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MG MGB Technical - Ignition light problem

Hi,
I'm new to this site...been on the MGOC UK for some time. Someone on there suggested that I post this thread here in the hope that someone might have an answer

I went to work in it the last sunny day we had and suddenly it started missing and stopped. After fiddling for a while I called the recovery folk. Nice guy arrived, said what a pleasure it was to work on a car that didn't have to be put on a computer. He was very thorough, and eventually it purred into life. He checked that the battery was charging and that the alternator was working, all fine.
I haven't used it since, until this morning I turned the key, ignition light came on, all good so far, then started it, purrrfect. Then noticed the fuel gauge was not working and yes there is fuel in it.
I went for a short run down the village, running fine, got back home and ignition light is on. I turned the ignition off, then turned it on again and the light dimmed as I watched, the car started and when I revved the engine it came on fully. Tried this several times, same result. Now very baffled as the guy checked that the battery was charging and that the alternator was working.
I keep the battery plugged into a battery conditioner via the cig. lighter, so it was fully charged when I used it this morning.
Sorry, seem to have waffled on, but just wanted to give as much detail as possible


Jeff Dodgson

What year? As it is a rubber bumper I suspect it is a 77 or later with two round relays by the fusebox. In which case it is probably a faulty ignition relay, stuck in the operated position, even though it clicks when the ignition is turned on and off. Your symptoms are very very similar to those on a friends car just this week, where the relay had failed as I describe. The ignition relay was the one nearer the radiator on his, with the brown/white and white wires, the other relay is the starter relay which has a thin white/red and a thick white/red or possibly white/brown.
Paul Hunt 2010

Thanks for that Paul.
Yes it is a '77 and does have the two round relays as you describe. They are actually fairly new, but from experience that means nothing with old cars.
I do also wonder if it could still be an alternator problem even thought the recovery guy thought that it was ok.

Jeff Dodgson

Hi Jeff, Welcome to the forum. There are some basic checks that are worth doing. Fan belt slipping, the connector into the back of the alternator, this can have a bad contact due to being loose, or verdigris on the contacts or even in the wiring.
Try turning the headlamps on and revving the engine, even with a good battery they should glow brighter, if they don't there is a charging problem.
If there is an Auto electrician near you. you can remove the alternator and take it in for bench testing, generally they don't charge.
c cummins

Alternator check is easy, just connect a voltmeter between the brown at the fusebox and earth. Ignition off you should see about 12.8v with a good battery (more reducing to that if the engine has only just been switched off and the alternator is charging. Ignition on it should show about 12.5v or so. Cranking about 10v, although if there are bad connections in the battery circuit this could be lower. With the engine started, revved to 1000 rpm or higher then idling you should see 14v or so. As more electrical stuff is turned on (headlights, brake lights, reversing lights etc.) this will probably drop but should still be more than 12.8v, and when the engine is revved should go higher. The voltage shouldn't go above 14.7v at any time.

I may have misunderstood something you wrote, I took 'got back home and ignition light is on' to mean when you had turned the ignition off. If it is glowing dimly with the engine running, and brightens when the engine is revved, and you still don't have fuel, temp, tach, brake lights etc. then it could still be the ignition relay but failing to operate rather than stuck operated (may still click).

If only the fuel gauge is not working, and the other things are, then that is another fault. In which case you need to look more closely at the warning light. It acts like a pair of balance scales between the alternator and the battery. When one is higher or lower than the other the light will glow. For example when the ignition is off and the engine stopped there is zero volts both sides and the light is out. With the ignition on but the engine stopped there is 12v from the battery but nothing from the alternator so it glows. Charging normally there is 13v-14v from both so it is extinguised. And when you turn off the ignition of a running engine you disconnect the battery supply, but the alternator is still spinning and generating voltage, so the light glows again then flickers and dies as the engine slows and stops. Measure the voltages on the brown/yellow at the alternator and the white/brown at the ignition relay and compare them for each of ignition off, ignition on, and engine running at 1000rpm at least. Rather than list all the possible variations and causes here tell us what you have, as it could be other things.

Paul Hunt 2010

Thanks again Paul,
I don't have much time this week, but I have a free weekend so will tinker then and let you know what I find....
Jeff Dodgson

You hear a lot about new parts in old cars failing after a short period of time. Usually poor modern quality and cheap parts are to blame. But maybe it's the old car sucking the newness out of the replacement parts. All that old and cruddiness drains their life force. Kind of like Ingrid Pitt in 'Countess Dracula'.

http://en.wikipedia.org/wiki/Countess_Dracula

Simon
Simon Jansen

Ignition light glowing at idle speed and getting brighter with increasing RPM is a faulty alternator rectifier (diode) allowing AC current to pass.

Hart
Hartley Mayers

OK, after spending an hour this morning it looks as if the problems might be solved.
I have cleaned all the contacts at the alternator and in the fuse box.
The end result is that the ignition light comes on nice and bright when key is turned, then goes out when the engine is started and the accelerator pressed, also the fuel and temperature gauges now work..time will if it is permanently solved.......looking forward to some good weather now !!!
Thank you for all the folk who have contributed, still not sure what Simon from NZ is on !!!
Jeff Dodgson

This thread was discussed between 27/03/2010 and 02/04/2010

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