earth/ground. The safest way to do this is to connect a voltmeter on its 12v scale in place of the battery earth strap. If the voltmeter shows any reading at all, there is something drawing current. Anything less that 12v shown is a tiny current, could be a clock or the 'keep alive' circuit of a radio. If it shows a full 12v it could be a small current e.g. something simple like a courtesy light left on, or it could be a full short. Connect a test-lamp or other 12v bulb (an old headlamp bulb is best) in place of the earth strap, and if it glows brightly it is a full short which must be investigated and fixed before you proceed.

A cruder check is to tap the battery earth strap very briefly on the -ve post of the battery. You should not get any kind of a spark. If you get a small spark maybe one of the courtesy lights or similar is still on. If you get a big flash then it looks like one of the browns is shorting to earth somewhere, which again must be investigated and fixed before you proceed.

With the brown at the alternator still not connected and protected from shorting to anything, and with the battery earth strap reconnected, connect the brown/yellow to one of the standard sized spades and turn on the ignition. If the warning light glows you can proceed. If it doesn't then turn off the ignition, move the brown/yellow to the other standard sized spade and try again. If the warning light glows now again you can proceed. If not, disconnect the brown/yellow from the alternator and connect an earth to instead end and try again. If the light glows now then possibly the alternator is faulty, or possibly the wire should go to yet another spade if you have non-Lucas alternator. If the light doesn't glow with the earth connected however, then either there is a problem where the brown/yellow joins the brown/black, or the bulb has failed, or there is some other open-circuit between the end of the wire at the alternator and where the white from the bulb joins the others at the ignition switch. This must be found and fixed before you proceed, or the new alternator probably won't charge.

With the bulb glowing with the ignition on, carefully connect the heavy gauge brown to the output spade, remembering it is live and unfused. You may prefer to disconnect the battery earth strap again while you attach the brown output wire, then go through the same tests for a short as before. With other types of alternator there can be different connection arrangements, some have an output stud as well as an output spade, use the stud as it will have a better current carrying capacity.

With both output and indicator wires connected to the alternator, start the engine revving it as little as possible, and watch the warning light. The warning light may still be glowing, so slowly raise the revs, and at about 900 rpm the light should go out. Now use a volt-meter on the brown at the fusebox and you should see around 14v. If so the new alternator is charging. With the engine idling turn on the lights, press the brake pedal, switch on any other electrical loads you can, and the voltage will drop to some extent, possibly towards 12v. Rev the engine to about 3k and the voltage should rise again above 12.5v. With everything turned off, and a fully charged battery, and the engine revved to about 3k, you should see a maximum of about 14.5v. With the engine idling again select 4th gear, handbrake and footbrake on, and slowly lift the clutch pedal up so the revs start to drop. The warning light should come on again at about 600 rpm. Dip the clutch again and take it out of gear, slowly raise the revs again and the light should go out again at about 900 rpm. If your normal idle causes the warning light to come on again anyway, it might be an idea to raise the revs a bit so it stays out, that way the alternator will still be charging at idle, rather than the electrical loads of the car draining the battery.

If the light doesn't go out when revved, and you have two standard sized spades on the alternator, switch off, and move the brown/yellow to the other standard spade. Turn on the ignition and if the light glows start up and try the tests above again. If the warning light doesn't go out when the engine is revved, with the brown/yellow on any of the standard sized spades that it glows on with the ignition on, or the voltages don't show as above, then possibly the alternator is faulty.

'One-wire' Alternators Added January 2010

  Safety First!
It is absolutely vital that batteries are securely clamped into their cradles. Many who switch to a single 12v, or fit battery boxes, seem to leave the batteries unsecured. Although it is an MOT requirement to have a secured battery the tester is not allowed to 'dismantle' any part of the car so unless he looks up into the cradle and prods the battery to see if it moves they are unlikely to spot an unsecured one. Roger Parker formerly of the West Midland Police motorway patrol unit has seen the effects of an unsecured battery and recounts the following:

Clamping arrangements:

• MGB batteries were always held down with a strip of angled metal as a retaining bar along one edge of the battery, and two hooked rods down from that to holes in the cradle, secured with double-nuts (GHF200) above the retaining bar. The batteries sit on rubber buffer strips (AHH6351) along the front and rear angles of the cradle base.
• CB cars have the retaining bar (AHH6353), with a strip of plastic packing (AHH5169) underneath, along the rear edges of the batteries. The rods (AHH6750) run down and forwards with hooks going through holes in the sides of the cradle. Care needs to be taken with the positioning of the retaining bars and battery connectors as with posts in diagonally opposite corners one of them on each battery will always be very close to the retaining bar. No problem with the earth terminal, but the other three are all 'live'. Originally there was an option of a larger battery that required a rectangular frame (AHH7109 - NLA) to sit on top of the battery and be clamped down by the rods.
• RB cars have the retaining bar (34G2065) and packing 37H3743 running fore and aft along one long side of the battery. 12v batteries have the terminals along one long side, and are normally positioned to put the -ve terminal towards the rear of the car. This means the terminals can be nearer the tunnel (Type 072) or nearer the off-side of the car (Type 075) depending on which type of battery you get, and the norm is to have the posts nearer the tunnel. The retaining bar goes on the opposite long side of the battery to the posts, i.e. normally towards the off-side of the car. The rods (BHH1586) run down almost vertically to one pair of two sets of holes in the angle pieces that form the base of the cradle. In theory the rods could be angled across the short sides of the battery to the other pair of holes, but they would need to be longer, and the ends of the retaining bar would also be at an angle to the rest of it, as they are on CB retainers, but RB retainers are the same shape all along. Physical battery size can impact on the security of the retainer.
• The physical size of both 12v batteries and 6v batteries can vary, and with the smaller ones you should make sure that the base is fully supported by the front and rear angles of the cradle base. My latest set of 6v are slightly smaller than those I've had before and I've had to add a strip of metal to the rear angle of the base to support the battery properly. With 12v batteries and the rods going straight down there is the additional problem that on a smaller battery they will be angled away from the battery when in the holes, which will pull the retaining bar off the battery. I've had to add some packing between the other side of the battery and the side of the cradle to move the battery further across. Yet another problem is where the battery is shorter top to bottom than the rods can accommodate, and you run out of threads. Many years ago I had to bend new 6v rods into an S-shape to take up excess length, and put thicker packing on top of the 12v battery. The latest 6v batteries are even shorter but I have dies now so have been able to cut more thread.
• Fitting the retainers can be a fiddle. I put the rods in the retaining bars with one nut started, then fit the assembly over the battery. By twiddling the upper end of the rods you can turn the hooks towards the holes and hook them through. The trick is to have the nuts on far enough so that the hook only just goes through, then tighten that a few more turns so it can't come out, while you fit the other rod in the same way. Slide the plastic packing under at that point, before tightening the first nuts and adding the second nuts, tightening the two against each either to lock them without applying too much pressure to the case. The irony is that this process is much easier with smaller batteries than larger as you can get your fingers down the sides more easily!

When doing any work involving any battery terminal, or the battery cable at the starter solenoid, or any 'always live' wiring such as brown wires, always remove the earth/ground cable from its battery post first and replace it last. This is regardless of whether the car is negative earth/ground or positive earth/ground, twin 6v batteries or single 12v. Many sources of automotive information say to always remove the negative connection first before the positive, but they are only thinking of 'modern' cars, not classics. I repeat, always remove the earth connection first and replace it last, regardless of polarity, and that applies to all cars i.e. moderns as well as classics. The reason for this is that if your spanner should happen to touch the body whilst it is also touching the earth/ground post of the battery nothing will happen. Once the earth/ground connection is removed it is now safe to undo the 12v (aka 'hot' or 'live') connection, because if your spanner should happen to touch the body while it is on the 12v post still nothing will happen because the earth/ground connection has already been removed. If you remove the 12v connection first and the spanner touches the body whilst doing so, you will generate a large spark which can ignite any battery gases that may be present, or maybe even cause the battery to explode in your face! So it's always earth cable off first, and back on last, regardless of polarity.

Car batteries contain a large amount of energy and can discharge it very rapidly under the wrong conditions, generating large sparks, toxic fumes, even exploding and showering corrosive liquid around. Not all batteries have polarity markings, on classic cars battery cables are not usually colour-coded for polarity, and the battery terminals and connectors do not usually have insulating covers. Great care must be taken to ensure the 'live' or 12 volt terminal does not accidentally contact the car body or a large spark can be generated which can ignite battery gases, and tools or other metal parts can become welded and glow red-hot.

Many MGBs and other classics of the era have two 6 volt batteries instead of a single 12 volt battery, and the two 6 volt batteries are connected together with a link cable. Both ends of this link cable must be considered as being 'live', as well as the 12 volt terminal, and need the same care to prevent accidental contact with the car body.

Another warning is never to run the engine without a battery connected, especially on alternator equipped cars. Although the alternator has a voltage regulator it actually generates pulses of AC which are then rectified to DC before being regulated, and will output pulses of high voltage if a battery is not connected. The battery acts as a very powerful smoothing device which prevent the high-voltage pulses reaching other circuits on the car. Without a battery the high-voltage pulses can blow bulbs and damage electronic circuits.

Twin 6v Link Cable

Battery Cover December 2014

Replacement Batteries

Battery Drains January 2014

Battery Types Added December 2008:

  Plastic battery bins

Fused Battery Connector January 2014

    Battery Chargers Updated November 2007

As on the V8 I installed the switch on the heel-board behind the drivers seat where I can reach it quickly in an emergency. The +ve lead passes right by here so that is the most convenient place to put it without buying new and extending the cable run. Some argue that it should be in the earth lead, but it makes no difference electrically and as far as cutting the power goes (but see below). One slight advantage of having it in the earth lead is that this is the lead that must be disconnected first (and reconnected last) when working on any of the battery connectors - twin 6v or single 12v. However unless all you are going to do is tighten one of the other posts, when you would need to remove the earth cable even though you may have a switch in the 12v cable, there is no advantage. If you are going to remove the battery you will have to disconnect the earth cable anyway, regardless of where any switch might be. As far as working on anything else goes i.e. the solenoid or anything with brown wires having the switch in either lead is equally safe, as its immobilisation function, and it's emergency disconnect function. The main advantage of being in the earth cable is that it avoids cutting the 12v cable and fitting two lugs. One slight disadvantage of having the switch in the earth cable but positioned on the (left-hand in this case) heelboard is that with single-12v battery cars you will need a longer cable to get from the battery to the switch, but the original cable could be used to get from the switch to earth. However the major disadvantage is that if you want to keep power to a clock and/or radio (but isolate power from everything else e.g. alarm) you would have to completely insulate said radio and clock from the car's body - which is not going to be easy - and run separate earth wires from the battery-side of the switch. In that event putting the switch in the 12v cable and running a fused bypass circuit to said clock/radio is the only way to go. Update January 2014: In a Bulletin Board discussion on this topic a pal mentioned another benefit, and this is if something goes wrong with your work, and it shorts to the body, then better the earth cable than the 12v cable. And a day later someone elsewhere posted that he had seen an unattended MGB burst into flames, over the back axle, so probably the battery cable shorted close to the battery. If that was the last few inches before the battery terminal, then a cut-off switch in the 12v cable at the heelboard would be no use, but one in the earth cable would be - if you could get to it. That person wondered if a battery terminal fuse might be even safer. But the biggest advantage of fitting it in the earth cable is that you just swing the existing earth cable round from the body to the switch, and provide a new one from the other side of the switch back to the body, instead of cutting the 12v cable. Much easier, and reversible should one wish. For twin-6v systems this means the switch is behind the passenger seat, but still easily accessible for the driver. End of update

In hindsight it would have been easier to install it in the earth cable, as both ends could have been dealt with off-car. You would need at least one new earth cable, plus perhaps a longer replacement for the original. In that case CB cars with twin 6v batteries would have it on the left-hand heelboard, with RB and CB cars converted to single 12v on the right-hand side, being careful to avoid the 12v cable. In this latter case if you still have the armoured sheath between the battery boxes you could run the (longer) earth cable through it and have the switch on the left-hand side. But if not, and you consider running the cable through the holes and being unsupported between them, be aware that it can rub on the prop-shaft and wear the insulation away. Not a huge problem on the face of it as both are at earth potential anyway - except that if you then need to turn the switch off for any reason power may still be connected.

Update April 2009: Very glad I had done this, and put the switch in the 12v cable instead of the earth, as when I replaced the link cable the new one was too short to go between the posts in their existing positions. The only possible way to make it reach comfortably then meant the 12v cable was too short. By having the switch in the 12v cable it was a relatively easy matter to remove the short piece between 12v terminal and switch, turn that battery round, then make up a new, longer cable to go between the new position of the 12v post and the switch.

Note 1: Unless otherwise indicated image are from Moss Europe.
Note 2: I have seen the dash harness for 77 and later UK models with capless/wedge-type bulb holders.
Note 3: Generally the 3-digit number following the GLB code is the industry code for the bulb style, fitting and output.
Note 4: It should be noted that LED bulbs are not always road-legal.
Note 5: RHD RB optional H4 headlamp - some suppliers describe this as 'flat glass', but whilst mine are shallower than the sealed beam, they are nowhere near flat.
Lucas Bulb Catalogue and Application Guide

  Bayonet bulb holders:
Bayonet bulbs use two pins one either side of the bulb base to push down into channels in the bulb holder, then turn a few degrees clockwise and come back a little way to lock the bulb in position. Dual filament bulbs have offset pins to ensure the bulb can only be installed in one orientation and the correct filament illuminated. With these even though the bulb can be pushed into the holder either way round it takes a particularly ham-fisted or brutal person to turn it and lock it in position, which has been known! If it doesn't turn easily then withdraw, rotate 180 degrees, and try again.

For both factory and after-market clocks, if you find it works with the lights off but not when the lights are on, then the earth supply to the clock is probably missing.

May 2020: Herb Adler has sent me some information on a classic Hillman clock that he tried to make use of, but in the end gave up. It's rather complex mechanically and electrically and the info document from the Hillman Car Club of South Australia recommends servicing one particular component every couple of years! No wonder Herb eventually installed an 80s VDO mechanism in the case.

Multiway Connectors:

If not moulded-on the connector pins and sockets can be teased out of the connector blocks using a bit of tubing the right size. The pins and sockets have two little sprung tabs that stick out and latch behind a flange on the connector block as they are pushed in, I've used a piece of brake pipe to get them out. For projects or modifications Autosparks (and maybe others) sell plug, socket and connector kits in various sizes and shapes.

With PO wiring or re-terminations also check the connection between the wire and the connector if you can. With PO 'repairs' using dodgy components and techniques or if the car has been abandoned in a field for years bad connections can develop that don't occur in normal use. When adding any wiring I really don't like those blue ScotchLok connectors, I've found on a number of occasions that after a while even in the cabin the bifurcated blade loses tension on the copper strands and they start to cause problems. If you are near a bullet connection then substitute a 4-way for a 2-way and put a bullet on your new wire, or if there is already a 4-way with 4 wires in it I'd rather make up a couple of inch length with bullets and add a second 4-way, but you can get 6-way connectors. If you want to tap into a wire going into a multi-way connector then really there is nothing for it but to cut the wire, put two bullets on the end, and use a 4-way. However if you are near a spade connection then piggy-back spade connectors are a good way of tapping into these.

The fusebox often causes problems in a number of places. The most obvious is where the fuse ends are clipped into the holders, they don't grip very tightly and corrosion can burrow underneath. There are also the spade connections, and on the 4-fuse type a rivet underneath the fusebox that connects the spade terminals to the fuse holders can corrode. Last but not least the fuse itself can corrode internally, even though the element appears to be whole. Generally speaking (but see below) problems in the fusebox will only affect the circuits fed by it i.e. the purple, green and red circuits, note that nothing to do with the starter or ignition goes through any fuses. However the white feed from the ignition switch may go onto one spade, and come off another spade for the coil on some years, and only in this case could fusebox problems cause ignition or starting problems. Any white circuit that gets its 12v supply from the 'other' spade on the fusebox could be affected by corrosion on the spades. More information on fuseboxes here.

There are a couple of areas that can cause problems even in regularly used and cared for cars. Horns, lights and electric fans can all suffer from poor connections as their connectors tend to be exposed to the worst of the weather - spade connections of horns, bullet connectors for all the front lights, and two-pin connectors for electric fans. Chrome bumper indicator/parking light units earth through their physical fixings to the wing, and these are exposed to the worst that the front wheels can throw at them by way of water and salt. And even though rubber bumper indicators have a wired earth it comes via a rather flimsy bullet-type connection on the light unit that is also exposed to all the weather. Rear light clusters can also develop earthing problems as they also rely on the mechanical fixings, but being in the boot and protected from weather they should be less likely.

Bullets:

I don't trust the mechanical strength of crimped connections, even when using the proper tool and doing a double crimp, so I always solder as well as crimp, using the semi-insulated female crimp spades or cutting the insulation off female crimp bullets which are only available fully insulated AFAIK. Some claim that heat and solder wicking affects the strength of the copper conductors causing it to fracture about 1/4" from the end of the connector, but I always use heat-shrink tubing over a soldered crimp connector and about the first inch of wire to stop it flexing at the connector anyway.

  Spade adapters:

  Branching connectors:
To connect several additional circuits to the some source i.e. 12v or earth these WAGO221 branching connectors are ideal.

Sealed Wiring Junctions Added November 2009

The chance of a fault developing inside one of these connections is highly unlikely bar severe abrasion of the insulation and hence shorting to metalwork, which is more likely to happen to a length of wire anyway.

Fuseboxes

In 1968 and 69 in-line fuses were added to protect the parking lights - one for the front and one for the rear, simply added to the bullet connectors where the red wires came out of the main harness for the rear harness for the rear lights, and back into the main harness for the front lights.

The 4-fuse fusebox is also 'handed' but in a different way to the 2-fuse, in that the front of the top two fuses are connected together as part of the splitting of the parking lights into two separate circuits with one fuse for each side. This link can only be seen from the rear, as shown here, but if you have some funny electrics and think you may have fitted it the wrong way round (which will put the linked pair at the bottom rear) you can check from the front by looking for the terminal numbers. These are quite small and easy to miss (circled on this image). In fact the Lucas Part No. and week of manufacture are easier to spot ('rectangled') and these should also be at the top of the fusebox when fitted to the car the correct way round.

Also shown are the riveted connections on the rear of the fusebox, which can suffer corrosion and bad connections. You may think that a solid connection here would be preferable, but the rivets allow the external spades to move from side to side while fitting the wiring connectors without twisting the fuse-holders, which would mis-align them with the end caps of the fuses. This could result in very small points of contact, so limiting current and resulting in volt-drops and hot-spots, which as well as affecting the performance of the electrics connected to that fuse this can also cause premature fuse failure. This image shows typical corrosion that can develop on the copper fuse holders.

The terminal numbers count from 1 at the top front to 8 at the bottom rear, slightly illogical when you consider that the bottom two fuses are the originals carried over from the 2-fuse fuseboxes. If you are wondering what the three circular holes in the fusebox are for and have lost your cover, then this image shows that the middle hole is for the cover locating peg and the two outer holes for spare fuses.

  Fusebox connections: April 2016 It's a common misconception that all wires at the fusebox are fused. Only the ones towards the rear are fused - outputs to the purple circuit (horns, interior lights etc.), green circuit (fused ignition stuff like instruments, brake lights, reversing lights etc.) and on fuseboxes with four fuses red circuits (parking lights one fuse per side). The ones towards the front are the unfused supplies to the fusebox - brown (powered all the time), white or white/brown (powered with the ignition on), and red/green (powered with the lights on).

It's also confusing as to why there can be two or more browns, whites or white/browns on the front of the fusebox. This is because the fusebox is being used as a branching point as an alternative to using a multi-way bullet connector with three or more wires. The power comes in on one of the wires, and goes out on the others as well as going through the fusebox. This happens elsewhere where there are two or more greens for example on a component - again one is bringing power in and the other is daisy-chaining it on to another component.

Prior to 1977 a white wire from the ignition switch supplies power to the fusebox, and there can be anything from none to three other white wires at the fusebox. These other whites are feeding things the coil, fuel pump, overdrive and heated rear window at various times. But where there is only one white wire on the fusebox, the ignition switch is feeding a bullet connector by the bulkhead, and further wires in that bullet connector are then feeding the fusebox, coil and fuel pump. There are many subtle differences over the years, and you have to be looking at the right diagram for your car to work out what is going on for diagnostic purposes.

In the case of the white/brown on 77 and later cars with the ignition relay, the feed is from the ignition relay to the fusebox, but after that there are differences between 77 and 79, with the change being made some time in 78.

It started off with there being three other white/browns at the fusebox - one to the coil ballast, one to an in-line fuse for the cooling fan, and one to the usual bullet connector by the bulkhead for the fuel pump and overdrive - see this schematic.

After the change there were only two other white/browns on the fusebox - one to the in-line fuse for the cooling fan, and the other to the bulkhead bullet connector. Now the coil ballast is fed by the ignition switch, but using a white/brown wire on the same ignition relay terminal as the white wire! This goes to the coil ballast then branches off to a new in-line fuse under the fusebox, which feeds things like the heater fan, indicators, and heated rear window - see this schematic.

If that weren't enough both these in-line fuses have white/brown one side and green the other, making three separately fused green circuits in all. Not only that but the way the wires run and the fuse holders have been connected, it's possible to connect both the white/browns together, and both the greens together (as a new harness that came to me was), which makes things very confusing indeed.

  Fusebox mounting: June 2016 The first thing to say is make sure you fit the 4-fuse fusebox the right way up! These have a link between one end of two of the fuses, and this must go at the top and the front or when everything is connected up the ignition will be permanently powered.

The 73 roadster fusebox is screwed to a plinth, with nylon sockets pushed into it, and pan-head self-tappers.

The 75 V8 fusebox is screwed to welded nuts on the inner wing, with a sheet of insulating material under the fusebox, then spacers between that and the wing. I'm not sure why the insulating sheet or the spacers were felt necessary, there is nothing loose under the fusebox and the electrical parts are spaced away from the base. The roadster doesn't have either, and the surface of the plinth is solid just like the inner wing.

Moss Europe and MGOC say the screws are SE910201 (3/16" UNF x 3/4") up to chassis 456250 (Feb 78), with AB610081 (No.10 self-tapper pan head 1") and hence the nylon socket after that. Brown and Gammons says they are PMZ316 (3/16" UNF but 1" long) from 70 to 77, with AB610081 (as above) from 77 on. Note that my 73 has the self-tappers and nylon sockets despite the above saying these weren't used until 77 or 78, and whilst they are 1 1/4" from tip to top of the head, they project from the nylon socket by about 1/4", so a 1" should be OK. The screws on the V8 are 3/16" UNF, but they are 1 1/2" long rather than the 3/4" or 1" mentioned above. When fitted the end of the screw just reaches the far face of the nut so they are not over-size, a 1" would not allow the spacers to be used. Since a nylon socket was used in the 73 and avoids problems of corrosion either causing the screw to seize or the panel to rust, it seems odd that the 75 should have the less desirable arrangement. No mention of spacers or insulating sheets by the above three suppliers. The Parts Catalogue has no information at all on screws, nylon sockets, spacers, or insulating sheets.

  Fusebox replacement: May 2016 I decided to replace Bee's fusebox as prior to the MOT the horn seemed a bit iffy, the fuse holder springs were well tarnished, and it isn't easy to clean them. But the new one doesn't grip the fuses anywhere near as tightly as the old - I had to tweak the springs closer together, and the neither does the cover hold the spare fuses as tightly as the old. One of the spares does seem to be a little undersized and isn't gripped at all, but was held by the old cover. Never mind, I'll just use the old cover on the new fusebox ... only to find that the new fusebox is a couple of mm longer and the old covers won't fit! No 'Lucas' branding or part number on the cover, so obviously after-market despite the OE part number being used by suppliers.

As mentioned above I decided to replace Vee's fusebox as well (at £10 from Moss it's not worth trying to clean up corrosion) but one of the screws sheared as they go into welded nuts on the back of the inner wing and hence are open to water and salt from the wheel arch, and none of the usual suspects show the correct screws i.e. 3/16" UNF x 1.5". But I got a pair of stainless from Stig Fasteners at £3.12 shipped.

  Fuses: As well as the two or four fuses in the fusebox there were a number of in-line fuses at various times.

• Firstly Mk2s prior to the 1970 model year had two where the rear harness joined the main harness and fused the parking lights - one 10A fuse for the front and one for the back. These were deleted at the fitting of the 4-fuse fusebox where the top two fuses are for the parking lights.
• From 1971 to the end of chrome bumper production the so-called 'accessories' circuit - so-called because it powers the wipers and heater fan (and North American screen washer) as well as the radio - had one fitted under the fusebox with white/green one side and green/pink the other. All V8s i.e. both chrome and rubber bumper had this system, which also powered the electric washers.
• 1973 North American cars gained an anti-run system utilising an electric valve fused with an in-line under the fusebox.
• The 1974 models gained hazard a flasher and these are powered with an in-line with brown wires both sides. Originally behind the centre console (with the flasher unit - hardly convenient). The fuse moved to under the fusebox on rubber bumper 4-cylinder cars, remaining behind the console on V8s.
• For 1974 only North American spec cars had the sequential seat-belt system with its own 500mA line fuse with brown one side and brown/purple the other, location unknown.
• 1977 models had a second separately fused ignition circuit from an in-line under the fusebox with white/brown one side and green the other.
• At some time in 1978 RHD cars gained a third fused ignition circuit, from another inline under the fusebox and also having white/brown and green wires. Someone must have been having a laugh with these as the fuseholders and the wiring is such that you can connect the two white/browns together and the two greens together, which is how a new harness came to me.
• One of these seems to have been replaced by a thermal cut-out - probably the cooling fan fuse. Clausager says that between January and March 1978 "Thermal cut-out switch instead of line fuse to eliminate fuseholders overheating". He indicates RHD as well as LHD, but I've not heard of them being found in the UK, just a couple of occurrences from America. As North America had twin fans but the UK 4-cylinder only a single it does make more sense if it were LHD only. Certainly the first fuseholder I fitted to the V8 (twin fans) melted in normal use.

There has also been some unnecessary worriting about the voltage rating of MGB fuses. Automotive fuses seem to be rated for '32v', or 32 volts, even though our cars are 12v. This seems to be simply because some august body has decided every electrical component must have a voltage rating, and (presumably) because automotive applications don't usually go above 24v they have decided on 32v! An MGB owner was concerned that as his system was 12v, should he be fitting a lower rated 32v fuse instead of a 35 amp? The answer of course is 'no', amps are amps and depend on the voltage and resistance in the circuit they are testing, not some notional maximum safe voltage which is what the 32v represents. But even that notional safe voltage is ridiculous - voltage ratings are supposed to represent the maximum (plus a safety factor) voltage the fuse can break without arcing occurring between the end caps so allowing current to continue to flow. 250v fuses are half the length or less of MGB fuses, as are modern blade-type fuses. The concept of something higher than 32 volts jumping between the end caps of an MGB fuse when the fuse blows is ridiculous, even 20kv HT voltage wouldn't jump that, and 250v fuse are less than half the length. The bottom line is that as long as you fit a fuse with the correct current rating, ideally one specifically for British cars of the era i.e. 17 amp continuous/35 amp blow and not a modern generic item of a similar physical size that seem to be available in America, you will be fine.

You would be well advised to add fuses to the fuel pump and overdrive circuits, as both these have been known to short out and cause major harness damage. See Pump Fusing and Overdrive Fusing.

Blowing Fuses
This can be a bit of a beggar, especially if it's intermittent. Even if it is constant you don't want to keep blowing fuses while you are diagnosing the problem, and I'll deal with this first. Temporarily replace the fuse with a high-wattage bulb. You can get away with a 21w indicator or brake light bulb, but a headlamp bulb (e.g. one with one blown filament but one good one) is better. Why not use a meter? If you used an ammeter, that has a very low resistance to current flow, which will still allow a very large current to flow. If in series with a fuse the fuse will still blow, if not it could damage wiring, or the meter. Solder a couple of wires to the bulb, and use it to replace the blowing fuse. Then while switching things on and off, waggling wires around, or parting and joining connectors, watch the bulb. If the bulb glows at full brightness you know the short-circuit is present. If it's out, or just dim, you know the short-circuit is not present. On any of the fused circuits if only one circuit at a time is powered the test bulb - especially if it is a headlamp bulb - won't glow at full brightness. It will get closer to full brightness if you turn all the circuits on the fuse on at once, like brake lights, reversing lights, heater fan, wipers etc., but not otherwise. The bulb is limiting the current that can flow to a safe level, and you aren't shelling-out for dozens of fuses. However don't think you can operate the car like this, as in the case of an intermittent fuse blow, as the bulb will be taking some of the voltage away from the circuits normally fed by the fuse, so either they won't work properly or they won't work at all. It's simply a diagnostic tool. The fault could be on the wire leading from your test bulb, in which case the bulb will be bright even though all circuits fed by the fuse might be switched off. In this case you will have to study the circuit diagrams and work out where the branches are, i.e. at bullet connectors, so you can isolate various branches to isolate the offending one. Unfortunately while pulling the wiring about to find, disconnect and reconnect these bullets you may well cause the fault to disappear, to appear again at the most inconvenient and inopportune moment. If the test bulb only goes bright when a particular circuit is switched on, then the fault is between that switch and the component it is powering - which should be easier to find.

For intermittent fuse blowing you have to be a bit cleverer. Get a couple of in-line fuseholders, with, say, 10 amp fuses in them, and use those to subdivide the fused circuit into separate sub-sections. It's then a matter of waiting until a fuse blows. As the factory fuses should all be 35 amp blow, your 10 amp sub-section fuse should blow first, leaving the main fuse intact. This does mean you will have to replace sub-section fuses as you go, but it's about the only way if waggling wires with the test bulb as above doesn't help by bringing the short on. If there is more than one spade used on the fused side of the fusebox (as is often the case), you can put one or more sub-section fuses on each of those first of all. Then by seeing which circuits work and which don't when the sub-section fuse is blown, and consulting the diagrams, you should be able to work out which 'branch' of the circuit has the problem, and so which parts of the branch to move the sub-section fuses to next, i.e. at bullet connectors. There are quite a few branches at bullet connectors in the green circuit, however some parts are daisy-chained, with two green wires in a single spade connector, meaning your sub-section fuse can isolate just one component or circuit at a time, and not a group of them. Hopefully, short of accident damage, only one circuit will be the cause, and it doesn't happen very often anyway.

Before the 1973 model year the heated rear window was always optional. Prior to 1971 an additional wire was run in to the front of the car. For Mk1 North American spec and prior to the 1972 model year elsewhere, a switch and warning light was provided on a separate small panel somewhere. For Mk2 North American spec and 1972-on for other markets, the switch moved to the centre console. From 1971 the wiring for the HRW seems to have been part of both main and rear harnesses (including roadster main harnesses, it not being worth producing a separate harness minus that one relatively short wire), even though prior to 1973 the HRW was still optional on all cars. From 1973 on HRW was standard on UK cars. In December 1974 production of the GT for North America ceased, and for other LHD markets in June 1976.

For most of the time there seems to have been a white tell-tale warning light associated with the switch, integral with pull-on switches, beside or above it with toggle/rocker switches. Mk2 non-North American cars had a pull-on switch until 1971 changing to separate switch and warning light in 1972, Mk2 North American cars had separate switch and warning light until 1973 when it changed to a pull-on type with integral warning lamp. Clausager says that for the 1977 model year until December 77 the switch had a built-in warning lamp - quite why is unknown as there was a blank position beside it. In December 77 the switch was changed to one with an external warning lamp - beside the switch, but the diagrams always show this switch with an external warning lamp. In 1980 the switch was changed back to one with an internal warning lamp, as there was now a rear fog lamp switch which took the place of the external warning lamp.

When powered from the main green circuit not only does this have a tortuous route through many components and connectors in the brown, white and green circuits but because of the very heavy drain of the HRW it reduces the voltage to these other components and results in a low voltage at the rear screen, only about 7 volts in my case. Most of the other circuits aren't that bothered by the lower voltage but the indicators are very sensitive to it and use of the HRW can stop them flashing at idle if headlights, fans etc are also on. Whilst this is usually due to one or more (probably several) bad connections, tired flasher unit, tired bulbs etc. even good connections result in low voltage at the rear screen. My flashers didn't stop with the use of the HRW but they did slow down so I decided to add a relay to remove the load from the green circuit and boost the voltage to the HRW at the same time

Updated September 2015: On my 75 V8 the window element measures about 1 ohm at the contacts on the sides of the glass, so from Ohm's Law with the engine running and the system voltage at about 14v one could expect about 14 amps to flow in the circuit. However as mentioned above taking its voltage from the green circuit, the long run, and the ageing connectors I was getting much less than that at the element, the rest being 'lost' elsewhere. Even with a (fused) relay direct off the brown circuit I'm still only getting about 10 volts at the connections to the harness under the rear cant rail, 8v at the element contacts, and a measured 8 amps. Even so that's about 80 watts, and the relay has made a significant improvement to screen clearing.

  Testing:

Note that due to the high current drawn by a working screen some voltage drop in the long wiring run and connections from the front of the car is inevitable, and will result in something less than full system voltage being seen at the screen spade contacts. Bad connections will result in significantly higher volt-drops leaving progressively less voltage to be measured at the screen spade contacts. But the more horizontal tracks that have failed the lower the volt-drop will be, and if the screen itself has completely failed (as opposed to just one or two tracks), or the earth connection on the other side of the screen is missing, you will see full system voltage at the right-hand spade connection with respect to a good body earth.

As well as the connections at the front of the car, and above the rear cant rail, there is another connection by the right-hand rear light where a 2-wire sub-harness with white/black (HRW) and purple (load-space light) joins the main rear harness.

  Note also that any bad connections in the earth connection will result in something higher than zero volts being measured at the spade connection in the middle of the left-hand side of the screen, with respect to a good body earth, as well as a higher voltage than normal at the spade connectors on the right-hand side.

For the earlier embedded element type that's about all you can do, but for the later surface-printed type you can go further, and should be able to measure voltage anywhere along the tracks. Connect the negative of your meter to a good body earth, then use the positive probe lightly (to prevent damage) on the exposed surface of the track. Right by the spade connectors at the side of the glass in the middle you should see the full voltage that is reaching the screen.

Assuming the screen is working, testing at various points up and down the main track at the right-hand side of the screen will show a small drop in voltage as you move away from the spade connections in the middle, in the order of a couple of tenths of a volt at the ends of the vertical track.

Moving along any horizontal track should show a gradual reduction from the voltage measured above, towards zero (but see above), being about half the voltage in the middle. Note that if you have a broken track, the voltage will NOT gradually reduce along that track, but will suddenly drop to zero as you cross the break.

  Adding a Relay:

The ignition, via the HRW switch as before, controls the relay which draws a very low current whereas the high current is carried by the relay and the short run of thick wire back to the purple (or brown) at the fusebox, and ensures that the HRW is only powered when the ignition is on. This increased the voltage at the rear screen from about 7 volts to about 10 volts. If you mount and insert the relay at the connector at the back of the car and run the thick brown direct to the battery you can get an even higher voltage, but even with my arrangement the screen clears noticeably quicker than before.

  Repair:

August 2016: The above paint is not aimed at HRWs, but another specifically for HRWs is Granville Electro Connector. They couldn't tell me the resistance of a typical bead of product, saying a typical repair is a 'thin bead to a preferred short length of 3-5mm although it is possible to mend longer breaks'. At the time of writing the typical price from Amazon and eBay is in the order of £15, but amazingly Halfords have it for £11.49! Mostly negative reviews, but one gave a detailed description of how he made a successful repair, so I thought it was worth a punt. The ZS has lost an element, testing with a meter located the fault but there is no visible break, so hopefully it is hairline and stands a chance of repair by bridging it. It took three goes ordering online before they managed to find it in the store, by which time they had reduced the price to about a tenner for my trouble! A single-coat test section about 1mm long and the same wide exhibits a resistance of about 1 ohm, which is significantly less than the other product. I had to wait until the weather was warmer and dryer before I could apply it, then wait for damp weather before I could test it! It didn't work, and despite re-testing and finding the break right at the end of the repair, and applying a longer repair, it still didn't work. Annoying, as I hate seeing that dead track in my rear-view mirror. Even more annoying is a second dead track a year later.

I also found this Loctite 'Rear Window Defogger Tab Adhesive', and a couple of other repair possibilities from Permatex.

  Replace:
March 2019: Graham Moore bought an unused NOS screen that was 24 years old. However it measured about three times the resistance mine does and two of the tracks were defective, when tested using steam from a kettle. Close examination shows copper oxide on parts of the tracks, i.e. corrosion, so I suspect the tracks have 'thinned' making them higher resistance than they should be, to the point of going open-circuit altogether on the two defective tracks.

As well as the usual considerations for front and rear screen glass replacement there are the electrical considerations for the HRW. The original has two small spades on each side of the element, one pointing up and one pointing down, concealed under the rubber seal. Wires run from these under the rubber seal to exit at the top near the hinges, for connection to the vehicle wiring, which is very unobtrusive. However replacement screens seem to have a completely different connection arrangement that is very much less than ideal.

But even better - and brave - is Joshua Taylor's approach which was to bend the spade over (while keeping the glass-end clamped to the screen top prevent it breaking off!) back on itself so the connector was under the seal.

Schematics

  Description:

Clausager says the horns moved to the inner wing in 1963, and that from 1970 'all cars now had' twin horns, implying that some markets e.g. North America may have had them earlier. I've not found that itemised in his book, but the Workshop Manual schematics do show twin horns for North America with the start of the Mk2 in 1969.

Originally the horn push was in the centre of the steering wheel. For North American MkII models and in 1970 for other markets it moved to the end of the indicator column stalk but was unpopular and reverted to the centre of the wheel for all markets in 1971. It remained there until the 77 model year when all markets moved back to the indicator column stalk until the end of production. (NB. Either arrangement is far preferable to that on the ZS, which has two little buttons at the edge of the large centre boss. This means that not only are they several inches away from fingers and thumbs when holding the wheel in the correct '10 to 2' position, but the buttons also move position as the wheel is turned. With their small size and changing position you have to look to see where they are before you can sound the horn, and you need to use a finger-tip rather than the palm of your hand, hardly ideal when you need to give an urgent warning of approach!

  Wheel-centre horn push: Updated October 2009

The sprung wire (69 and earlier) and 'pencil'/sprung rod (71 to 76) that connects to the live side of the horn contact are often described as a 'brush' but that is incorrect. A brush is something that typically provides a rubbing contact between a fixed component and a rotating one, e.g. on the commutator of a dynamo or motor or the slip-rings of an alternator. Up to 76 MGB centre-push horns do have a brush, but it is a contact fixed to the column that as the wheel and column turns either rubs on an insulated brass cylinder on the column (69 and earlier) or on a slip-ring on the back of the wheel (71 to 76). This brass cylinder and slip-ring are connected to the sprung wire or pencil, which then connects to the horn push. Some think that the pencil rubs on the back of the brass ring that is on the rear face of the wheel as it is turned, but a moments thought and a turn of the wheel with the horn push removed will reveal that the wheel, sprung wire/pencil, and horn push all rotate as a unit. The sprung wire and the pencil simply sit between the horn contact and the column or wheel slip ring, under spring tension, but rubbing against neither. They are spring-loaded to absorb the movement of the wheel centre when the horn is sounded, as well as press on the slip-ring and horn switch to make a good connection between them. The pencil in particular seems quite a complicated component for what it does, the same thing could have been achieved (without detachability) by soldering a wire between the slip-ring and the horn contact. Detachability could have been achieved by having a pull-apart connector in this wire. Some after-market wheels dispense with the instant detachability of the factory 71-76 horn-push. The Moto-Lita wheel on my V8 has a wire soldered to the back of the slip-ring which connects to a threaded stud on the back of the wheel centre with a nut, removing this nut allows the horn-push to be detached from the wheel, and the fitting of pushes with different logos as required. The 71 to 76 pencils can be fitted either way round and the horn will function, but the correct way round is with the long hex brass rod pointing at the slip-ring on the back of the wheel and the end with the insulating sleeve facing the horn push. This way round means that the brass ends of the pencil can't come into contact with the frame of the horn push, which is at earth or earth potential (and would sound the horn) when any of the springs are touching the wheel, when refitting the push to the wheel or if the push is rotated once fitted. The factory horn push has four bosses which fit between the heads of the bolts securing the rim and spokes to the hub, and these only allow the horn button to be rotated a small amount in either direction which in practice keeps the pencil away from anything at earth potential even if it has been fitted the wrong way round, as well as keeping the logo centralised (once fitted correctly in the first place).

April 2019:
I discovered during a very rare need to sound the roadster horn that it needed to be pushed in a certain way, whereas pushing anywhere on its periphery (tilting it) or pushing it straight down should work, so an opportunity to inspect it. Three screws connect the earthed base to the rubber push via springs, and another screw attaches the 'live' contact to the rubber push, and both that and the copper ring round the hole in the earthed centre were pretty manky. A bit of fine wire wool cleaned both up, and reassembled and refitted it now works as it should.

August 2019: At least I thought it had. Something made me try it while driving, and again some positions worked and some didn't. Back home I tested the horn push with an ohmmeter (not ideal) and all the way round it was about a couple of ohms which is nothing with the relay I have fitted. That relay is powered from the purple circuit behind the dash i.e. for the courtesy lights and headlamp flasher, whereas I discovered just the other day that of the three purple wires at the fusebox, two are standard-size in one spade connector and power the courtesy/boot lights and headlamp flasher, and a thicker wire is in it's own spade connector and goes direct to the horns. With that wire pulled off and the other two connected I realised I could hear the relay operating without the horns blaring out. And working round the edge of the horn push I could tell that the relay was operating better in some positions than others, and it varied if I turned the wheel a little. So this time use a voltmeter on the horn brush behind the wheel (after removing the cowl) and find the 12v is only dropping to 2 or 3v instead of zero volts. So off comes the wheel and I clean up the brass slip-ring on the back - which is pretty manky - with Solvol Autosol, and the stud on the brush. Peering into the horn-push pencil hole the back of the slip-ring looked pretty manky as well, so removed the slip-ring and its carrier from the wheel, and bent back the three tabs to remove the brass ring from the carrier. Tried cleaning that with Solvol Autosol but it is pretty pitted from having carried horn current for many years before I fitted a relay. So polished up another section and refitted it to the carrier rotated 120 degrees to use a 'new' area. At £17 for a new slip-ring (BHA5042) it was worth a few minutes of my time. Fitted the carrier to the wheel (has to go in one of two possible ways for the pencil holes to line up), the wheel to the column, and the horn-push to the wheel, tested the voltage on the brush and now it's dropping to zero volts all the way round and the relay sounds much 'stronger'. Checked the headlamp flasher and indicators to make sure I hadn't disturbed anything else, tightened the steering wheel nut and refitted the cowl - all hunky-dory now ... until the next time maybe.

  Horn Mounting September 2013

I bought new horns for Bee (why I forget) and as the existing ones didn't have brackets I bolted them direct to the inner wings as before, which positioned them vertically instead of horizontally. Originally I fitted them facing backwards to keep water and dirt out, and with the spades uppermost for accessibility. But they were never as loud as the ones I subsequently fitted to Vee (which needed a relay from the start or they didn't work at all), and thinking this was partly due to them facing backwards I turned them round to face forwards, which meant to keep the spades at the top again for accessibility they changed sides. Several years later and the horns seem to be getting worse if anything, so I do the voltage tests indicated below and find that whilst I'm only losing about 0.5v in the purple feed I'm losing about 3v in the earth feed i.e. the tortuous path back through the rack and steering column to the wheel centre button, so fitted a relay to Bee as well and that made a huge difference.

I had wondered whether I could 'tune' the horns to be louder, some have an adjuster screw and locknut in the centre of one side, so took one of mine off. Nothing as simple as a screw and locknut, just a plastic stud under a rubber cover, so I decided to leave that alone rather than risk damage. However while I was turning the horn over I became aware of a rustling noise, and when I tapped it on the bench all sorts of rubbish started falling out! Much tapping, shaking, turning, and poking a length of stiff copper wire up the trumpet of both horns extracted quite a pile of dead flies and debris that wouldn't have helped at all.

I then started thinking about the orientation of the trumpet, which curves around the edge of the horn body. My dual Mixo horns i.e. one high note and one low note are mirror images of each other. The construction is such that installed as mine were facing forwards with the spades uppermost, the curve of the trumpet is downwards and so any water, dirt, dead flies etc. that find their way in will remain lodged inside. They need to be mounted such that the curve of the trumpet points upwards, with the trumpet itself angled downwards to some extent, and both aspects will naturally resist stuff going in and getting stuck. The downside is that the spades now point downwards, but one can't have everything. The upshot is that my low note Mixo has to be installed on the left as you look at the front of the car, and the high note on the right.

  Fault Diagnosis

  Repair September 2014

The casing is usually in two halves, with the 'trumpet' in one half and the active stuff in the other, with a diaphragm clamped between them. The two halves are usually riveted together, in this case with six rivets, one side usually being easier to get at all six than the other as some may be in the mouth of the trumpet. Use a drill the next size up from the head of the rivet and drill the head off - it may be easier to start with a small drill to drill a pilot hole part way through first, then use a nail punch to punch the remainder of the rivet out. If the two halves haven't parted by now, carefully lever them. This horn had a paper gasket either side of the diaphragm, you may be able to separate the halves and remove the diaphragm without ripping the gasket as I did, if not it's no big deal to cut new ones out of thin paper.

The adjuster screw acts on the assembly that holds the contacts, moving the contact lever closer to or further away from the operating pin of the diaphragm, to get the most effective movement of the diaphragm, and hence the loudest noise! Whilst there is some pitch change as adjustment is made, the primary difference between high-note and low-note horns is in other aspects of the design.

With the innards exposed the first thing to do is check the continuity of the coils, because if one of those is open-circuit you may not be able to go any further, it should be easy to see where the ends of the coils are terminated. If those test OK - typically 4 or 5 ohms - then test the continuity of the contact. This one was open-circuit, possibly through oxidisation during a long restoration of the car. A little bit of wiggling and manually opening and closing the contact was enough to restore continuity in this case.

The only real way to test is to clamp the two halves together again, as the diaphragm has to be held firmly and at the correct distance from the coils and the contact lever in order to function properly. I used three nuts and bolts in case I had to take it apart again, although I intended to re-rivet when I was happy with the repair. This is when I discovered just how small the operating range of the adjuster screw is - just a quarter turn. With that I was happy with the sound, so fitted three pop-rivets and backing-washers in the so-far unused holes before removing the three temporary nuts and bolts, as I didn't want to disturb the alignment of the two halves and the diaphragm, then fitted the remaining three rivets and backing-washers. Retested, final tweak of the adjuster screw, and returned it to the very satisfied owner.

  Adding a Relay: One fairly common problem with the earlier 2-wire horns, particularly with collapsible columns, is a weak or non-operating horn even all the right voltages seem to be present. This is sometimes caused by a bad earth to the column itself - it relies on its mechanical fixings between the chassis rails, crossmember, rack, UJ and the upper and lower halves of the later collapsible steering columns for this and not a dedicated earth wire. I've even had one where the bad connection was where the UJ clamped onto the shaft at one of the splined joints! This earth path is only a problem with the earlier 2-wire horns. I used a relay to 'boost' the earth to operate the horns, but I have heard of others connecting an earth-strap between chassis and rack. Before going to the bother of adding a relay or earth strap check the other connections first.

The relay spades are shown with modern markings and the diagram also shows which pair are the winding and which are the contacts. If you use an older-style Lucas relay the 'W1' and 'W2' spades are the winding and the 'C1' and 'C2' are the contacts. It doesn't matter (on either type of relay) if you reverse the winding pair or reverse the contacts pair, as long as you don't get any of the winding wires on the contact spades and vice-versa.

The relay is operated from the earth from the horn-push and 12v from the purple (the purple is always hot and fused for safety) and will operate reliably even with quite large resistances in the horn-push circuitry. The contacts push out a good earth, taken from a tag secured under the relay lug, onto the purple/black to the horns themselves.

Footnote: Some time later I decided to see just where the high resistance connection on Vee was and the results were interesting. I was losing 0.5v between the body and the outer column despite all the fixing bolts, and another 0.5v between the outer column and the inner column. But the greatest loss was inside the horn button itself. As this was a Moto-Lita wheel and the two halves of the switch casing were held together with spire clips on three small plastic pins that always break when you try to remove them, discretion was the better part of valour. Given that, there didn't seem much point in making a better connection between the outer column and the body, even less trying to fabricate another brush to get a good connection between outer and inner columns. The relay has been working perfectly well for a number of years so I left well alone. Subsequently I replaced the Moto-Lita wheel with an original but left the relay in place.

September 2013: Bee's horns have never seemed as loud as Vee's with the relay, and yet more testing showed an iffy earth through the column. So without any more messing about I fitted a relay to Bee as well, and a noticeable improvement.

  August 2014: I've been helping a pal finish off the restoration of a TR3 and one of the last things is to deal with some electrical problems to get it ready for the MOT. One of those is the horns not working - "should be easy" I thought. The principle is the same as on the MGB i.e. an earth up the column, through the horn button in the centre of the wheel, out to the horns, then back via a fuse to the 12v supply. The TR3 has two steering column UJs, and they are rubber doughnuts, so there is an earthing wire from one yoke to the other, around the doughnut. There is also an earthing wire going direct from the rack to the chassis, even though the rack is bolted to brackets on the chassis and not a removable cross-member like in the MGB. The rack earth wire was broken, the lower UJ earth wire seemed to be missing altogether, and the upper one was iffy being a bodge of wire strands wrapped round the UJ clamp bolt. First job was to replace the rack and lower UJ earth wires. Still didn't work. Must be that iffy upper earth wire, but did some testing, and to our amazement the earth wire was fine, the fault was where the upper yoke was clamped onto the steering column at the splined joint! Removed both UJ clamp bolts with the intention of tapping the yokes up and down the shafts to clean the splines, but the upper one didn't move. Nothing for it but to remove the UJ and the column. Four bolts and the doughnut comes out, and we found bolt-through terminals under one bolt-head each side - for the original but missing earth wire! We'd put the earth wire on the lower UJ between the two UJ clamp bolt heads, as the bodged wire on the upper UJ had been, thinking that was the correct position, but we aren't going to move the lower one now. Also the four yoke bolts screw into the opposite yoke, no nuts, but have locking-wire through the ends of the bolts. There is also a weird clamp with two bolts and an Allen screw and lock-nut on the column shaft. Another difference to the MGB is there is no outer tube as part of the column, it is part of the bulkhead. So the column needs to be removed via the engine compartment as the yoke was stuck on the lower end, but it can only go forwards along the line of the column as the outer tube is fixed. Would there be enough room? We removed the steering wheel, but didn't get very far as something was stopping it going forwards, possibly the indicator cancelling cam hitting the fixed outer tube. But then we found that the column was in two halves - a long section that withdrew into the cabin leaving just a short section to withdraw into the engine compartment, so easier than feared. It took some pounding with hammer and drift to get the shaft out of the yoke, so we could clean up the splines with wire brushes. With it all apart we could see that the clamp with the Allen screw clamps the two halves of the column together, a) to get a good earth going all the way up, b) to position the upper part and the steering wheel correctly in relation to the indicator switch cowl, and c) to set the fore and aft free play of the column shaft in the outer tube that is part of the bulkhead. The clamp with the two bolts goes around the upper half of the column, which has the lower half of the column sliding inside it, and the Allen screw tightens through a slot in the upper column onto the lower column. We weren't sure if the Allen screw had been adjusted correctly before so went to slacken the lock-nut but it was stuck fast, and needed heat before it would shift.

After that it was 'just' a matter of putting it all back together again. I'd decided to fit the new earth wire to where we had found the original bolt-through terminals, and not where the bodge had been or where we had put it on the lower UJ. That meant the wire could run through the middle of the doughnut rather than being round the outside, and so not scrape on the shelf or various tubes and pipes nearby. Also when removing the UJ it was apparent that because the column and intermediate rack shaft were not directly in line, the doughnut would have to be 'bent' to get at least two of the bolts to line up correctly with the opposite yoke when refitting it. So I decided to assemble the UJ off the shafts, but even that needed the doughnut to be squashed a bit in a vice to get the fourth bolt started. By leaving the Allen screw clamp off I was able to slide the lower half of the column up out of the way enough to get the two sets of splines engaged, then fitted the Allen clamp, setting the fore and aft free-play in the process. Time for a test ... and we have a horn! Actually we should have two as there are twin horns, but one isn't working. More voltage tests on the horn spades and it is apparent that horn itself is faulty, but one is good enough for the MOT. That's taken all morning, so the wipers will have to wait for another day.

Schematics

Tachometer
Fuel Gauge
Tank Sender
Instrument Voltage Stabiliser
Calibrating the gauge
Electric Temperature Gauge
Electric Oil Gauge
Speedometers
Dual oil-pressure/temperature
Instrument Lighting

The smaller gauges were secured in the dash with a small 3BA knurled nut 17H932 and various U-straps. Early gauges had external illumination with strap AJH5185 (fuel) and AJH5186 (dual) that carried the bulb, later gauges with internal illumination used AJH5187.

The electric gauges are usually powered from the green circuit (fused ignition), the one exception is the early electric tach from 64-67 which was powered from the white (unfused ignition) as well as having another white coming in to the pickup from the ignition switch and going out to the coil. I have no experience of electric temp and oil gauges in MGBs but the following info on fuel gauges may be of some use in faulting them. What I can give is the wiring colours. All run off the green circuit, either direct or via the voltage 'stabiliser' as follows:

• Fuel: 62-64: green circuit - fuel gauge - green/black - tank unit - black - boot earth
• Fuel: 65-on: green circuit - stabiliser - light-green/green - fuel gauge - green/black - tank unit - black - boot earth. From about 75 on for North America, and 77 on for the UK, there was no longer a wired earth at the tank sender.
• Temp: (North American spec 67-on, UK spec 77-on) green circuit - stabiliser - light-green/green - temp gauge - green/blue - temperature sender
• Oil: North American 67/68: green circuit - stabiliser - light-green/green - oil gauge - white/brown - oil pressure sender
• Oil: North American 69-71: green circuit oil gauge - white/brown - oil pressure sender. From 72-on North American spec reverted to a mechanical gauge

  Tachometers:

Schematics
Description
Problems
Testing October 2013
Internal and external seals

Description: There have been two types of electronic tachometers - the earlier RVI inductively-coupled type (which came in two versions - positive earth and negative earth) and the later RVC directly connected type. The inductively coupled type uses the white wire that goes from the ignition switch via the tach to the coil for triggering. The wire is looped round the tach pick-up i.e. it passes through it twice, and the direction is critical. The directly connected type was only used on negative earth cars and uses a black/white from the coil -ve (the same terminal as the points wire from the distributor) which terminates on a bullet-type connector at the tach rather than a flat spade. 1962-64 cars used a mechanical rev-counter. Secured into the dash with two large 3BA knurled nuts 17H1304 and spring-washers, with a U-strap AJH5176 to September 64, separate 'legs' 17H3744 on each threaded stud from then until July 74, and 17H1339 after that until the 1977 model year.

1977 and later have a much better arrangement where the tach and speedo have three studs in the instrument case, and by turning them clockwise 30 degrees these studs will align with cut-outs in the dashboard to allow the instrument to be withdrawn. Unfortunately LHD only.

The tach was powered from the white (unfused) ignition circuit on Mk1 cars, then for Mk2 cars onwards from one of two green (fused) circuits. Until 1978 this was the green circuit in the fusebox, but when the ignition relay circuit on RHD cars was modified sometime in 1978 and a second in-line fuse between brown/white and green wires was added under the fusebox, the tach was powered from one of these - the one with the thinner wires, the other with thick wires being for the cooling fan. For more information see the ignition schematics.

Tachometers were marked with the original polarity - positive and negative - from inception, at least until they changed from chrome bezels to plastic for the 1977 model year. But bear in mind that a PO may have changed the internal wiring of a positive earth tach and not changed the legend on the dial.

Serial numbers (on the faceplate) were as follows:

Year	Chassis Nos.	Market	Battery Earth/Ground	Sensing	Pickup Location	Size	Reference No.
1964-67	48766-138360	All	Positive	Current	External	4"	RVI/2401/00B
1968	138401-153877	Canada	Negative	Current	Internal	80mm	RVI/2430/00
1968-71	138401-256646	USA	Negative	Current	Internal	80mm	RVI/1433/00
1968-71	153878-256646	Canada	Negative	Current	Internal	80mm	RVI/1433/00
1968-72	138401-294250	Not North America	Negative	Current	Internal	4"	RVI/2430/00
1972	258001-294250	North America, Sweden, Germany	Negative	Current	Internal	80mm	RVI/1439/00
1973-74	294521-360069	not North America, Sweden, Germany	Negative	Voltage	N/A	4"	RVC/2415/00AF
1974.5-76	360301-409401	not North America, Sweden, Germany	Negative	Voltage	N/A	80mm	RVC/1410/00AF
1973-76	294251-409401	North America, Sweden, Germany	Negative	Voltage	N/A	80mm	RVC/1410/00AR
1977-80	410001-on	RHD	Negative	Voltage	N/A	80mm	RVC/1414/00F
1977-80	410001-on	LHD	Negative	Voltage	N/A	4"	RVC/2432/00F
All	All	V8	Negative	Voltage	N/A	80mm	RVC/1810/00

Note 1: The RVI external either uses a continuous loop of white wire from the harness fed through the pickup, or has short flying leads with bullet ends already in the pickup, in this latter case the harness also has two separate whites with bullet ends. The RVC internal has male and female bullets on the back of the case, and corresponding bullets on the two white wires. These tachs will only work with the current going through the pickup in one direction - a fiddle to reverse with the continuous loop if you convert a positive-earth car to negative, easy with the bullets. You will not harm the tach if it is the wrong way round.
Note 2: The change to 1968 Canadian models occurred part way through the year.
Note 3: There were gaps in chassis numbers. From November 1967 with the advent of the Mk2, most model years thereafter started at a 'round number' e.g. the last 1971 model was 256646 (a GT) and the first 72 model was 258001 (a roadster).

In the RVI current-operated tach coil current flows via the inductive pick-up on the back of or inside the tach, so the tach responds to the current pulses through the coil as the points open and close. If that circuit breaks the engine stops, if it shorts to earth you fry the harness! 1964-67 types had the pickup mounted on the outside of the case and the later one (68-72) had it mounted inside the case. For the earlier type a continuous loop of white wire comes out of the main harness from the coil SW terminal, through the pickup as indicated in the top picture on the left (click to expand), then back into the harness again towards the ignition switch. This is for positive earth cars. If the car is converted to negative earth the wire must be removed from the pickup and routed through in the other direction. If fitting a new harness I'm not sure if the ends of the loop are marked to indicate which is coil end and which is the switch end, if not you will just have to try first one way then the other. It won't hurt the tach if you get it the wrong way round. The earth wire for both the electronics and the instrument lighting is under one of the knurled wheels holding the tach into the dash.

For the later type with internal pickup (negative earth cars) the ends of the loop of wire through the pickup are brought out to male and female bullet connectors mounted on the back of the case, see the lower picture on the left. Note that these seem to be smaller than the standard wiring bullet and connectors at 4.5mm instead of 5mm (Malc Gilliver). The spade for the 12 supply to the tach electronics is close by the bullet connectors. The harness now has separate white wires with female and male respectively bullet connectors, meaning incorrect connection is not possible.

  See here for a description of the early version with external pickup from Mark Olsen's Sunbeam Tiger pages, and here for the later 2430/00 RVI with internal pickup by Herb Adler. However neither say much about the thermistor, this thread from The Sunbeam Owners Club of America states the original should have a value of 150 ohms at room temperature, but items in the 200 to 500 ohms should be able to be used successfully. A negative temperature coefficient (NTC) item is required, there are positive temperature coefficient (PTC) items around which are not suitable. Unfortunately suppliers in the UK only seem to stock items in the thousands of ohms, not hundreds of ohms.

With the RVC voltage-operated type the ignition current goes direct from the switch to the coil, and a tapping off the coil -ve/points connection goes to the tach on a white/black wire, which responds to the voltage changes as the points open and close. If this circuit breaks the tach ceases to register and the engine continues to run. If it should happen to short to earth the engine will stop, but not fry the harness. (In fact this makes a nifty anti-theft device using a hidden normally open switch connected to the wire at the tach rather than in the engine compartment.) Since the current flowing through the coil has a direct relationship with the voltage at the coil CB or -ve terminal it follows that the two types indicate the same thing. Both types have a threshold above which any voltage or current pulse will register on the needle, and the relative sensitivities of the tach electronics and the engine are such that under normal circumstances weak ignition pulses will affect the engine before they affect the tach reading.

  See here for information about the RVC tach by Herb Adler, Franz Hubl and Rick Astley.

  Problems: Typical problems are sticking, wavering, or simply not working at all. Sticking, where a rap with a knuckle on the glass fixes it, and it only occurs after being parked for a while or at certain times of year, is almost certainly a mechanical problem with the movement itself.

Wavering or flicking about, if accompanied by changes in the idle speed, have a good chance of being caused by bad connections in the ignition LT circuit that is ignition switch - coil (via tach where appropriate) - points - earth.

Wavering or flicking about not accompanied by changes in idle speed, randomly dropping to zero for longer periods, or not working at all, could be either the 12v supply to the tach, the connection between coil and tach on the later voltage-operated types, or electronic problems inside the tach itself. From 64 to 67 the tach was powered from a third white (unfused ignition) wire and black earth but after that it was from a green (fused ignition) and black earth for both current- and voltage operated types. In no case is the tach powered from the instrument voltage stabiliser as the output from this is 12v switched on and off about once a second and so is unsuitable for the tach for obvious reasons. Get a multi-meter with an rpm range, connect it to the points-side of the coil, and compare that with the cars tach. If they shows similar variations then there is a problem in the ignition LT circuit through the ignition switch, coil and points. If it is steady when the cars varies, and you have the voltage operated tach, then connect the multi-meter to the white/black at the tach. Variation here but not before would indicate problems with the white/black wire or connections between tach and coil. If that is steady too, or if it was steady at the coil and you have a current-operated tach, monitor the 12v supply and earth at the tach. If these are steady too then the problem must be inside the tach itself.

If it works normally with the lights off, but doesn't work at all with them on, then the earth supply is probably missing. Early tachs have the earth connection as a tag under one of the knurled securing nuts (as on the other gauges). Later cars have a spade spot-welded to the back of the case, as well as the insulated 12v spade.

Very occasionally there are reports of the tach shooting up when the ignition is turned on but before the engine is started, or swinging right round to max with the engine running. About all you can do here is to unplug the illumination bulb and disconnect the trigger wire or wires, and see what happens with just the green 12v supply wire (white on Mk1 cars) and black wires connected. If you have 12v between the spade the green (or white) is connected to and the case, and zero volts between the case and a known good earth elsewhere on the car, and the tach is still registering incorrectly, then it is an internal problem. If it has suddenly happened it is quite likely to be a dry joint or cracked PCB track which may be visible, or a component fault which probably won't be. If you have less than 12v between the green (white) and the case, or any voltage at all between the case and a known good earth elsewhere, then the 12v supply or earth is faulty.

  Updated May 2009: There are several sites around showing how the original tach circuit can be replaced with one using a 555 timer - two examples here and here. This can be done to both RVI and RVC tachs, although in the former case this would effectively convert it to an RVC and would need a trigger wire from the coil CB or -ve instead of sensing the current pulses in the white wire going to the coil SW or +ve. Initial testing and calibration can be done using a basic battery charger as described here, then comparison with a separate instrument such as in a strobe light or automotive multi-meter which are likely more accessible.

  Updated July 2011: If your RVI tach (64 to 72) doesn't work with your shiny new electronic ignition system, there are a couple of things you can try. Originally the 'fix' was to try changing the wire going through the pickup from two passes (one turn) to one pass (half a turn) and recalibrating. You will need to dismantle the tach to get at the pick-up on later versions. Subsequently Herb Adler reported that a 123 used with an RVI tach caused problems when following the instructions to connect the red (power) lead of the 123 to the Batt (+12v) terminal of the coil. He found that connecting this to an alternative 12v ignition source that didn't come through the tach pickup, e.g. the white at the fusebox, solved the problem. This shouldn't be necessary with the later RVC tachs, but is worth trying if you have other electronic ignition systems and tach problems. Note that this alternative connection is often recommended when putting electronic ignition on rubber bumper cars (i.e. with the ballasted ignition system), as the electronics are then fed with the full 12v and not the reduced and varying voltage.

Herb also reports that an RVI tach he had modified for 6-cylinders has R6 (see the above Tiger pages) at 820 ohms instead of 470 ohms.

Updated April 2012: In response to yet another complaint of an RVI tach not working with electronic ignition I dug out an old tachometer adapter that came with my Sparkrite ignition kit in the late 60s to see what it involved. It's no less than four transistors that basically provide a clean signal to drive the tach, probably together with powering the module from an alternative ignition supply. I no longer have the connection information for it but I suspect the adapter gets its input from the coil CB/Points terminal, has a second terminal going to earth, and the output terminal connects to the ignition feed via the tach. And whilst searching for information on connecting it I found these Sparkrite SX2000 instruction sheets. Steps 9 & 10 of the first one set and page 5 e) of the second basically say if you have problems with an RVI tach, the Sparkrite unit has two wires that would normally connect to the coil SW or +ve and you move the red one to an alternative ignition supply, i.e. basically the same thing, so worth trying first. 123 distributor, and the Pertronix, Aldon Ignitor and Powerspark under-cap modules are 3-wire systems that use a connection to each of the two coil spades and an earth from it's physical installation to the block or distributor, suggested wiring of these with the RVI tach is shown here.

July 2014: I suggested moving the red wire to new owner Bob Warwood, who had installed an Accuspark electronic ignition system, where the tach was reading about 800rpm high after installation ... but not if the lights were on! It seemed to me that he probably had a tach earthing problem, and with the lights on the voltage to the tach electronics was being reduced, and that overcame the problem caused by the electronic ignition. Bob reported that moving the red wire to the fusebox fixed the over-reading problem.

  Testing: October 2013 I've always noticed my tachs give a little pulse of the needle if I turn the ignition on and off without starting the engine. I'd imagined this was from the trigger pulse as a 4-cylinder engine usually stops in one of two positions with the points closed, but a recent topic on the Yahoo MG-MGB list about this jump got me thinking. So I disconnected the coil and tried again, and still got the jump, therefore it's the power being disconnected from the tach electronics that causes the jump. If your tach suddenly stops working in the car turning the ignition on and off and looking for the jump as you turn it off would be the first test, which would indicate that the electronics were working - partially at least, and that the movement is working. It would also prove that the tach had the 12v and earth supply to the electronics. However both my tachs are the later voltage pulse triggered type with an RVC reference number on the dial, not the early current pulse type. A sample of one RVI tach is a little less clear. The first and second times the ignition was turned on and off the tach needle did jump, but on the third it did not. Neither did it jump when the white wire was disconnected from the coil and the ignition turned on and off again. On a subsequent test with the white wire disconnected the tach didn't jump, but when it was reconnected it did jump. As I say it's not clear yet, but the possibility is that the tach only jumps when the trigger circuit is complete i.e. the white is connected to the coil, so would be no help in diagnosing a non-functioning RVI tach.

You can go further with a bench-test and a conventional battery charger (i.e. not one of the more modern conditioning chargers that are designed to be left connected to the battery long-term, I don't know what results these would give). For the earlier RVI-type tachometer connect the output of the charger to a coil or similar load, with one of the wires going through the tach pick-up in the appropriate direction to give the correct current pulse. Connect a 12v supply from a battery to the tach electronics via the usual insulated spade and case, observing the polarity of both the battery and your tachometer. With the charger switched on a 4-cylinder tach on 50 Hz in the UK (60Hz in the USA it seems, so in the remainder of this section figures in brackets refer to 60Hz mains supplies) should read 3000 (3600) rpm. I've not been able to confirm this as I don't have an early tach, but there are plenty of web sources indicating that this is the case. A 50 (60) Hz mains supply has 50 (60) positive pulses per second and the same number of negative pulses per second. If only the positive pulses are used (half-wave rectification), 50 (60) pulses per second, times 60 seconds, gives 3000 (3600) positive pulses per minute. A 4-stroke engine has two firing strokes per revolution so ordinarily you would have to divide that by two to get the expected reading on the tach, i.e. 1500 (1800) rpm. But most battery chargers should convert both positive and negative half-cycles of the AC supply to positive output pulses (full-wave rectification), which will give 100 (120) positive pulses per second, hence 100 (120) pulses times 60 seconds gives you 3000 (3600) rpm. If you have a charger with low and high settings, you might find that the low setting only has half-wave rectification, hence only 50 (60) positive pulses per second, and in this case you would see 1500 (1800) rpm on the tach.

The later RVC-type tachometer can be tested in a similar way, but in this case the charger negative needs to be connected to the car body (in-car test) or the tachometer case (bench-test). Remove the wires from the coil +ve and connect the charger to the coil +ve. Disconnect the black/white from the distributor (25D4) or open the points. Turn on the ignition (in-car test) or connect 12v to the tachometer (bench-test) from a battery with +ve going to the insulated spade on the tach body and -ve to the tach case. With the charger switched on you should see the same tach reading as above i.e. 3000 (3600) rpm with a full-wave rectifier in the charger, or 1500 (1800) rpm with a half-wave rectifier. Note that unless the points are disconnected or opened the tach will not register. This test can be performed on rubber bumper cars with ballasted ignition and 6v coils as well as chrome bumper with unballasted ignition and 12v coils.

This test also gives you a very accurate way of checking the calibration of a tach. V6 engines have three firing strokes per revolution, so the V6 tachs readings would be 2000 (2400) rpm on full wave and 1000 (1200) rpm on half-wave. A V8 has four firing strokes per revolution, so those tachs would read 1500 (1800) rpm on full-wave, and 750 (900) on half-wave.

Fuel Gauge:

Schematics
Early vs late systems
Fault diagnosis
Tank sender
Voltage stabiliser
Calibration
Gauge identification
Internal and external seals

Originally Jaeger gauge BHA4214E was used with a screwed tank sender, changing to Smiths BHA4470 in October 64 for the remainder of Mk1 production also with a screwed sender but electrically very different. Both these gauges have external night-time illumination with holes round the edge of the case to let the light in, and a fully exposed pointer. Mk2 cars changed to one with a partially shrouded pointer and internal illumination (BHA4736 or BHA4685 according to market with leaded fuel, BHA 5342 for the USA with unleaded fuel). All these have 'hanging' pointers, and use a sender with a locking ring. The final change was for the 77 model year and the plastic dash - to AAU3032 for leaded fuel and AAU3031 for USA unleaded fuel, with the indicator pointing upwards. The sender for these changed to one combined with the fuel outlet pipe, and as the North American electric temp gauge and sender both changed at the same time, and it is known mismatches between that sender and gauge cause incorrect readings, I had wondered if the same would apply to the fuel gauge and sender, but the resistances are very similar. All the Smiths gauges require an earth for night-time illumination, which if missing will not affect the gauge reading (it does with the Jaeger gauges).

Early vs late systems: July 2015: The 62-Oct 64 cars use one system (similar to the MGA), after that it was a completely different system. The two systems use different ways of ignoring changes in system voltage, which can vary between 12v and 15v and would otherwise cause a significant variation in readings. The early Jaeger system uses a kind of Wheatstone Bridge which is a method of determining the value of an unknown resistance where you have an unstabilised voltage supply, by comparing it in a particular way to three other resistances of known value. This has several components inside the gauge which is technically quite complex and expensive and requires the gauge to be earthed both in order to indicate the fuel level and for the night-time illumination. A missing earth will probably vary the reading as the parking lights are turned on and off, and it may not work at all if the panel lights are switched on. The later Smiths system uses an external (to the gauge) voltage stabiliser and a much simpler and cheaper method that uses the heating effect of the current to bend a bi-metallic strip which moves the pointer.

Another difference between the two systems is that the early is 'fast-acting', i.e. it indicates the fuel level immediately you switch on the ignition. The later system is 'slow-acting' or damped and needs several seconds to indicate the true fuel level. The later system does have the benefit of not swinging about as the fuel sloshes around in the tank, but that is usually controlled by the tank being baffled. However a drawback of the later system is that on a long bend such as on a motorway the fuel level will vary considerably between a left and a right, and take time to settle again once you have straightened up.

A further difference between Jaeger and Smiths systems is that whilst on both the tank sender is a variable resistance that changes as the fuel level goes up and down, on the early system the resistance is high (about 70 ohms) when the tank is full, and low (close to zero) when the tank is empty. On the later system the resistance is lowest when the tank is full (20 ohms or so e.g. 25 for Vee and 22 for Bee) increasing towards 250 ohms or so as the tank empties.

A third difference is that on the early gauge the wires must be connected to the correct terminals, but on the later gauge can be connected either way round. The early gauge has one terminal marked 'B' - for Battery, the green wire goes on this. This terminal is on the upper right part of the back of the gauge, and usually has two spades, so you can use it as a pick-up point for a fused ignition supply for something else. The other terminal is marked 'T' - for Tank, the green/black wire goes here, and is in the upper left area of the back of the gauge.

A quick test of the early system is to disconnect the green/black sender wire from the back of the gauge, and the gauge should register above F. Earthing the sender terminal should cause the gauge to read Empty or just below. Connecting a 70 resistor to the T terminal (tank wire disconnected) should cause it to read close to Full, and a 35 ohm resistance should cause it to read slightly below half. For further information on the early system see Barney Gaylord's web site, the remainder of this section largely relates to the later system.

From the above it should be obvious that a late sender with an early gauge will not work correctly, nor vice-versa. The correct tanks, senders and gauges are normally available with one exception, and that is for the brief period where the tank still used the screwed sender, but electrically used the later system. This sender is NLA so would normally mean you had to use an early Jaeger gauge, or change the tank to the later type with the locking ring. See here for an alternative to either of those.

Diagnosis: Problems with the fuel gauge are frequently caused by the connections at the tank unit (they are exposed to a great deal of dirt and spray from the back wheel) or the tank unit itself failing. Problems can be non- or erratic operation, or being wildly inaccurate at E and/or F. You need the ignition on and a reasonable amount of petrol in the tank (i.e. 1/4 tank minimum) for the following tests. You also need to make sure the two connectors and their spades on the tank unit are clean and bright and making good electrical contact.

Non- or erratic operation: Briefly connect an earth to the green/black wire on the back of the gauge. Does the gauge pointer move smartly towards to 'Full' (remove the earth as soon as you see the pointer moving)?

Tank Sender: Updated December 2015 Schematics
Early cars up to October 64 had sender BHA 4292 attached to tank ARH 176 with six screws, and was for the unstabilised Jaeger gauge BHA4214E. A stabilised system was introduced over a period from chassis number 47112 to 48767 in October 64 using sender BHA4471E which also secured with six screws and operated Smiths gauge BHA4381E with the same exposed pointer and external illumination as the Jaeger gauge. However the two systems are not compatible i.e. the correct sender has to be used for each gauge or the readings will be completely incorrect. But that only lasted a few months until chassis number 56742 in March 65 when the tank (variously ARH 223, NRP 2 or NRP 1132) secured sender ARA 966 with a locking ring.

  The final change was shortly after the start of the 77 model year in August 76 where it was combined with the pick-up tube, but again with the locking ring. As that sender and gauge changed at the same time as the temp gauge and sender on North American spec, and if you get those mismatched the readings are incorrect, and I had wondered whether the same applied to the fuel. This was after reading about someone who had been getting strange reading with a late tank and sender and the earlier gauge, but it turned out his sender was at fault cutting out part-way through the range (as my four failures on two cars had been). A replacement OE (i.e. metal) late sender with output pipe measured 20 to 258 ohms before fitting (Colin Tozer), and an after-market plastic without outlet pipe measured 17.5 ohms to 250 (John Holland), although since then two people have posted that their pre-77 plastic ranges between 6 and 250 ohms. Vee's measures 25 ohms with a full tank and Bee's 22 ohms - both OE metal types. Colin was concerned that his late tank with earlier gauge would read incorrectly, but given those figures this seems unlikely.

November 2019: Colin Parkinson has received a plastic sender from Moss and has measured it at 18 to 185 ohms, so hugely different to the others at the 'empty' end. This could well result in showing a couple of gallons left when it runs out.

February 2020: As well as the significant variation in readings from plastic senders someone on a forum elsewhere returned a plastic sender to the MGOC saying it was way out, and the response was inevitably "not had any problems on the other 2000 we have sold.... ". Then later on they said they had had a faulty batch!

Bizarrely plotting resistance and voltage against gauge calibration marks showed that the voltage was linear with the gauge markings, but the resistances weren't. How can that be, Mr Ohm? The resistance wire used in the gauge to heat a bi-metal strip to move the pointer has a positive temperature coefficient i.e. the hotter it gets the higher its resistance goes, and there is also the changing force needed to bend the bi-metal strip along its range of movement, and the system of levers that causes the pointer to move. In the graphs attached I had to use 12v as the supply voltage as I have the original thermal stabiliser which switches 12v on and off to average 10v and not an aftermarket electronic that outputs a steady 10v.

What follows relates to the later Smiths system, for the earlier Jaeger system see here.

David Jackson wrote to me with a problem he was having with a new Heritage tank ARH176 with the screwed sender for the early Smiths stabilised gauge. When fitting the original sender it seems to foul something internally and does not freely move up and down from Empty to Full. This sender float moves in a different arc to both the Jaeger and the later locking-ring senders, as shown here. He has had to fit the earlier Jaeger sender, but the unstabilised system works in reverse to the stabilised system i.e. Empty is low resistance and Full is high resistance, whereas for the stabilised system Empty is high resistance and Full is low resistance. The upshot is that his gauge was working in reverse. However he found this Spiyder module that as well as allowing calibration of E and F, also reverses the operation to correct the indication on the gauge.

For details of tanks, senders and gauges see here.

  Replacing the sender:
The sender can be replaced with up to about a 1/4 tank of petrol in the tank if you raise the right-hand rear corner of the car. Early cars had the sender screwed into the tank, from about 1964 it is held in place with a locking ring that locates under three lugs on the tank. The locking ring consists of three tapered sections that locate under the lugs and as the locking ring is rotated in a clockwise direction the tapers cause the tank unit to be pressed in towards the tank making a seal. To remove the tank unit rotate the locking ring in an anti-clockwise direction, having come across a picture of the factory tool after many years I made my own. But by alternately tapping on the thin end of a couple of the tapered sections of the locking ring with a hammer and drift you can achieve the same effect. The use of steel tools is usually quite safe unless the area is wet with petrol.

Check the new tank unit by connecting it up to the green/black and earth wires and checking the movement of the gauge pointer as you move the float up and down on the tank unit. And before paying for it make sure it makes a smooth and quiet transition from Full to Empty and back again. If the movement is at all 'scratchy' reject it as a sharp edge on the wiper is probably catching on the turns of the resistance wire and will break them in a relatively short time.

Install the new tank unit by putting a new rubber sealing ring against the tank, then the sender unit on top of the seal, and finally the locking ring on top of the sender unit, rotating it in a clockwise direction to tighten it.

A new tank or tank unit will often throw the gauge 'accuracy' out, check the 'empty' indication and readjust as soon as possible (personal experience!)

  January 2015:

Herb Adler's experiences with tank senders.

Voltage Stabiliser: Updated July 2013 Schematics

Where is it?
Is my stabiliser working?

The purpose of the instrument voltage stabiliser is to supply a 'fixed' voltage to the instruments so that the only variation in readings comes from the senders, and not from changes in supply voltage, which can vary between 12v and 15v. Note that MGBs up to October 64 with 18G and GA 3-bearing engines and Jaeger instruments did not have a stabiliser, as the fuel gauge used a different method of compensation for supply voltage variations, see Barney Gaylord's MGA site. Also note that the tachometer is fed with full and varying system voltage, not by the stabilised voltage. The diagrams do show that the North American electric oil gauge used from the start of Mk2 production was fed from the stabiliser, but only for the first year after which it was also fed with full system voltage. This is almost certainly an error in the drawing as the gauge and sender did not change after one year, and the electric sender is of a type used on other makes which has its own method of stabilisation as well as indicating pressure. In any event the electric gauge reverted to mechanical with a capillary tube for the 1971 model year on.

'Stabiliser', when applied to the original component, seems an inappropriate term since when operating it switches 12v on and off once or twice per second, after a 2 or 3 second 'warm up' period of being on all the time when first switching on the ignition. The relative lengths of the 'on' and 'off' periods change as the system voltage changes - as the system voltage rises the on period gets shorter, and as the system voltage falls it gets longer. The result is that over time, the average output voltage to the gauges is kept at about 10v, and the gauges give a stable reading determined solely by the sender resistance, and not by changing system voltage. Since the gauges it supplies are thermal instruments (the current flowing through a coil heats up a bi-metallic strip which bends and moves a pointer) they are slow to move large distances and so the relatively short on and off periods don't allow the pointer to move very much at all. But if you watch a gauge carefully, when it is showing about 1/2 a tank or normal engine temperature, with the ignition on but the engine off, you can sometimes see the very small movements up and down as the stabiliser switches on and off.

The stabiliser consists of a bi-metallic strip with its fixed end connected to the I(instruments) terminal i.e. to the gauge(s), and its moving end breaks and makes contact with the B(attery) terminal i.e. the 12v supply. A heating element is wound round the bi-metal strip and connected between it and earth i.e. the case. With the bi-metal strip cold the contact is closed, so system voltage is passed to the gauges and flows through the heating element to earth. The heating element causes the bi-metal strip to bend, which opens the contact, which disconnects system voltage from the gauges. It also disconnects system voltage from the heating element, so the bi-metal strip cools, closes the contact again, which reconnects system voltage to the gauge(s) and the heating element, and the cycle is repeated all the time the ignition is on. With high system voltages the heating current is higher, which causes the bi-metal strip to bend faster, and open the contact sooner. However it cools at a relatively constant rate, so the contact takes about the same time to close again regardless of system voltage, which is how high system voltages result in shorter 'on' times than low system voltages. Pulse-width modulation, if you want to get technical.

You may wonder about the effects of ambient temperature changes on the bimetal strip in the stabiliser, and indeed the gauges, as they could be fitted to cars operating in temperatures from well below freezing to above 45C/115F. Some have said the ambient changes are much less than the changes due to the heating coil, but even if they were only 10% of the change, a range of 55C/100F in ambient would need the heating coil to be operating at ten times that i.e. 550C/1000F which is ridiculous. Instead the stabiliser and gauges are designed to ignore ambient temperature changes, as described here.

January 2020:
As far as the 'TOP' marking goes, I was surprised at just how much difference the orientation of the original stabiliser makes to the fuel gauge reading.

Updated July 2010:

  Some (Many? All?) suppliers only have electronic stabilisers available, and these are often polarity sensitive e.g. Moss 128484 for positive earth cars, and BHA4602 for negative earth, so you have to get the correct type. However Moss state the earlier one is for 62 to 67, which is incorrect, MGBs didn't get the stabilised system until 1964. Ironically if you installed an electronic stabiliser to the early cars it probably wouldn't make any difference, but if you connected an original the fuel gauge would go up and down about once per second! Incidentally Moss are naughty as they have retained the original part numbers even though it is not an original component. These stabilisers don't have the adjuster stud. Brown and Gammons (Mk1, Mk2 on showing the correct spades in each case) have also used the original part numbers and also implies the early ones were used from the beginning. Their Mk2 stabiliser at least has the 'TOP' markings, which implies it is the original thermal device. But from what I can see they don't have the adjuster studs, so almost certainly electronic as well (and equally naughty). Googling I can't find any pictures that show the adjuster stud, so maybe they are all electronic these days (you can certainly see the printed circuit tracks in many images). Many of those are labelled 'NEG' or 'POS' indicating they are polarity sensitive, so you must get the correct polarity, as well as one with the correct spades for your wiring. The Moss and B&G items don't show polarity, but the early one will be positive earth and the later negative, so unless they are polarity independent the early one wouldn't work on an early car that had been converted to negative earth.

Some people are tempted to replace the original stabiliser with an electronic one, thinking it must be better. In fact they are not so good, as they output 10v as soon as the ignition is switched on, whereas the factory stabiliser outputs full battery voltage for several seconds, resulting in a faster gauge rise time.

Herb Adler describes how he made his own electronic version.

Where is it?

Is my stabiliser working?

Calibrating the gauge:
We are in hallowed company, this from a Hawker Hurricane cockpit:

1977 and later: February 2020 For some reason the factory put the temp gauge where the fuel gauge had always been and the fuel gauge in the centre of the binnacle above the steering column. One could possibly understand it if there was still a dual gauge, but not when they are both electric. Or to put the temp gauge out of direct line of vision as drivers of RB cars tended to be paranoid. Any road up, short of dropping the steering column which isn't a trivial exercise and could upset the UJ alignment, the next best thing is probably to remove the tach, which should only take few minutes, and go in through that hole to release the fuel gauge.

Update May 2007: Gary Alpern contacted me to say while he was calibrating his gauge he noticed that the pointer moved another 1/8" or so when he tapped the glass, and wondered whether anything could be done to eliminate this. I doubt it, and I think it is a 'feature' of the design - the bimetal and spring strips are connected together by nothing more than what is basically a 'hook and eye' hinge. I think the normal vibration of driving the car will continually 'tap' the gauge, however during calibration you may want to tap it after each tweak of the slotted plates. 'Tapping the gauge' is a long-standing and honourable part of living with machinery of this technology, as anyone who has seen 1940's, 50's and 60's films will know :o)

Update September 2007: I debunked my long-held theory that the thermal stabiliser was needed with the thermal gauge to eliminate fluctuations caused by ambient temperature variation, click here to see why. However the thermal stabiliser does result in a faster initial movement of the gauge from rest than would be the case with an electronic stabiliser, as full system voltage is available to the gauge for the first few seconds after switching on the ignition with the thermal stabiliser, whereas with the electronic it is limited to 10v from the beginning.

Gauge identification:

Electric Temperature Gauge: Added November 2008 Schematic

The electric temperature gauge is very similar to the fuel gauge, but using a sender on the cylinder head instead of the tank of course. Diagnosis and calibration is the same, substituting green/blue for the wire from the gauge to the sender. There is no earth/ground wire for the temp sender as it is screwed directly into the head.

The early 180 degree gauges (both numeric and 'C-N-H') were capillary and dualled with the oil pressure, the later 'narrow angle' gauges were independent, electrically operated, and all 'C-N-H'. These gauges use a 'thermistor' (negative temperature coefficient resistor) sender in the cylinder head in which the resistance reduces as the temperature increases, so driving more current through the gauge to give a higher reading.

Note that there were at least three gauges and three senders (part numbers, may be just two colours red and black) used. The red and white senders give fairly similar readings over most of the range (white starting off higher at low temps, but changing over at about 20C to be lower and hence give higher readings), but black gives lower gauge readings for a given temperature than the other two. For example at 82C (one of the standard stat temps) a red sender may put the gauge in the middle of the 'N', but the black sender would be just below N. Conversely a red sender would show Hot at 90 degrees, whereas for a black sender it would have to be 100 degrees, and that ten degree difference is pretty constant through the range. Also note that like the fuel gauge it is possible to adjust the gauge to some extent.

Also note that there are other colours, green possibly being for early Minis which did not have an instrument voltage stabiliser, and the temp sender like the fuel gauge sender operated in reverse and so would have a positive temperature coefficient i.e. increasing resistance with temperature.

From the Leyland Parts Catalogue and Clausager the senders and gauges changed as follows (only North American spec cars got the first three variants, all models had the final variant):

As you can see the only correlation between a sender change and a gauge change is for the 77 model year onwards, which is when UK and other non-North American spec cars got the electric gauge. One source gives the date of change as 71/72 i.e. when the gauge did change and the other (Roadster Factory) gives it as 74 i.e. the same as the Leyland Parts Catalogue when the gauge didn't change. Apart from the Leyland Parts Catalogue the online catalogues of Roadster factory, Moss and Victoria British only indicate two different types of sender - Roadster Factory changing at 74 as previously indicated, the other two changing for the 77 year. September 2015: Further research revealed multiple online parts sources showing pictures of BMK 1644 with the red insulator for 68 to 74 or 75, and 13H 6602 with the black insulator for 1975 on. This gives possible change dates of variously 71/72, 74, 75, and 77. However if there is the potential for a mis-match between gauge and sender, September 76 for the 77 model year (as indicated by the Parts Catalogue) is the most likely date for the change, as the fascias and the gauges changed on all cars at that point.

There has been another suggestion that perhaps the sender changed at the same time as the thermostat, to keep the needle centred on the gauge, but that doesn't tie in either as the thermostats changed shortly after September 64 then back again in March 69, which doesn't tie in with any of the gauge or sender changes from any source. Neither does it make logical sense when higher and lower stats were available for colder and hotter countries respectively, the gauge is only a general indication, and anywhere from just below the C to just below the H is part of the 'Normal' range depending on ambient temperature and usage.

Yet another possibility that has occurred to me is that the gauges have basically the same electrical/movement characteristics but the black sender was fitted as that has higher resistance across the range compared to the red, so would give lower gauge readings helping to avoid owner paranoia!

Electric Oil Gauge: Added November 2008 Schematic
The situation with the electric oil gauge is a little more complex. For the first year of Mk2 production for North America the schematics show it was wired the same as the fuel and temp gauges i.e. from the stabiliser, to the gauge, to the sender. After that full system voltage was applied to the gauge as it was the oil sender itself that contained the stabiliser. As there seems to only have been one sender for the electric oil gauge for both wiring arrangements I'm assuming that the wiring for 67 was in error. With the oil sender instead of a continuously varying resistance with changing oil pressure as is the case for changing fuel level and temperature for the other two gauges, the earth/ground signal from the oil sender switches on and off much like the voltage from the voltage stabiliser for the other two gauges. However the duty cycle i.e. the time it is on compared to the time it is off, varies with pressure as well as with system voltage. As it has no connection with the system voltage other than through the gauge it must sense this somehow and vary it accordingly, unfortunately I haven't had one to investigate internally, although my 1989 Toyota Celica had the same system (same physical appearance of the sender) so I was able to determine what the output from that looked like, at least. The gauge reverted to mechanical with a capillary pipe for the 1972 model year for the remainder of production.

Dual oil pressure/temperature: This gauge is mechanically-operated. The temperature part consists of a sealed system containing a gas or fluid. It is usually the temperature part that fails first, and usually due to a fractured tube. In this case you will have to get an exchange unit. Information on internal and external gauge seals can be found here.