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MG MGB Technical - Reverse light switch

Hi All
I've a '78 roadster, the reversing lights are not working, it looks like the gearbox switch, has anyone changed one of these? the Haynes manual isn't very clear exactly where it's located and how accessible it is.
Are working reversing lights needed for the MOT on a car this age?
Ron
R. Algie

IIRC the reverse light switch isn't easy to get at so check that it's not a bad or crudded up connection somewhere and that the rear units do work

a spare battery and test lead with various connectors can be useful for checking electrics or clamp connectors to your fitted battery with test lead and/or test lamp that you can connect in to existing wiring

AFAIK reverse lights aren't in the MoT test - http://www.transportoffice.gov.uk/crt/doitonline/bl/mottestingmanualsandguides/mottestingmanualsandguides.htm

my reversing lights don't work and I'm sure it's the connector just off the switch which I tucked up inside the transmission tunnel so that it can only be reached with the car raised but just before any time (about twice a year) I'm about to raise the car the reverse lights start working again and I forget to check the connection until the next time they stop working again

it was from when I first got my current midget that I made up a long test lead to go from the fitted battery in the middle of the engine bay to get to the reversing lights in the boot, I later added various alternatives of connectors
Nigel Atkins

The reverse light switch is easier to get at than the OD switch, at least, It's high up on the front of the gearbox remote housing, on the right-hand side, as indicated in the attached which is a top view.

But first check the green and green/purple connectors where the gearbox harness joins the main harness by the fusebox, and bridge those two with the ignition on. If the reverse lights work then, but still not in reverse gear, then it does look like the switch or its connections.

But if they still don't light it's probably a fault at the 4-way bullets by the right-hand rear light in the green/purple wires, or you have other faults in or near both rear light clusters.

If it looks like the switch or its connections you might need to undo the bolts holding the rear crossmember and lower the tail of the gearbox to give you more access. Check the wires are firmly on the spades, and see if you can rotate the switch. If the switch and the wires are tight and they still don't work remove the wires and unscrew the switch. These are not adjustable as such but originally were usually fitted with two spacer washers. Try removing these one at a time and seeing if that makes a difference, after testing the switch on the bench. If the switch doesn't work then it obviously needs replacing, but if it does there but still doesn't with no spacers then either the ball on the end of the operating plunger has worn too far, or there is some other problem inside the gearbox, which is a pretty major job to investigate. Non-working reversing lights aren't an MOT failure




Paul Hunt

Paul has said it well. I have replaced the reverse light switch with the transmission in place in spite of relatively large hands.

BUT, the removal of any fibre washers will not cause the non-working switch to function. The switch is a "normally on" type with a plunger ball at the lower end which is pushed upwards by the cross rod in the shifter to keep it in the "off" position. When the reverse gear is selected, the rod has a depression cut into it which removes the pressure on the plunger ball, allowing the switch to turn on. If the switch is too low in its mounting, the depth of the notch is not sufficient to allow the plunger ball to fully extend and the switch remains held in the off position. A quick check is to simply remove the wired going to the switch, unscrew it two turns, and replace the wires. If the back up lights are illuminated, it is a problem with lack of sufficient spacers. If the back up lights do not come on, it is a problem with the switch or wires. The wires are easily tested using a test light. If the power wires does not illuminate the test light, there is no power to the system. If it does illuminate the test light, there is power to the system, but not through the switch. If the switch is rotated two, or more, turns, the wires reattached, and the system does not work, the switch is not functioning properly. Remove, clean, and bench test.

About one half of the switches I have tested over the years were non-functional.

Les
Les Bengtson

Hi
Thanks for the answers, I put the car up on stands yesterday and had a good look, the switch is easier to reach than I expected. The weather here is rotten just now so I'll put it in for an MOT next week and sort out the reversing lights when it improves, from a quick check it does look like the switch, it's probably been like that for years if it's not required for the MOT, I can't remember when I last checked it!
Ron
R. Algie

Les - are you sure that the reverse light switch is the normally-on type? Removing spacers to allow a non-working switch to start working again has been the standard advice from many people for years.
For a normally-on switch to work the selector bar would have to remain in the same place for all four forward gears plus neutral, and only move away from the switch in reverse. The same would have to happen for the overdrive switch which is the same part. As wear occurred the switch would be more likely to stay on, as does happen with the OD switch, not fail to come on.

Are you thinking of the brake light switch? That is a normally-on type, and tends to stay on with wear.
Paul Hunt

Paul. I had remembered that the back up light/overdrive switches were the normally on style--similar to the brake light switch on the Mark II cars. However, your question caused me to dig through my parts and equipment until I found my test gear and two spare switches.

You are correct and I was incorrect. The switch is a push on type which is turned on when rod, having a ground out portion in normal use, is moved by the shifter causing it to cam the switch on. Thus, as you have noted, remove shims to cause the switch to turn on sooner and add shims to cause the switch to turn off sooner.

Sorry for the confusion and thanks for the reminder to check. Les
Les Bengtson

Pleased to say it passed it's MOT today, they weren't bothered about the reversing lights, so saying I would prefer that they worked even if they don't look as if they would be much use. I've a NOS TR6 overdrive inhibitor switch which I think is the same unit, I'll check the part numbers.
Ron
R. Algie

Yes, same part for Triumph and MG Overdrive and reverse light switches.
Paul Hunt

This thread was discussed between 19/02/2014 and 05/03/2014

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