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MG MGB Technical - Electrical Problem, Possibly Earth Related

Hi All
I've recently discovered a problem with the lights, when the headlights are on and I press the brake pedal I get a dim glow in the indicators, this only happens when the headlights are on it's OK otherwise, I presume this is probably an earth fault, any suggestions where to start looking, car's a late rubber bumper Roadster.
R. Algie

The headlights are earthed properly, but the rear lights are expected to earth through the lamp body to the car via the fixing screws. Corrosion often makes this earth poor. Try running a temporary earth wire - make it up with a croc clip on each end. Clip one end firmly onto one of the rear lamp screws and the other end onto a clean earth point. Try both rear lamps one at a time. If the problem goes away, you have your answer.

It can also be corrosion in the bulb holders of the rear lights and also corrosion in any of the many bullet connectors that are used in all the lighting circuits.
Mike Howlett

If it's only when the headlights are on and a rubber bumper it will be the earthing for the headlights and the front indicators, which on those have a common wired earth.

Chrome bumper cars front and all rear light clusters earth through their physical fixings. If there is a problem with these earths then the interactions are visible within those lights, usually going dim and bright as the indicators and brake are used together, not the headlights.

If both indicators are doing it the problem is either the earth bullet connectors by the right-hand headlight, where the earth wire connects to the body by the fusebox and relays, or the bullets both sides are faulty. These bullets are the most fault-prone on the whole car as they are exposed to the worst weather conditions. Buy new connectors, polish the bullets with fine emery, and reassemble with Vaseline. Make sure each bullet is pushed fully into the connector, it goes so far then appears to stop, but needs the head of the bullet pushed past a pip in the connector to lock. That is, it should, 'modern' connectors barely have any pip, making it all too easy to pull a bullet out.
PaulH Solihull

Thanks for the info, I'll clean up these connectors and see if that sorts it out,
R. Algie

While all that stuff Paul mentions is mostly correct, if the brakes affect it then the primary problem is earthing of the rear lamps. Bad earths there make the brake lamps earth through the indicator and side/tail circuits. The brake lamp earths are mostly common with the rear indicator/tail earths, except where the brake sockets are crimped into the lamp bases; all these are then partially earthing through the front lamp bulbs & earths of any lamp that is not ON. When the headlamps are ON, sharing a common dubious earth with the side/indicators, there is power everywhere, and it is earthing through the dash indicators.

FR Millmore

As well as cleaning or renewing connectors, I ran separate earth wires to each bulb holder, front and back, to ensure I had good earthing. I preferred this to the alternative of scraping paint back to bare metal to ensure good contact between the fittings and the body especially as I had just paid good money for a respray.

John Minchin

Hi All
Spent a couple of hours at it today and the problem eventually boiled down to a fault in the earth between the front of the car and the earth point behind the fuse box below the brake cylinder. I can't get continuity even after cleaning the terminal a couple of times so it looks like the fault is in the wire, I've run a new earth and it's OK now, it also looks as if there has been a bit of botching in the past so that probably didn't help either.
Thanks for the sage advices.
R. Algie

This thread was discussed between 20/03/2012 and 23/03/2012

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