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MG MGB Technical - Blinking headlamps

I've recently gone into co-ownership of my Mother-In-Law's '73 MGB and have thrown myself into the job in hand. Two queries (so far) have emerged:

The headlamps appear to "throb", ie vary in brightness when switched on. This does not seem to correspond to revving the throttle, as it happens on idle. Faulty coil? Earthing? Any ideas?

Also, could anyone please explain the function of the switch between the speedo and the rev-counter. It is marked "P" with an clockwise arrow. I've tried everything and am lost, but I'm pretty sure it's not for the ejector seat.

Thanks.
B J Donaldson

Dear BJ
To answer your first question I would like to know a bit more about the fault you have. For instance does it happen on both high and low beam? Does it happen when you have the engine running? Does it happen only when you apply the brakes? Does it only happen when you have another electrical item turned on?
Given the basic details of the problem I would suggest you start by looking at the connections to the lights, including the light switch. Ensure the earth, along with the power side have good connections.
The switch you refer to is the panel light rheostat. They are not very good and as a consequence most have been disconnected by a previous owner.
Cecil Kimber

Cecil,
I noticed the fault when returning the MG to the garage after giving it a good clean (my insurance doesn't kick in until the 1st of May, so up and down the drive is as far as I can go for now!).
I'm pretty sure it happened on both dipped and high beams; the engine was running; only the handbrake was applied; nothing else electrical was switched on.

I will check connections forthwith.

Regarding the rheostat, if it has been disconnected, would that explain a lack of instrumentation lighting? If so, how can I get the lights to work?

Thanks for the prompt reply.
B J Donaldson

I wouldn't worry about the headlights, mine do this and I've seen it brought up by others on this board as well. It's all down to antique Lucas alternators. Usually, they run at a continous brightness once you've got some revs on, but pulse slightly at idle.
On the panel light rheostat, if it doesn't seem to do anything and you have no panel lights, then it's probably not disconnected, just U/S. They're not the most reliable piece of kit in the world and are one of the first places to look when all your panel lights disappear at once. Try bypassing it with a piece of wire and see if you get some lighting then.
T J C Cuthill

Check the voltage on a brown at the fusebox and see if it is going up and down in time with the lights. If so it is the alternator (although neither of mine have every done it) if not a bad connection in the headlamp wiring.

My 73 rheostat has two wires and four spades, so it is a simple mattery to move one of the wires to the 'spare' spade on the other connection so bypassing the rheostat - size of hands permitting. The rheostat in every used car I have ever bought has been faulty. Even with a good rheostat the lights were never bright, and particularly after 30 years or so of accumulated dust inside the cases and on the glass. But I doubt anyone would go to the bother of disconnecting the rheostat and disabling the lights altogether just because they were dim. But maybe if the PO was dim ...
Paul Hunt

I had the same problem, and yes it is usually an alternator problem. You could also be seeing this if your battery is dying, but it would have to be so far gone that you'd have problems starting the car to begin with. The Lucas alternator is a pretty anemic unit, and in my case at least wouldn't show even power output at idle unless I set the idle way too high - over 1000rpm. I've since made the switch to a Bosch alternator and the issue has disappeared. And happily, with the more powerful alternator I can also hook up more accessories (eg my nice Blaupunkt stereo) without worrying about insufficient power. The conversion is very easy, but of course isn't an option if you're concerned with originality.
Adam Birnbaum

Thank you everyone for your suggestions.

Adam; originality is not a priority, but legality and practicality most definately are! Maybe I'll look around for a Bosch alternative. Actually, the car hadn't been started since last August and the battery was (predictably) rather flat. A good over-night charge sorted it out, but would this explain the pulsating headlamps?

Paul; I will attempt a by-pass operation on the stat. I am an avid "fiddler" when it comes to car electrics and am sure I will eventually find a solution.

Just a small point - the guy that produced the wonderfully helpful electrical problem page attached to this site: he's not THE Rick Astley, is he?
B J Donaldson

My non-pulsating headlamps have been both with and without flat batteries.
Paul Hunt

I have a new post concerning headlight relays today. The headlight switch is the culprit(on my 73B). Since full amperage to the headlamps goes through the switch the contacs arc hard every time they are switched on or off. Full amperage also runs through the dip beam switch on the steering column. Eventually these contacts burn enough from arcing so that full voltage/ amperage sometimes gets through and sometimes it doesn't. Easy way to check will be a switch that may be hotter than hell to the touch with headlights on. With no fusing at all in that circuit, be careful with this problem. Switch, car and all could be in flames before long (though likely you'll just burn the switch clear up). Regardless, I think retrofit relays is the way to go. At least the switch will live longer, and the headlights will get all the voltage and amperage they deserve. I just would like to know where the best source is to tap into the brown circuit. But that's on a new post.

Mark Thomas

This thread was discussed between 24/04/2003 and 27/04/2003

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