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MG MGB Technical - Intermittent headlight problem

My car (1978 MGB GT) has an intermittent fault which is causing me a real headache as I rely on it for daily trnnsport.

When I turn the sidelights on everything is fine but when I try to go to headlights the indicator lights on the dashboard and the main beam indicator light up. At the same time the side lights go out and the turn indicators glow steadily. The rear lights are unaffected.

By jiggling the turn indicator stalk I can get any number of combinations of lights across the dashboard and if I twiddle the hazard flasher switch enough I can get the sidelights to turn on and off in time to the rear turn signals. I think, but I am not sure, that the petrol pump and sometimes the electric engine fan are being activated. This happens without the ignition being on.

I thought that I had an earthing problem but the earth strap on the battery and the earth strap to the headlamps both feel solid.

This has been going on for three successive evenings. The first night a bit of twiddling the fuses and flicking various switches got the headlights back on. They worked fine during the daylight hours of the next day and I thought it was one of those MGB idiosyncracies. The next evening the problem was back and this time I ended up running home on sidelights. The next morning the headlights worked fine again but I changed the switch anyway and I asked my friendly MG mechanic what he thought. He suggested a bad earth somewhere but he reckoned the switch had cured the problem. The headlights were still working fine as I parked at around 4pm but when I set off for home about 3 hours later the problem was back. This time I abandoned the car at work.

It is Saturday tomorrow and the chance of getting anyone to have a look at it are pretty remote so I need some suggestions for things to check or ways to get the headlamps to run as a temporary measure.

Chris
Chris Thompson

My bet is on dirt in the headlamp switch. I'm not sure if you can take one apart to clean it on a 78 but i did on my 73 and it cured my problem. Also you might want to look in the archives for more info. Good luck with it.
James Skinner

Does sound like bad earthing for the front lights to me. On an RB (unlike a CB) the indicators and sidelights share the wired ground with the headlights and the cooling fan. With just sidelights on enough current can flow to light the lamps but they will be dimmer than they should. With headlights the current tries to find anyway it can to get back to ground, which will be backwards through the indicators to the repeaters on the dash, and similarly through the main beams to the tell-tale on the dash. There is only a single wire at the body tag for all these, so although that connection may be sound there could still be a problem at either or both of the 4-way connector behind the grill.

PaulH.
Paul Hunt

Thanks for the help. I think I've got it beaten with changing the switch, the four way connector and a dodgy in line fuse holder.

Chris

Chris Thompson

This thread was discussed between 30/11/2001 and 03/12/2001

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.