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MG MGB Technical - 1979. Tail light problem

On my 79 MG B I have a problem with the signal/parking lights. When all the lights are off the turn signals work fine. When I put on the parking or headlights the right rear signal light goes on and the signal lights don't work. I have looked into this but to no avail. Is there a fix?

Jim
JMcHugh

hey Jim, every time I have had weird issues with MGB electrics it has been a bad ground. Try looking for such. Hope this helps, Tom
Thomas McNamara

Yes, probably bad earth.

The rear light clusters get their earth from the physical fixings to the body - three serrated Dotloc special nuts on the back.

However if the parking and signal filaments are in the same bulb it can be where the bulb body earths to the socket, or even where the socket is mounted in the light-unit casting.

With the lights on look for voltage on the bulb body, the socket, or the light unit body. Should be zero volts on all.
paulh4

Sorry it has taken so long to respond. I have looked at the grounds and they seem OK. Specifically I checked the lights. They all seemed good. Then I unhooked the signal light and all the lights stayed on. So the grounds were OK.

Is there anything else to check?


Jim
JMcHugh

It does sound like the stop/tail light globe isn't earthing properly. I'd give the tailight assembly mountings a good clean up, pull the light out and clean the body where the nuts tighten up to. If that doesn't work, try swapping the globes out from the other side light, if that doesn't fix it try swapping the whole light assembly and see if the problem follows the light or if it syays on that side---process of elimination.

willy
William Revit

I did all rhat. As a matter of fact I bought anew Tail light assembly and, when it had the same results, switched it. The results were the same, the problem remained where it was. Thank you.

Jim
JMcHugh

"When I put on the parking or headlights the right rear signal light goes on"

What are the front lights doing? A bad earth there can result in stray voltages getting into wires that go to the back.

Unless both front and rear do the same thing at the same time either they are not connected together as they should be e.g. cross-connected with something else, or there is a bad earth somewhere.

paulh4

The front lights go on at the same time as rear lights. The only difference that I can see us that when I put on the parking or headlights the right rear signal light goes on but the signals don't work. Only the right rear one stays on.

If it is a ground I can't find the problem.

Jim
JMcHugh

How did you check the earths? It has to be done with lights on and measuring at all the points I mentioned looking for voltage, it should be zero. Of course zero volts could also be that one or other probes isn't touching correctly, or the meter is not working, so that has to be confirmed by testing against the 12v feed to one of the lit bulbs.
paulh4

Jim--If you get a length of electrical wire and bare both ends, remove the coloured lens and turn the lights on--then jam one end of the wire in between the side of the taillight globe and it's holder and put the other end of the wire to a good earth (boot catch)---IF the signal light goes out with the taillight globe earthed like this there's definately an earthing issue with the taillight globe

willy
William Revit

This thread was discussed between 09/07/2024 and 08/08/2024

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