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MG MGB Technical - no running lights

I have a 74 B (rubber bumper) and the running lights do not work--none of them, front, rear, or side. The headlights do work.

I looked at the headlight switch, and there are three wires: blue, brown, and red/green. It looks like the red/green wire sends juice to the running lights. I took that wire off and flipped the headlight switch on. I got about 12 volts from that red/green terminal. I replaced that red/green wire and then took off a taillight lens and checked for voltage at that bulb. I got about 1/2 volt.

So, where am I most likely losing voltage? If this is a ground problem, is there a likely spot to look?

Many thanks for any and all help.
Robert Fox

Your first step is to go to http://www.advanceautowire.com/mgb.pdf and print out the wiring diagram for your car. Have it enlarged and laminated and keep it with your tools.
Dan Hiltz

What about connecting the red green wire to the battery +ve terminal? If the lights all work then it would appear to be the switch.

It could well be that the switch is broken or dirty inside. It can supply 12V (low current) when there is nothing connected but cannot supply enough current to light the bulbs in the sidelights (running lights). The voltage drops across the dirt in the switch and you only see 1/2 Volt at the tail-light.

Mike
Mike Standring

They are fused separately check and clean the fuses and spades for verdigris.
c cummins

Thanks again for the help. I had hoped there was a wiring diagram available online, so that link really helped. I'm glad to report I fixed the problem. Turned out to be very minor, though. I removed the red/green wire leading into the fuse box. My plan was to check for voltage at that spot. However, I put the wire back onto the terminal, and suddenly the lights worked! I guess it was just dirty enough to break contact.

Anyway, I cleaned it off with contact cleaner and replaced everything. Hate that I posted a message for such a simple problem, but I do appreciate the feedback.

Definitely the cheapest in a long line of expensive repairs: two squirts of contact cleaner.
Robert Fox

Do the others as well, and the fuses and holders, and check the rivetted connections on the bottom between the spades and the holders. With the possible exception of the fuses they are all the same age and had the same exposure to the elements.
Paul Hunt

This thread was discussed between 03/09/2013 and 05/09/2013

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