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Electrical System

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Automotive Electrics Basics - Part 1 - Terminology and Part 2 - Typical faults, symptoms, and diagnostic techniques

Ammeters_and_Voltmeters    Alternator/Dynamo    Anti_Run-on_Valve    Batteries_and_Chargers    Battery_Cut-off_Switch    Brake_Balance_and_Handbrake_Warning    Bulbs    Cable_and_Pipe_Routing    Clocks    Connectors_and_Terminals    Cooling_Fans    Cruise_Control    Fan_Belt    Fuel_Pumps    Fuses_and_Fusebox    Gauges    Hazard_Flashers    Heated_Rear_Window    Heater_Fan    Horns    Ignition_Switch    Ignition_System    Ignition_Warning_Light    Indicators/Turn_Signals    Instruments    Lighter_Socket    Lighting    North_American_'Key_in'_Warning    Overdrive    Polarity    Radio    Relays    Schematics    Screen_Washers    Seat_Belt_Warning    Sealed_Wiring_Junctions    Starter    Steering_Lock    Switches_in_General    Tachometer    Wipers    Wire_Colours,_Terminal_Numbering    Wiring_Harness_Replacement    Won't_Start    Won't_Switch_Off!    Torque_Values    Links   

(Image posted by Geoff Hutton on the MGOC forum)

Probably more problems crop up with electrics than anything else, possibly everything else put together. Not surprising, considering the number of electrical components and connectors in the car. For those new to classic car electrics basic electrics terminology is covered here, and further generalised information including typical faults, symptoms, causes and diagnosis can be found here.

Bad connections are a frequent cause of problems in classic cars, and high-resistance connections can be the most confusing to deal with, small increases in resistance having a disproportionate effect on the circuits affected. The Lucas Fault Diagnosis Service Manual states:

"The acceptable volt-drop figure for most circuits is 10% of system voltage (1-2v on a 12v system) but there are exceptions to this rule as in the case of the starter circuit where the maximum voltage drop is 0.5v." The manual also states: "The majority of procedures involve circuit testing and the principle used will be that of checking for 'voltage drop' where a voltmeter is connected in parallel with the particular circuit to be tested.

"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."

That is exactly the principle that is followed in these pages. And as the Lucas manual says the majority will be using a voltmeter in parallel with a circuit to look for a volt-drop, which will indicate a bad connection or an open-circuit fault. About the only case where an ammeter will be used is for testing the LH overdrive circuit, as that is the only way of determining if current is passing through the solenoid or not, as well as whether it is to the correct level or not. Practically every other circuit has a visual (lighting, gauges) or audible (heater fan) indication that something is happening, even if it is not to the correct level, and a voltmeter on the device terminals will show whether the full voltage is reaching it or not. Similarly use of an ohmmeter is very rare - like for the D-type overdrive (this initially takes 17 amps which is beyond the capability of a typical automotive multi-meters), or testing the internal resistance of the two-speed heater assembly, although they can also be used for doing a simple go/no-go check of a condenser, and even checking the dwell/gap of points-based ignition systems.

Understanding how the circuit is wired and what it shares its supply and earth with will help immensely, and for that you will need the Workshop Manual, glovebox handbook, or Haynes wiring diagrams. The colour codes for your model and year are essential, but the factory diagrams can be difficult to follow as they generally place the components on the page as they are in the car, which means a lot of wiring snaking about. Where a component simply isn't working you can use either the individual circuit elements as here, or these Advance Autowire diagrams. But if you are getting strange interactions between various circuits you will need the factory diagrams to see how the supply and earths are shared, as bad connections in these are a frequent cause of problems.

The information that follows is mainly geared towards situations where the car has been working but now has a fault. Obviously, if there are faults when you buy the car, or after someone has been making changes, then absolutely anything could have happened, i.e. multiple faults and incorrect wiring, but the basic diagnosis techniques should allow you to resolve the problems.

I have created individual schematics of virtually every circuit in all variations of the MGB - hopefully you will find them a little clearer than the official diagrams. If you hover your cursor over a wire it should change shape to indicate a link, and then display a 'tool-tip' to confirm the wire colour. Where such a schematic exists you will see an icon somewhere in the main text that talks about that circuit, click on this to see the schematic in a separate window. Clicking on the graphic here displays a list of available schematics.

Ammeters and Voltmeters

As John Twist has said: "Except in the RAREST of circumstances, the ignition warning light indicates any problem with the charging. AMMETERS properly connected into alternator circuits provide at least two more connections which can corrode and cause the alternator to fail. Buy some driving gloves instead." That presupposes that your warning light is working, of course ...

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 - ideally fused - and earth. Switched as the classic thermal (slow-acting) type can be drawing up to 160mA which while only equivalent to an instrument bulb will slowly discharge the battery on an infrequently used car, digital instruments take less current but I still wouldn't want to leave them powered all the time. Fused in case of shorts. 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! An alternative is the accessories circuit off the ignition switch on 1969 and later models, which has its own fuse on 71 to 74 models feeding the radio (bullet connection behind the centre console) wipers and heater fan. Beware of voltmeter vendor claims that the instrument will tell you the 'strength' of your battery, this type of 'slow-acting' gauge cannot.

The classic type above are rather expensive at around 50, there are some with modern internals at around 10, installation and use described by clicking the image. I'd never bothered about fitting one before but having recently bought a small digital unit to check the charging voltage in the A-Class when that became available I thought I might as well use it. Fits in nicely beside the courtesy light on the roadster, but as Vee came to me with an unused gauge at the far right of the dash I decided to get one of the modern analogue type for there. This type is a fast-acting stepper-motor type (but without the wobbling about of the early fuel gauge) and hence shows cranking voltage i.e. the condition of your battery and its connections as well as ignition and charging voltages.

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 15.2v is shown as 'green' and dynamo-equipped cars can show this although an alternator should not get that high. At the lower end 11.5v is also shows as green but for me if it drops to 12v for any length of time I'd be concerned. 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

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.

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 are 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!

Bulbs Added July 2009

Bulb holders

Part No.LocationTypeWattsUsage
GLU101HeadlampSealed Beam60/45101-187210 RHD
GLU106HeadlampSealed Beam75/50187211-360300 RHD and CB V8
BFS415HeadlampBulb50/40101-360300 LHD except Europe and North America
GLB410HeadlampBulb45/40101-360300 LHD Europe except France and Germany as below
GLB233Headlamp pilotBayonetBA9457028-59462 Germany
GLB411HeadlampYellow bulb45/40101-360300 France
17H9472HeadlampSealed Beam60/45101-410000 North America
GLU123HeadlampSealed Beam with pilot window75/50360301-410000 RHD and RB V8
GLB501Headlamp pilotWedgeT10
5360301-410000 RHD and RB V8
GLU114HeadlampSealed Beam?360301-410000 LHD except France, Germany and North America
GLB501Headlamp pilotWedgeT10
5360301-410000 LHD except France, Germany and North America
GLB411HeadlampYellow bulb45/40360301-410000 France
GLB233Headlamp pilotBayonetBA94360301-410000 France
BHA5387HeadlampSealed Beam?360301-410000 Germany
GLB233Headlamp pilotBayonetBA94360301-410000 Germany
GLB472HeadlampHalogenH460/55410001 on RHD
GLB233Headlamp pilotBayonetBA94410001 on RHD
17H9472HeadlampSealed Beam60/45410001 on North America
GLB989Front ParkingBayonetBA95101-187170 North America
101-360300 Not North America
GLB382Front FlasherBayonetBA1521101-187170 North America
All, not North America
GLB380Front Parking/
Offset bayonetBA156/21187170-on North America
GLB323Front FogBulbP36s48101-187210
GLB185Front Long-rangeBulbP36s48101-187210
GLB380Stop/tailOffset bayonetBA155/21All
GLB207Number plateBayonetBA95101-339964 and V8 to 1247 except as below
GLB501Number plateWedgeT10
5187211-219000 North America
GLB989Number plateBayonetBA95339965-360300 and V8 1247-2100 except North America
GLB233Number plateBayonetBA94All RB except North America and Germany
GLB254Number plateFestoonFestoon6339095 on North America
GLB233Number plateBayonetBA94339095-410000 Germany
GLB382Rear FogBulbBA15211980 UK models
GLB987Map lightScrewMES E102.2101-258000
GLB989Gear lightBayonetBA95Automatic only
GLB273ReverseFestoonSU8, 5-821101-410000 and V8 except as below
GLB270ReverseFestoonSU8, 5-818268698-410000 North America
37H 1547ReverseFestoonSU8, 5-8?France, possibly yellow
GLB254Load spaceFestoonFestoon5GT and V8
GLB239Interior/Courtesy & BootFestoonFestoon5219001-410000
GLB989Side markerBayonetBA9S5187211 on North America
GLB987InstrumentsScrewMES E102.2
GLB987Ignition warning,
main-beam and
indicator tell-tale
ScrewMES E102.2Tin dash, chrome bumper, not V8
GLB643Indicator tell-taleBayonetBA9S2Early padded dash, claw holder
GLB281Ignition warning,
main-beam and
indicator tell-tale
BayonetBA7S2Other padded dash, all V8, all RB. Push-in holder with spade connectors
GLB921Switches and controlsScrewLES E51.2410000 on, use GLB280
GLB643Cigar lighterBayonetBA92.2 
GLB280Brake warningScrewLES E51.5 

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

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.

The bulbs can be a fiddle to fit as the claws get in the way of fingers trying to turn the small globe, but there is a technique to make it easier.

  Clocks February 2016

A clock was standard equipment for the 77 and later models, powered from the purple circuit, in the centre console on UK cars but in the main instrument panel on export models. I don't know the 'technology' of the original clocks i.e. whether it is modern electronics taking a brief pulse of current every second, or the older escapement-style that wind themselves up with a motor every few seconds.

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.

A PO had fitted a clock (electronic) to the V8 by cutting a hole in the extreme left hand end of the dash, which doesn't sound very convenient for the driver but is surprisingly easy to read. Originally powered from the purple circuit, when I fitted the battery cut-off switch I soon got fed-up with (not 'of' ...) having to reset the clock each time, so ran a wire from an in-line fuse attached to the battery cable connector direct to the clock, disconnecting the original purple feed. Why didn't I fit a switch with a bypass fuse? Because the switch was to prevent the alarm from flattening the battery when I stopped using the car every day. I did a similar thing when I fitted a cut-off switch to the ZS for the same reason.

I wouldn't contemplate cutting a hole in Bee's dash for a clock, nor wanted one in a separate bracket, and for years struggled with pushing my sleeve back - often being a coat and a jumper to see my watch. Eventually a pal with an interest in wood-turning mentioned he had made a holder for a watch insert and case that fits the cigar lighter socket, so I got him to make me one as well and get me an insert from Stiles and Bates (so he could make the holder a snug fit for the insert), which is a company he uses for wood-turning tools and materials, that just happens to sell clock and watch inserts! I chose one with a black face and bezel with bright hands as being most in keeping with the rest of the dashboard, and sprayed the 'holder' with several coats of satin black as well. 4.85 for the insert which isn't bad, but another 4.75 for delivery. However with the 'clock' installed I found that the bright hands reflected the black seat covers and I couldn't see the time! So I took the insert out of its case and painted the hands with a thin smear of Snopake. Much better, but having painted the seconds hand as well, I found I was having to look at it for two or three seconds, or several times in that time, in order to see where the other two hands were pointing to get the correct time! Also at about that time the insert stopped working, replacing the battery didn't help, which was very annoying. As well as having 'defaced' the insert if they wanted it back, I didn't want to put my pal to more trouble and expense in getting it replaced and posting it up to me, so ordered one direct at another 10, then complained that one didn't work. In the event they didn't want the old one back and sent me two more (one the same and one with a bright blue face in a bright surround) as well as two replacement batteries. So good service, but it would have been cheaper for my pal to complain then me pay him to send them up to me. Any road up, with a working clock, I painted just the hour and minute hands white, and finally I can see the time at a glance ... in the day time anyway.

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.

  Connectors and Terminals:

Sealed Wiring Junctions
Earth/ground Connections

Always test for voltage with the connectors in position, parting the connection may reveal an open-circuit but not a high-resistance i.e. lowered voltage at the component.

Test the voltage on each side of bullet connectors as either bullet could be making a poor connection with the connector sleeve. Bullets are crimped onto the wires in factory harnesses, usually OK, but at the front of the car you can get corrosion running under the insulation for several inches. In one case I've had the conductor strands corrode right through where the insulation was damaged on an unsheathed headlight wire under the wing.

When checking voltages at components check the component terminal i.e. its spade as well as the terminal on the end of the wire that slides onto the spade as corrosion can develop between the two. Factory wires are usually spot-welded to the spade connectors so pretty robust and the least likely to fail.

I always assemble bullet connectors and fan plugs/sockets at the front of the engine compartment using Vaseline which makes assembly easier as well as providing a seal against moisture. Even so it can take some force to push bullets right home into the connectors, so I modified the handles of a pair of pliers. I subsequently discovered there is a specialised tool for this, but at 20 I'll stick with my modification, thank you very much. However replacement connectors seem to be rather inferior to the originals, not retaining bullets firmly enough, allowing one to be pushed in too far so it's opposite number isn't pushed in far enough, and plastic sleeves that slide all too easily to expose metal parts.

Sometimes it is necessary to replace electrical connectors, or you may be fitting additional electrical components. There are after-market, crimp-on spades and bullets available, male and female in both cases, colour-coded for current carrying capacity - red (5amp), blue (15amp) and yellow in increasing capacity and conductor size. There are also brass solder bullets. Some are shown in the picture on the left, click to enlarge. The only places the small red female spades fit on the MGB that I am aware of is the fuel tank sender (without fuel feed pipe) earth, some tach earths, and the 'boost' contact on rubber bumper solenoids that provides a full 12v to the ignition coil on cranking (however many rebuilt starters seem to have the medium sized spade for both the solenoid 'operate' connection and its 'boost' connection). The medium sized blue spade is correct for virtually everywhere else on the MGB. The large yellow female spade may fit the large output spades on alternators but I have not used them. Use of male spades attached to wires is very rare on the MGB, I can only think of the handbrake diode on later MGBs with male on one side and female on the other to ensure it is connected the correct way round. Similarly with female bullets on wires, possibly only the signal input on the later electronic tach. Female spades come in two varieties - fully insulated and partly insulated. On the face of it the fully insulated are best for anything other than earth connections, but that precludes soldering the wire (see below). Bullet connectors are a problem. The red males are too small for the standard bullet connector on the MGB and the blue ones are slightly too big. They can be forced in, but this distorts the connector and weakens it. Once a blue bullet has been forced in the connector will have opened up such that a standard bullet is now a loose fit, and crimping it tighter just weakens it still further. The brass bullets are the correct size for the standard connector, and are themselves too loose a fit in the blue females (confirming that the blue crimp type are the wrong size for the standard bullet connector). I only ever use crimp bullets for new work if I have the opposite gender on the wire it is connecting to, for anything connecting to existing wiring I always use a standard connectors and brass bullet. There are also crimp connectors for 'permanently' joining two wires together. I never use these, preferring to solder and heat-shrink - remembering to put the heat-shrink tubing on first and sliding it away from the heat of the iron!

Spades: The standard size of spade is 6.3mm width but the starter solenoid on rubber bumper cars has a coil boost contact which is smaller. Andrew Edwards asked if I knew what size it was, he had got a 2.8mm wire terminal but that was way too small. 12v Planet show 4.8mm (as well 2.8, 6.3 and 9.5mm used on alternator output terminals) but that is too big, it seems to be somewhere in between i.e. between 2.8 and 4.8mm. When I replaced the V8 starter (with coil boost system) with a rebuilt unit that came with two standard spades instead of one standard for the solenoid and one smaller for the ballast bypass, so I thought I had nothing to compare it with. But while writing this I remembered that later CB cars (i.e. Bee) have the same starter but the coil boost contact is unused, and that measures 0.187" which converts to 4.7498, or 4.8mm. So why Andrew's is smaller is a mystery. He's managed to pinch up a 4.8mm to fit, but it's not ideal. The spades on reversing light units are also 4.8mm but have a 2-pin connector on the harness, and the night-time illumination on the dash switches of 77-80 cars are the same size. I don't have the switches but the females from an old 1980 harness fit the reversing lights and the bypass spade on Bee's starter.

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. 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:
These piggy-back and 'Y' adapters are very useful for wiring modifications.

Multiway Connectors: Translucent multi-plugs can be tested by pushing a probe in along side the wires, again test both sides with the connector fully assembled. But moulded-on types can only be tested by parting the connector just enough to get a probe in, and even then only the pin-side.

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.

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
There are a number of sealed junctions in wiring harnesses, where a number of wires of the same colour come together in a sealed and permanent junction rather than a multi-way bullet connector. This seems to have started in the 1970 model years, and there could be one in the brown, black, green, red/white (instrument lighting) and maybe the green/orange of a late 1980 UK model, and as far as I'm aware all are behind the dash i.e. where there are the greatest number of components in close proximity. Understandable in the brown circuit because of the high currents you can get and the need for low contact resistances, and where there are five or more wires coming together, but for other circuits where there are four or less wires in the junction it doesn't seem to make sense. Also 68 to 72 North American models were making use of 6-way bullet connectors in the white circuit at the same time as sealed junctions with six or less wires on other circuits - most odd.

The wires are crimped into a brass 'staple', for want of a better word, the wires and staple then being soldered. A heat-shrink end-cap is fitted over the junction first, then a length of conventional heat-shrink tubing over that, the two being shrunk over the soldered junction and that is all there is to it.

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.

Earth/ground connections:

by Felix Weschitz

Two types - a wired earth, and an earth derived from the physical mounting of the component. Although useful for other reasons the Dan Masters simplified drawings do not differentiate between the two and neither do they show which components share earths with other components, and some of those shared paths use bullet connections for branching. Earth faults can cause some very strange interactions between components and the Leyland/Haynes drawings will be essential to work out where the actual fault may lie.

Wired earths: From the Leyland schematics for the 62 to 64 MGB there seem to be quite a few wired earth points in various places:

  • Headlights
  • Wiper motor and instruments
  • Dynamo control box
  • Wiper motor switch and indicator switch (tell-tale contacts)
  • Heater fan
  • Fuel tank sender
  • Fuel pump
After that there are generally only three covering all the above - one in the engine compartment for stuff at the front, one in the cabin above the wiper motor for stuff in the middle, and one (or two) on the rear panel for stuff at the back. Variations on this were:
  • Early Mk2 and the 68 model year look like the engine compartment and cabin earth points were combined into one.
  • Although the fan motor is in the engine compartment it always seems to have used the cabin earth point.
  • North American Mk2 and rubber bumper cars use wired earths for the indicator tell-tales, prior to that it was picked up from their mountings.
  • From around 1971 or 72 although there were only a couple of wires on the cabin earth point one of them fed a sealed junction behind the dash that could have many more wires, and some of those could be daisy-chained off various components and be branched in a series of 4-way bullet connectors with three or four wires.
  • The schematic show that until 1972 models with the seat-belt warning the fuel tank sender used a wired earth abut picked up an earth from it's mounting to the tank and the tank to the body after that. RHD cars are shown as using a wired earth until the start of the 77 model year, but in fact all RB cars also used picked up an earth from its mounting even though the sender still had the earth terminal. Note that current stock plastic senders need a wired earth to a convenient point.
  • From 1974 North American cars with the sequential seat-belt warning system used a wired earth for the number-plate lights on the number-plate backing panel, before that when they were in the overriders they picked it up from their mountings. This is via light unit - overrider - bumper - bumper iron - body and can fail to work with freshly painted components. RHD cars gained wired earths with rubber bumpers for the same reason.

Physical earths: A number of components pick up an earth for their electrical operation from their physical mounting:

  • Starter motor and starter solenoid
  • dynamo and alternator
  • Distributor
  • Chrome bumper front parking lights and indicators
  • All rear light clusters
  • Overrider-mounted number-plate lights
  • CB (not V8) ignition, main-beam and indicator warning lights
  • Horn button where it is wheel-mounted
  • Overdrive solenoid
  • Factory fog/driving lamps
  • Early map light
  • Instrument voltage stabiliser
  • Courtesy light switches
  • GT load space light
  • Electric oil and temperature gauge senders
  • Brake balance failure switch
  • RB and all V8 fuel tank sender
  • North American ignition switch with 'key in' warning
  • North American anti-runon system oil pressure switch
  • North American induction heater
  • North American TCSA solenoid
  • Horns 1977 on
  •   Fuses and Fusebox

    Fusebox Connections April 2016
    Fusebox Mounting June 2016
    Fusebox Replacement May 2016
    In-line fuses July 2023
    Blowing Fuses
    Fused Battery Connector January 2014

    Originally a 2-fuse unit was used, with one fuse protecting 'always powered' circuits like the interior light and horns and the other certain ignition-powered circuits. This fusebox is 'handed' in that one fuse has four spades at each end, and the other has four spades at one end and six at the other. It seems that the main harness for 1969 has five individual green wires at the fusebox, even though the Workshop Manual schematic only shows four, and in this case the fusebox needs to be orientated such that the six spades are pointing at the bulkhead and the five greens go on these, with the whites on the front of this fuse.

    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.

    In 1970 this was changed to a 4-fuse unit with the additional fuses protecting the parking lights, one fuse per side. This fusebox only has two spades per fuse end, and to accommodate additional wires two were sometimes put in one spade connector.

    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.

    Pondering replacement of Vee's fusebox as well as Bee's I thought I'd take it off and have a look at the back. However the lower screw was seized and the cross-head slots stripped. Eventually I cut a slot across the head with a junior hacksaw (space limited for drilling given the V8) which got it turning with a large flat-blade screwdriver, until it sheared. Fortunately it did so leaving a stub long enough to get a Mole wrench on, and with the previous application of WD40 and Shock and Unlock it unscrewed, so a new screw needed - and that's where the interesting bit started, in that the 73 roadster and the 75 V8 have quite different mounting arrangements, let alone earlier and later models.

    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.

    In-line Fuses:

    • First used in 1968 and 1969 to fuse the parking lights before the 4-fuse fusebox, one for the front and one for the rear, with 10A fuses.
    • None used in 1970, but in 1971 one was used in the 'accessories' circuit for the heater fan, wipers and radio where fitted. Positioned under the main fusebox between the white/green from the ignition switch and the green/pink on to the aforementioned components, with a 35A fuse. Deleted on rubber bumper cars when the heater fan and wipers were powered from the ignition green circuit again and only the radio was powered from the accessories circuit direct off the unfused white/green, presumably with it's own in-line fuse.
    • One was added for the hazard warning lights in 1974 (North America for Mk2), very inconveniently located behind the center-console initially, subsequently moved to under the main fusebox, brown wires both sides, 35A.
    • In 1977 the ignition green circuit was subdivided with an inline fuse feeding the electric cooling fans. Mounted under the main fusebox, white/brown on one side and green on the others, with a 35A fuse.
    • In 1978 RHD cars gained a second subdivision feeding the tach, indicators, heater fan and GT HRW, white/brown to green and 35A as before. Note that these two have been arranged such that it's possible to connect green to green and white/brown to white/brown which will cause some head-scratching, a new harness came to me like that!

    As well as for new circuits it's a very good idea to fuse the fuel pump and the overdrive circuits as shorts have occurred in both causing damage to the various harnesses. The V8 came to me with the cooling fans unfused - they were originally powered from the main green circuit as was the heated rear window but those two alone take 20 amps i.e. more than the 17A rated fuse. I powered the HRW from a fused relay off the brown circuit which boosted it's performance as well as taking load off the main green circuit and ignition switch. I suspect there was an official change to power the fans directly off the brown circuit which left them unfused. I didn't like that so added an in-line with a female spade one and and a male the other so it could be reversed at any time.

    In-line fuse holder failures:
    Good job as I did, as three times now fuse-holders have suffered heat damage and installing them how I had I was able to take the fuse out of circuit. The first one was a 'recovered' blade-type where the fuse had partially melted but still worked, only discovered by chance when for some reason I removed the cap! The second and third were new 20A eBay items that came complete with fuses, and both suffered heat damage in the holders where the ends of the fuses made contact with the holder terminals. After the second one I looked at these holders and fuses more closely, using an unused one from the same batch and made some interesting discoveries.

    I opened up an unused one and the fuse stayed wedged inside, refitted the cap and realised the cap terminal wasn't in contact with the fuse. Trying different fuses they didn't jam, so I started measuring them and discovered that the included fuse end cap diameters were up to 0.2465" whereas the separately bought ones were slightly smaller at 0.242". Aha - too big for the holder? The larger ones the other way round, or twisted slightly, they didn't jam so it was really marginal. So was that the cause i.e. the fuse not free not making proper contact with the terminals? But then I compared the length and found the included fuses were 1.27" and the separately bought ones were 1.195" and a noticeable difference. Then I realised it was a separately bought i.e. shorter fuse in the second holder, so was it a case of the spring not pushing hard enough?

    When I fitted headlamp relays and fuses to Vee I bought the bits from 12v Planet and have a spare fuse holder I can test the fuses with. These seem to have a larger internal diameter than the eBay ones as there is no sign of the larger fuse sticking, and it has a stronger spring which should mean even the shorter fuse is making a better contact. So buyer beware.

    November 2023: Needing to pump a gallon out of Bee's tank using the on-board fuel pump I was surprised to find the fuse warm to the touch.

      Fuses: As well as the two or four fuses in the fusebox there were a number of in-line fuses at various times. Unless otherwise stated these were always 17 amp rated, 35 amp blow.

    • Firstly Mk2s prior to the 1970 model year had two in-line fuses where the rear harness joined the main harness for 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 an in-line fuse 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.
    The tubular glass fuses used on the MGB usually show quite clearly when they are blown, especially if they have a slip of paper inside, and it can be confirmed with an ohmmeter. But it is not unknown for a bad connection to develop between the fuse wire or tape and the end cap, and appear to be sound. An ohmmeter may also show good continuity, but ohmmeters are not a reliable indication of a bad connection. The fuse needs to be in its holder, and the circuit powered and drawing current from one or more of the components it is supply, then to test both sides of the fuse with a voltmeter. There can also be bad connections at the fusebox between the end caps and the fuse holders, between the fuse holders and the spade terminals, and between the spade terminals and the spade connectors, so each of these points need testing as well, more info here.

    With the exception of those specified in the above list MGB fuses are glass 17 amp continuous/hold rated, 35 amp blow rated, GFS3035. The Parts Catalogue specifies 35 amp i.e. the blow rating, and some replacements are only described with one rating so you would have to be sure that relates to the blow rating and not the continuous rating or you could find that in a short-circuit situation the fuse wouldn't blow and the wiring would be damaged. Most of the usual MG suppliers only give the 35A rating, Moss in some places gives the continuous rating as well but in others only the blow rating. One source (who I refuse to use anyway) states 35A but shows 25A! It's claimed that generic American fuses only state the continuous rating, so you would need to use a 15 amp or 17 amp, not a 30 or 35 amp. Also these generic American fuses are said to be physically slightly longer than UK fuses at 1.25" as opposed to 1.17" so at first glance look the same and will fit the fusebox, but could cause a problem with in-line fuseholders. However US MG parts suppliers such as Moss, Victoria British and LBCarCo do supply the correct 17 amp continuous/35 amp blow fuses. Fuses can come with a slip of paper inside giving the rating, or it is screen-printed on the outside of the glass, or stamped into one of the end-caps. The first two can give either one or both values, the third one only one of the values.

    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.

    Heated Rear Window

    Adding a Relay

    Before the 1973 model year the heated rear window was always optional. For Mk1 North American spec and prior to the 1972 model year elsewhere a pull-switch with integral warning light was provided on a separate small panel somewhere. For Mk2 North American spec and 1972-on for other markets the switch and separate warning light moved to the centre console, and for 1977-on to the main switch panel. 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 for an external lamp beside it, and there is no reference to it in the Parts Catalogue. In December 77 the switch was changed to AAU3210 with an external warning lamp beside the switch, and the diagrams only show this arrangement. In 1980 there was now a rear fog lamp switch which took the place of the external warning lamp and the HRW switch was changed back (?) to one with an internal warning lamp. This switch does not appear in the Parts Catalogue, wiring diagrams or suppliers catalogues so I don't have a part number, however the Mini switch of the period YUF101680 may suffice although it has a yellow lens in the rocker whereas it looks like the original had a green lens.

    Prior to 1971 (Mk2 for North America) a separate wire (green/black in the schematics) was run from the switch to the back of the car, after that it seems to have been part of both main and rear harnesses (white/black, 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 although the Leyland schematics continue to show them as optional and Haynes only shows it as standard in the final 'later models' diagram. In December 1974 production of the GT for North America ceased, and for other LHD markets in June 1976.

    Up to 1970 they were powered off the white circuit via their own in-line fuse - white to green/black (white/black Mk2 North America) - near the fusebox, the remainder of chrome-bumper cars were powered from the green circuit, sharing this with many other components.

    Rubber-bumper cars up to December 1977 (but not V8s) had a dedicated relay and powered the HRW from the purple (fused) circuit. After that it reverted to no relay and was powered from the green circuit again as there was now an ignition relay which took the load off the ignition switch. However that was only for one year, apparently there were problems with the relays sticking on and leaving all the ignition powered stuff running even with the key in your hand. So some time in 1978 on RHD cars things like the ignition, heated rear window, indicators and heater fan were moved back to the ignition switch, but the fuel pump was still left on the relay, see the ignition schematics for more info. At that point the HRW (plus indicators, heater fan and tach) were powered from one of two inline fuses under the fusebox that connected a white/brown wire to a green wire. One of these has thick wires but this is for the cooling fan, the HRW etc. are powered from the other one with the thinner wires.

    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 about half that at the HRW connections, the rest being 'lost' en route.

    There often questions about the wiring and connections to the HRW on the tailgate itself. There were two types of HRW - the earlier embedded wire element type and the later surface-printed element type, I can only speak for the latter. On my V8 the wires exit from a hole in the rubber seal surrounding the glass very near the hinges, and terminate in bullets. The wires run down under the seal to the element connection points which are about mid-way down each side. The rubber seal is pretty hard to lift, and I don't want to damage the connections, so I've been unable to determine what lies beneath, but word is that it is a small spade connector (October 2011: it is, see here). You should be able to test at these points with a voltmeter to see if non-functioning of the HRW is due to a break in a wire feeding the element, or a problem with the elements themselves.

    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:
    I added a fused relay direct off the brown circuit and that increased the voltage to 10 volts at the connections to the harness under the rear cant rail, 8v at the element contacts, and a measured 8 amps. Despite that being quite a bit less than the theoretical 14v it's still about 80 watts and has made a significant improvement to screen clearing.

    It is convenient to interrupt the white/black at the bullet connector where the main harness joins the rear harness near where the firewall joins the right-hand inner wing. Mount the relay near the fusebox so there is a short run of thick brown wire between the two. Use a relay with an integral fuse or an in-line fuse close to the fusebox. Pick up the earth from the physical mounting, then run two wires from the relay - one to the existing bullet connector still on one of the wires and a new connector on the other. It would be preferable to use the new one on the wire to the rear harness as that carries the greatest current, and clean up the bullets. You could add a thick purple back to the fusebox instead of a fused relay or separate in-line fuse, although I used a brown as I was not aware of the factory relay arrangement at the time. Also make sure the connectors and earth at the back of the car are clean and sound.

    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.

    Peter Ugle reports that after trying various repair paints for the surface tracks without much success he obtained a custom-made replacement kit from DS Demist which works really well. The link I had for DS-Demist now displays a web page that will infect your computer if you click anything on it, and you have to force close the browser or shut down the computer. On no account click anything on a page like that. The same link is displayed on Citroen DS car sites, so take care there as well. April 2016: One such paint is from Bare Conductive, but from their data sheet you can see a 70mm strip 3mm wide has a resistance of 473 ohms. Now this is much longer than one would hopefully need to repair a track, but even if it were only 1mm long it would have a resistance of nearly 7 ohms. And if one kept to the track width of about 0.75mm it goes up to 28 ohms, although increasing the thickness will reduce it. As my screen measures about 1 ohm, which represents 9 elements of 9 ohms each, you can see that even one 'repair' is going to leave the track virtually useless for screen clearing. There is also this HRW kit from Holdens, which consists of lengths of self-adhesive foil that can be cut into narrow strips and stuck to the glass. Expensive at 60, it's intended for providing full HRW rather than a repair, and I don't think I could reasonably cut anything as narrow as my strips, and there is the problem of connecting the ends to the existing connectors. They say it is suitable for front screens as well, but getting it thin enough not to cause annoying if not dangerous obstructions would be difficult to say the least.

    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.

    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.

    There is only one spade connection each side of the element which is fair enough, but it protrudes across the face of the glass in line with the elements, quite a long way. As the wire connector has to push onto this spade, the wire will protrude even further, and be quite unsightly. Anything catching on the wires could well rip the connection off the glass, and you would be back where you started. It might be possible to bend the spade back on itself so it is pointing at the seal instead and the wire could come out from the seal straight onto the spade, but great care would be need to hold the 'glass' end of the spade steady while you did so. Probably the best bet is to use a right-angle wiring connector instead, the wire making another right-angle from that to go under the seal. You will still need to take great care when pushing the wiring connector onto the spade, I'd advise trying it on another spade elsewhere several times to make sure it slides on easily.

    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.

    Another possibility is a fan heater - many different types, all more powerful than the screen, some several times more so. But don't go too mad or it could crack the glass!

    Horns Updated September 2013

    Wheel-centre Horn Push
    Fault Diagnosis
    Adding a Relay

    Originally the horn push put out an earth to one side of the horns, the other side being wired back to the purple fuse, these are known as '2-wire horns'. Apart from 1970, 77 and 78 the horn push was on the steering wheel picking up its earth from the steering column, in the other years it was on the end of the indicator stalk and had a wired earth.

    For the 1979 model year on the horn push was fed with 12v from the purple fuse, and thence to one side of the horns, the horns picking up a local earth from their physical mountings. These are known as '1-wire horns' and as well as saving about 3 feet of wire they eliminate the problem of the poor earth connection through the steering column mentioned below. There is conflicting information from various sources as to when this change occurred - all rubber bumper cars; the start of the 77 model year; a few months into the 77 model year; but Clausager dates it to chassis number 471001 in May 78 for the start of the 79 model year.

    Also originally there was only one pair of wires for the horn as the second horn was optional. This came out by the right-hand headlight and went to the horn mounted on a flat bracket on the slam panel. This harness also only had one set of bullets for the front lights, which reached the middle of the grille, the tails from the light clusters reaching that point also. If you had the optional second horn there was a sub-harness between the two. However it seems that the indicator and headlamp tails currently available have shorter tails for the later harness that went right across the car, so for the earlier harness you will need to make up extenders to reach the near-side lights units at least.

    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.

    Note that with the early harness where a sub-harness is needed to power the second horn, you need a horn with two sets of spades on each terminal for the right-hand horn at least, these horns were BHA4514 and BHA4515. Later horns only have one spade per terminal, so there is nowhere to connect the sub-harness. On cars with twin horns from the factory the wiring to the right hand horn has two wires in each spade connector, so horns with only one spade per terminal can be used. You can use a male-male-female spade adapter on a later horn with the early harness.

    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
    Replacing a Moto-Lita steering wheel with an original
    The various arrangements for connecting to the wheel-centre horn contacts can be seen by clicking the thumbnails to the left. Note that North American Mk2s, and other markets for the 1970 model year, moved the horn push to a column stalk. But this was not liked and it moved back to the horn centre for all markets for the 1971 model year. However it moved to a column stalk again for the 77 model year to the end of production.

    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. used so infrequently - usually just at MOT-time, it's a good idea to occasionally test the horn with no traffic or people round just to check it still works.

      Horn Mounting September 2013
    There were five different types of horns over the years and two different brackets. Originally a flat bracket (57H5309) mounted the horns vertically on the edge of the slam panel, before changing to an angled bracket (originally 17H8641, now GGE110) basically identical to the earlier flat bracket but with the bend, on the inner wing in January 1963 which positioned them horizontally. Also originally the harness only had one horn tail and there was a sub-harness to extend the wires across to the other side if you had the optional second horn. In late 69 dual horns and hence two tails in the main harness became standard. Both locations - when used with the appropriate brackets - position the horns horizontally, but going by the illustration in the Leyland Parts Catalogue you have very little choice about orientation - they have to point forwards or the spades will foul the top of the bracket. It's true that being horizontal water and dirt can't fall down inside the trumpet, but it will still be driven inside. However at least two horn types have the spades in a different position and so allow the horns to point across the car towards each other rather than forwards. Vee's are like this and it protects them to some extent from driving rain and dirt, whereas the position of the spades on at least one other type means they have to point forwards for the terminals to be accessible.

    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

    Two-wire horns:
  • If the fuse is blowing or the horns sound continuously with the purple/black connection removed check for continuity between the horn terminals and the body. Any continuity here is a fault, as Steve Penkethman found in June 2021 with his 1970 which was blowing fuses but only when mounted.
  • Check the voltage on the purple at the horns with the horn button both released and operated. If there is no 12v at all, or you see 12v with the button released but significantly less with the button pushed, then check the purple fuse (bottom in the 2- or 4-way fusebox) and the wiring back to it for broken or bad connections.
  • Check the voltage on the purple/black with the horn-push released. If you don't see 12v at all then the horn itself is bad.
  • If you see 12v then measure again with the button pushed. This should drop to almost 0 volts (earth). If it does but the horn doesn't sound then again the horn is bad.
  • If the voltage doesn't drop, or doesn't drop very far, check the wiring back towards the horn button for broken or bad connections, and test on the connector going to the brush, the brush itself, and the brass ring on the back of the wheel. If you get the same 'less than zero' voltage at the body of the wheel then the column earth is bad, fit an earth strap or relay.
  • (Updated May 2008)If the wheel shows zero volts, but the voltage on the brass ring on the back of the wheel is higher than this, then there is a problem inside the wheel and horn push. There are several possible places this could happen - the pressure contact between the back of the brass ring on the wheel and one end of the pencil, the braided wire between the two brass ends of the pencil, the pressure contact between the end of the pencil and the brass contact inside the horn push, the connection between the brass contact and the copper ring attached to the horn push frame (this is the operative part of the horn switch), the copper ring where it is attached to the horn push frame, where the springs are attached to the horn push frame, and these springs and the wheel when the push is fitted. Unless these springs have been bent and are slack this last is very unlikely unless the wheel is badly corroded.

    One-wire horns: Check the voltage on the purple/black spade with the horn button pushed. If there is no 12v or something less than 12v check the wiring and connectors back to the horn button for broken and bad connections the horn button itself, and 12v on the purple wire feeding the button. If there is a good 12v on the purple/black measure the voltage on the horn body. If you see more than 0v then the horn earth is bad. If you see 0v (earth) then the horn is bad.

  •   Repair September 2014
    This horn is from a TR4, but the principle should be the same for the MGB as the originals were electro-mechanical as this is. This horn was completely dead, but another fault could be that it just clicks when power is applied. Both could be down to adjustment, so before opening the horn up try twiddling the adjuster screw that is usually present. On this horn just a quarter-turn of the screw is enough to change the fault from 'completely dead', through sounding to some degree, to 'just clicking'. If adjusting the screw doesn't bring it back to sounding normally then you might as well open it up and see if it can be fixed, you have nothing to lose. Note that if it just clicks, leaving the power connected for any length of time will probably burn the internal coils out, the horn will get very hot in the process.

    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.

    Inside you should find a couple of electromagnet coils, and some kind of interrupter contact, in series with the two spades (or the single spade and the body of the horn in the case of 77 and later MGB horns). The principle of operation is that current flows through the coils and attracts the diaphragm towards it. This diaphragm has a 'crinkle' in it, which means that rather than it moving gradually as the magnetic field builds up (electromagnet coils are effectively inductors and there is a finite period over which the magnetic field builds to its maximum, it is not instantaneous) the field has to reach a certain level before the diaphragm starts to move, then it moves suddenly as the force overcomes the resistance of the crinkle. This is exactly the principle used in the 'D-Day Cricket' used by paratroopers in WW2 to tell friend from foe when they couldn't see someone, rather than risk showing themselves. The crinkle makes the diaphragm move further and faster with a snap action than it otherwise would, which greatly amplifies the sound over a simple flat diaphragm. However, unlike the Cricket, as soon as the diaphragm has moved it's operating pin moves a lever which opens a contact, which breaks the electrical circuit through the coils, so the diaphragm is released, again with a snap action. As soon as it has released the diaphragm the operating pin releases the lever, so the contacts close again, re-energising the electromagnet, attracting the diaphragm again, and so on, vibrating the diaphragm.

    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 prior to 1977, but not on cars with the horn push on the column stalk (1970 model year) as these have a wired earth. 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.

    Note that this circuit will only get round a poor earth from the column through the switch and onto the purple/black. I opted to install the relay as the column on my V8 had a low-grade earth that would not even operate one horn (I wondered why it came to me with what looked like a moped horn!)let alone two. I mounted it behind the dash on the firewall close by the indicator flasher and voltage stabiliser where all the wires that are needed are close by (on an RHD, at least). The purple/black is cut between the column multi-way connector and the main harness, and will need short extensions to the relay, I opted to use a 2-way 'chocolate block' connector rather than solder bullets or spades. Inside the cabin is also a better environment than under the bonnet. Subsequently when I found the roadster would benefit from a relay as well as it was losing about 3v in the earth path, to avoid cutting this wire I did think about fitting it behind the radiator diaphragm, diverting and extending the purple/black from the right-hand horn back to the relay winding, then extending the relay contact to the horn. However that would only have helped the right-hand horn, I would need to extend the new purple/black to the left-hand horn to benefit both. So I decided to put that one in Bee's cabin as well.

    February 2021:
    Even more subsequently I realised that for 1970 to 76 inclusive there is a spade connection to the horn brush under the cowl, so with a relay by the column the existing wire can be removed and extended down to the relay contact, and a new wire run from the brush to the relay coil, although on CB cars at least the cowl is pretty snug round the column and harness.

    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.