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MG MGB Technical - Reverse light switch

Recently I replaced my reverse light switch for my 1980 MGB. Now The lights only lite when I put my transmission in 4th gear and not in reverse. Any suggestions to what is going on? By the way, I'm not sure that this was going on before I replaced the switch in that the switch ball was frozen and did not move at all so I could not check it. I have only owned the car for 1 year. Thanks for your help
WRD William

Sounds like you might have the OD and reverse light switch wiring reversed, on a 1980 OD only works in 4th. The OD switch should be on the left, reverse light switch on the right. Don't reverse with the OD manual switch on until you have resolved it one way or the other.
PaulH Solihull

Lots of misinformation out there on which switch is which as I discovered a while back. Paul is correct, the reverse switch is on the right or passenger side of the transmission assuming you most likely have a North American left hand drive model. It should be easily visible with the second switch on the shifting tower and less visible. There should be no problem with switched wiring if you have the original wiring harness. The reverse light pair should break out of the wiring a foot or so shorter and not be able to reach the other switch. I learned this while reinstalling my transmission using incorrect internet sourced diagrams and couldn't get the wiring to reach. The second switch might be the TCSA (transmission controlled spark advance) switch considering your car model year. The TCSA switch engages only in fourth gear.
Rick Penland

My MG does not have OD and I am on the correct side ( right ) of the transmission. No other wires are visible and I replaced them as soon as I replaced the reverse switch. Any other ideas?
WRD William

Why did you replace the switch?

Was any other work done on the gearbox?

Were the new and old switches identical? There were different switches for different functions on various years. However whilst I can see a different switch not working at all, being stuck on all the time, or restricting movement of the gear lever, it is difficult to see how a different switch would change from working in reverse to working in 4th gear as the two movements are at opposite sides of the gate, albeit in the same direction.

Forgive me, but you are trying reverse in the left and back position, and 4th in the right and back position? If so, then it can only be a problem with the new switch having its activation pin in line with the selectors in 4th gear instead of in line with reverse gear. That is, unless something has gone wrong inside the extension tower where the switch screws, or other work has been done incorrectly in there.

PaulH Solihull

I replaced the switch because the back up lights weree not working . When I removed the switch from the transmission I found that the activator pin was stuck and would not move at all. At that time the back up lights were coming on , but only when i placed it in a forward gear. No other work was done to the gearbox that I know of. There is only one place to put the switch in . The switch is basically the same design and size being that it is aftermarket.
WRD William

William. Actually, there are two places to put the switch in. The right side (as seated in the car) has the switch for the back up lights. The left side (as seated in the car) has the switch for the Transmission Controlled Spark Advance (TCSA), on the North American specification cars. The TCSA system is, also, used as the overdrive inhibit switch on the NA spec cars.

If your back up lights were connected to the TCSA switch, which only functions in 4th gear, the product of such connection would be identical to the observations you are reporting.

That is why everyone keeps asking you if you have the system connected correctly.

Les
Les Bengtson

WRD does say he is working on the right side of the gearbox, presumably this is the right when facing forwards, i.e. on the passenger side of a LHD car.

However I'm now confused by "At that time the back up lights were coming on, but only when i placed it in a forward gear."

At what time - before you changed the switch? Or only afterwards? And any forward gear? Or only 4th gear as you said originally?
PaulH Solihull

Yes, before I changed the switch the back up lights would come on only in 4 th gear. I thought that the reason was because the switch was not working properly. The lights still come on in 4th gear only. And yes I am on the passenger side of the car(and transmission ) ANY POSSIBILITY THAT IT COULD BE A RIGHT HAND DRIVE TRANSMISSION? Would it make a difference?
WRD William

William. "Yes, before I changed the switch the back up lights would come on only in 4 th gear."

This really seems to say it all. You seem to have the TCSA switch wiring hooked up to the back up light switch and the back up light wiring hooked up to the TCSA switch. That is all that could account for the problem as you describe it.

TCSA (also controls overdrive on cars having it, but installed on all of the NA specification cars from, at least, 1977 onwards) is the switch on the left hand side of the transmission. Back up light switch is on the right hand side of the transmission. There are, or should be, two wiring harnesses going back to the transmission, one for the TCSA system and one for the back up lights. I think they have been switched and, if the system is still in original condition, you will only have vacuum advance in reverse!

Les
Les Bengtson

Right or left hand drive makes no difference, the switches don't move. If WRD is absolutely positive that he is working with the switch on the right-hand side of the gearbox, i.e. the same side that the distributor and starter motor are on in the engine compartment, then there can only be a problem inside his gear lever remote tower with the levers, especially as the reverse lights coming on in 4th gear instead of reverse was the problem before he changed the switch as well as after.

“When you eliminate the impossible, whatever remains--however improbable--must be the truth.” --Sir Arthur Conan Doyle.
PaulH Solihull

Thanks guys, problem fixed! After reviewing the last few blogs I had my wife look at the colors of the wiring ( since I am color blind ) and discovered that the wires to the reverse switch were in fact the ones belonging to the TCSA switch. Apparently before I purchased the car , someone had done some " work " on the transmission and reversed the wiring. Thanks to your suggestions I was able to fix the problem especially after reading Les from Arizona's blog. I was confused about the overdrive switch being a problem since I do not have overdrive. LONG LIVE BRITISH CARS!!!
WRD William

This thread was discussed between 15/08/2011 and 23/08/2011

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