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MG MGB Technical - ignition problems

Guys I have a problem I have started my 1975 MGB for the first time following a total rebuild over the last 18 months, it was last on the Road in 1987. Anyway, put key in the ignition turned it over and everything worked really well the car started and ran all be it a little rough. During this test period the ignition started playing up. The ignition light came on whilst the car was running, I thought this was the alternator however the car was kicking out that much s**t after 20 odd years the wife told me to turn it off so I took the key out but the car kept running, What happens now is as soon as I connect the battery even with the key out the ignition light comes on and when I remove the key the car just keeps running . I have changed the ignition relay but the same happens, the battery appears to be charging but the damn ignition light won't go out has anyone got any idea what I need to do
RJ Neale

Is this a UK market car or is it a USA model car? If it is a USA model car, you are likely having trouble with the anti-run on valve. This is controlled by your oil pressure switch and will keep the engine running as long as it registers oil pressure. They were always problematic, and were known for feeding power back to the ignition switch. The factory cure was to install a diode, in the wiring, to allow power to flow in only one direction. RAY
rjm RAY

Hi thanks for the reply its a UK model so I don't think it has an anti run on valve unless you know otherwise?. I have found that tonight just to add to the confusion when I connected the battery the ignition light did not come on, it worked as it should, I turned the key and it came on then went out when the car began to run but then after a few minutes it came on and wouldn't go out and the car kept running even when I stood outside the car with the ignition key in my hand. Thinking of calling her "Christine" Rich
RJ Neale

Check to see if you have an ignition relay. This was added on later cars. Cars with an ignition relay tend to have the ignition relay stay on due to current from the alternator to the ignition light. Some relays are worse than others. The cure is to put a power diode in series from the alternator to the ignition light. I think the arrow should point away from the alternator, but I'm not sure.

You can check if this is the problem. If the car stops by removing the ignition light then this is likely the problem.
werner haussmann

After thinking about it, I think the diode points toward the alternator. Maybe someone can give a definitive answer.
werner haussmann

Werner- I thot is was bleed back through the e-brake switch as well at least in the 77-80 cars. In series, or in line? Cheers, Vic
vem myers

That my be the case. But I've not run into that.
My experience is that on cars with a ignition relay (don't know the years, but later than '74) there is enough 'bleed back' from the running alternator through the ignition light to keep the relay on with the ignition switch off. The easiest test for this condition is to remove the ignition light light bulb. If removing the bulb does not fix it it is a different problem.

On US cars this is not a problem if the run-on circuit is working properly. It will kill the engine.

Some relays don't exhibit the problem. The more sensitive the relay, the more likely the problem. Another fix is to put a power resistor or light bulb across the relay coil to load the circuit so the ignition relay will cut out.
werner haussmann

Ignition relays weren't provided until 1977 according to the schematics. How many relays do you have? If only one that is the starter relay.

UK cars never had the problem mentioned by the American contributors, as they never had the anti-runon valve, and hence never had the wiring error that allows the engine to continue to run normally when the ignition is switched off if the valve or any of the emissions plumbing is faulty or removed. What *is* correct (for American cars from 1973) is that without a working anti-runon system, the current through the alternator warning light is sufficient to keep the ignition relay operated when the key is turned off, and it is the relay that supplies power to the coil. Ordinarily it is the anti-runon valve that stops the engine, the alternator then stops outputting, and it is that which releases the ignition relay to disconnect power from the coil.

No cars had a diode fitted by the factory to cure this. American cars never had a factory cure as far as I'm aware, UK cars were wired such that the ignition warning light was connected to the relay contacts along with everything else - at least to start with. A year later several things like coil, fuel pump were moved back to be powered directly from the ignition switch as there had been problems with the relays sticking on from a relay defect!

A diode *will* stop American cars running normally when switched off if the anti-runon valve or emissions plumbing is faulty or removed, but it is a bit naff as it blocks one of the diagnostic functions of the light. Better to move the warning light white wire as above. If you *do* choose to fit a diode then it should conduct to allow current to flow from the white to the brown/yellow, but not the other way. This would normally be achieved by having the marked or +ve end of the diode pointing towards the alternator.

Likewise the comment about the brake test diode is incorrect, when that goes short-circuit it causes the *starter* to crank continuously, unless the handbrake is dropped!

None of which helps RJ Neale who presumably has a UK car.
PaulH Solihull

This thread was discussed between 03/12/2011 and 09/12/2011

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