Right up front I give you this warning: The MGB overdrive system consists of a manual switch that the driver can use when they want overdrive to be engaged, and a gearbox over-ride switch that prevents overdrive being engaged when it shouldn't be, even if the driver has the manual switch on. Anything that prevents this over-ride switch doing its job can wreck the overdrive if the car is reversed. Ordinarily the only thing that could cause this is if the switch failed in the closed position but the switch is very robust and I have never heard of one failing in the closed position (open, yes, but that will not cause damage to the overdrive). However if any wiring is added to the solenoid side of this switch there is a slightly increased risk. This modification does add wiring to the solenoid side of the switch. If anyone performs this modification and subsequently has a problem with the overdrive, I shall be very sorry to hear it but cannot be held liable in any way. I am already looking at an alternative method that does not increase the risk in any way, watch this space!
But enough of that. It may be my age but from time to time I find that I move from 3rd or 4th with overdrive engaged to 1st or 2nd and forget to switch overdrive off. All is well until I change up from 2nd to 3rd, then all of a sudden the overdrive engages again, usually under conditions where it is inappropriate. This means not only do I get the mild and unexpected jolt as it engages but another one when I manually switch it off until I need it again. This has bugged me for some time and I felt sure I could knock up a circuit to prevent it.
The intention of the circuit is to allow overdrive to be engaged if the manual switch is operated whilst in 3rd or 4th, but to lock it out when I change to any other gear even when I change back to 3rd or 4th, until the manual switch is turned off and on again. I started thinking I would need a couple of relays with a couple of contacts that would variously latch themselves and lock out the other relay, one of the relay contacts being in series with the existing manual and gearbox switches, and it was all getting a bit complicated. Then I had an inspiration and suddenly realised I could do it with just one relay with a single normally-closed contact.
What happens is that the normally-closed contact of the relay is wired in series with the gearbox and manual switches. This means that if any of the manual switch, the gearbox switch, or the relay contact are open overdrive is disengaged, and they all have to be closed before overdrive will engage. So as long as the relay remains released the overdrive operates completely normally, but when the relay operates it prevents the overdrive engaging regardless of the state of the gearbox and manual switches. The relay is wired in such a way that if the manual switch is closed and the gearbox switch is open then the relay will operate, and will remain so until either the manual switch is opened or the ignition is switched off. Remember that when the relay is operated its normally closed contact is open, and this prevents the overdrive from engaging.
If you look at the diagram you will see that when the relay operates it does so in series with the solenoid, i.e. current is still flowing through the solenoid. At first sight it may appear that because current is flowing through the solenoid it must be operated and hence overdrive will be engaged. The LH solenoid (in a sample of two) has a resistance of about 15 ohms, and a typical relay measures 75 ohms, but the solenoid needs most of the available 12v to operate it whereas the relay doesn't. Put the two in series and the relay will operate but the overdrive solenoid will not. In fact you have to make sure that the solenoid will release when the relay is introduced into the circuit, as the solenoid may well hold in with a lower current than is needed to operate it. I checked mine carefully and found that with 25 ohms in series the solenoid overdrive (one rear wheel off the ground) slowly switches in and out by itself. With 20 ohms in series overdrive stays engaged and with 35 ohms it disengages and stays disengaged. The relay I am using is 75 ohms and there is an additional 15 ohms in series with it to reduce the current still further yet reliably operate the relay, giving me a safety factor of about 3 (i.e. the solenoid needs three times the current I am passing through it).
Once the relay has operated its normally closed contact is now open, and this means that even if the gearbox switch closes again the only current that is flowing through the solenoid is the small current through the relay, so the relay remains operated and the overdrive disengaged. It is only when the manual switch is opened (or the ignition is switched off) that current ceases to flow through the relay, it releases and its contact closes again. But of course because the manual switch is open (or the ignition switched off) there is no current at all flowing through the solenoid so overdrive is still not engaged.
The next time the manual switch is closed one of two things will happen:
Choosing a relay - this must be done with care. There are Lucas 6RA relays with a normally closed contact instead of a normally open, I happened to have one of these so used that. However Lucas 6RA relays are like 25D4 distributors - there are many different types that all look outwardly the same but they have detail differences indicated by the reference number. As well as contact differences as mentioned above there are winding differences. I have ones with 40 ohm windings and others with 75 ohm (the one I have used). I suspect this relates to whether they are an 'intermittent use' (40 ohm) relay like the starter relay or a 'constant use' (75 ohm) relay like the ignition or fan relays. You must use a 'constant use' relay as it may be operated for some time, and the 'intermittent use' type may overheat if powered for more than a couple of minutes. An aftermarket relay I have has a resistance of about 80 ohms which also would be suitable. If the relay winding is too low a resistance it may allow too much current to flow through the solenoid which may not release. If this is the case, and you go into reverse, you are likely to wreck the overdrive.
The following installation notes relate to a 1973 UK car. There will be differences in some other years and markets, some of these are discussed below. The only tricky thing in installation was getting at the gearbox switch. I had to remove the centre console, fold back the tunnel carpet, remove the access plate from the tunnel, and drop the rear cross-member. Support this on a jack while undoing the bolts then lower it carefully to make sure the speedo cable doesn't get damaged against the fixed crossmember. I used a piggy-back spade connector on the new yellow/purple wire which goes on the gearbox switch in place of the wire to the solenoid, which plugs onto the piggy-back connector. The new wire needs sleeving all the way up to the relay to avoid chafing and the risk of shorting out. If you can't get the sleeved wire through the existing cable supports then use cable ties in the appropriate places. I mounted the relay by the fusebox, which is also where the existing yellow/red wire to the gearbox switch joins the yellow wire from the manual switch with a bunch of other bullet connectors. Connecting the relay to the existing wiring in this way means that it can be restored to normal very easily. I also took the opportunity to add an inline fuse to the overdrive circuit, this consists of a standard inline fuse with a male spade one end and a female at the other, and simply plugs in between the white and the manual switch on the dashboard, and again means it can be restored to normal very quickly and easily.
Differences in other years and markets:
IMPORTANT: If any of these tests fail do not drive, especially in reverse, with the manual switch in the 'ON' position. Remove the relay and restore your wiring to normal. Either your relay is too low a resistance, you have wired it incorrectly, or your solenoid stays operated with a lower current than mine does.
I tested mine over a 3-day 470-mile trip and it worked well. There has been some discussion recently about having a warning light glowing when overdrive is engaged. With the dash and gear lever switches there doesn't seem much point as it is easy to see and feel whether it is on or off. Less easy with the column switch, and in fact my V8 (column switch) came with a warning light courtesy of a PO. But on the roadster with the sequencer relay there have been a couple of occasions I wasn't sure if overdrive was engaged or not, even though the manual switch was on, i.e. I can't tell if the sequencer relay has operated and locked it out without turning the manual switch off and on again. So a possible enhancement is to have a warning light that glows when the relay has operated. This would either require a relay with a change-over contact and any kind of warning lamp, or be a LED wired in series with the relay. Again, watch this space.