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MG MGB Technical - Reverse Light Switch

I reinstalled the transmission in my 80LE after new seals and general clean up. There is an electrical run to the transmission with 2 pair of wires. One pair for the reverse switch and the other for transmission interlock. When I looked in the Moss catalog at the wiring diagram (page 38 in a couple of recent issues) it clearly shows the reverse switch on top of the transmission as part of the remote tower. On my transmission, there is a switch on the tower as shown and one on the passenger side of the transmission (north American, left hand drive model). Based on the length of the wiring harness, the correct color wires for the reverse lamp can only go to the switch on the passenger side and the switch on the tower must be the transmission interlock. Note, this is a non-overdrive transmission. Is the Moss catalog diagram wrong?
Rick Penland

The driver's side switch is the reverse light switch, according to my manuals. The one on the passenger's side is the overdrive isolation switch, that prevents the overdrive from being engaged in any gear except 4th on the '80 model B. RAY
rjm RAY

The reverse light switch is on the right side of the gearbox casting spades pointing mostly upwards. The overdrive switch is on the left side of the tower assembly spades pointing backwards, and because it is over part of the OD/gearbox is more difficult to get at from underneath. It's possible the late 4th gear only switch, used for TCSA (plus OD when provided), is on a different part of the tower. See http://www.mgb-stuff.org.uk/images/gbs2.jpg for a UK RB OD gearbox.
PaulH Solihull

Thanks, Paul. I found the same pic you inserted in a Google search last night and was then 95% positive of my conclusion. With your comment, it's 100%. Considering the wiring harness which is factory standard, that's the only possible conclusion. I resorted to the Moss catalog diagram when reinstalling the switches. I had forgotten which one had the 2 fiber washers which is exclusive to the reverse light switch. Quite simply, the Moss catalog diagram is wrong. At least it's wrong for my car.
Rick Penland

Not wishing to confuse matters, but AFAIK both switches could have the same fibre washers as they are the same part and need to operate under very similar circumstances. The washers are said to be there as spacers to ensure that they do reliably operate the switch, but don't obstruct the movement of the rods. The Parts Catalogue states only one washer for each, I've never actually removed one to find two, that is from tales of yore and has been repeated many times. Moss Europe is similarly confusing in that it appears to show the 4-synch box with a reverse light switch blanking plug going into the left-hand overdrive switch hole, hence it would be assumed that's where the switch itself would go. Not only is that wrong, but all cars had reversing lights several months before the 4-synch box was introduced anyway hence the blanking plug was never an option.
PaulH Solihull

The factory drawings must have been in error and repeated, in print, over the years. I have a BMC dealer's parts book from '65 that shows the reverse light blanking plug on the right side of the shift tower. In other shop manuals that I've seen, these errors are never corrected. One of them, for a Subaru, shows the layout for the timing belt and both camshafts, complete with timing marks. Unfortunately, one of the camshafts is shown 180 degrees out of phase, and the engine runs quite poorly when set according to the factory specs. I set the cam timing up twice before I discovered the misprint. All of the new manuals, that I've checked, still show the same incorrect picture. RAY
rjm RAY

Ray, the Moss catalog diagram that I found notes that it is for 1968-1980 models and subtitled for "late model" gearbox versus early gearbox. The Haynes manual on page 118 in my edition shows the reverse light switch in its drawing, but the location is undefined. I might be opening a "can of worms" with all this, but there are likely several minor variations of transmissions throughout those years to accomodate (1) overdrive or not and (2) introduction of the TCSA (transmission controlled spark advance). I've found some information here and there on the internet, but no definitive discussion.

Paul, to address the washers. I recall both were on one switch and it appears to be commonly done with the reverse light switch to insure proper engagement per a comment I read somewhere on the internet. I'll leave both on the reverse switch and check again when the car is all together. I realize I wasn't very methodical in disassembly, but the transmission was an oil and grease soaked mess. Now you can handle it without washing your hands afterwards.

To both of you, thanks for for your insight. I'm sure there are decades of combined experience between you fellows. But, it's sort of pleasing to know that we still have things to learn about our favorite cars.
Rick Penland

It's only logical that the reverse switch would be on the right side of the transmission, in hindsight. The base of the shift lever, when pulled towards reverse, actuates a plunger that completes the reverse light switch's electrical circuit. This should apply to all B transmissions. I'm in the process of re-assembling a '73 Yamaha 2 cycle motorcycle engine that tore up a bearing and destroyed the right case. No parts have been available for over 25 years, but the original owner wants the bike repaired. I located a complete engine, in Arizona, but after tearing it down it's not much better than the one I was working on. If only parts and information were still available for these old bikes, like the B, my life would be much easier. RAY
rjm RAY

The reverse light switch *is* on the right side. Did you mean to say your books showed it on the left? Don't forget manufacturers drawings are often generic, even in the workshop manual, and it could have been copied from a drawing of a model with reverse to the right (lever-wise!).
PaulH Solihull

Paul, I have the Bentley shop manual, the BMH cd as well as a parts book that all show the reverse switch on the left hand side of the transmission. I know that they are wrong, and the error was probably never corrected after the first printing. As I mentioned above, I have seen this many times before, in the manuals of other makes of cars. I never saw a B with reverse to the right, however. RAY
rjm RAY

It looks like I opened a can of worms by asking this question. However, the conflicting information out there was why I posed it. The overriding evidence that I can provide to support the reverse switch on the right side of the transmission is the configuration of the factory wiring drop to the transmission. The green and green/brown break out of the harness about a foot before it terminates in the yellow and yellow/purple pair. There is no physical way to connect the green pair to the switch on the left side that is part of the remote tower. I didn't have any concern about hooking it all back up during removal since the harness configuration seemed so apparent. It was only after noting the Moss catalog parts diagram that I was doubting my conclusion. From what I've read, the TCSA switch was introduced sometime in the 1976 model year. I've found the Moss catalog diagram duplicated several places on the internet. Let me be fair to Moss, they are not the source of that diagram.
Rick Penland

This thread was discussed between 24/02/2011 and 28/02/2011

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