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MG MGB Technical - Dash ignition light?!?!? huh?

Trying to get my head wrapped around the function of the red dash ignition light.
Please straighten me out on this someone.

First allow me to qualify;
stock 73 US roadster, It isn't green so it can't be that. I've read through the hundreds of articles in the archives and now I am knowledgable but confused about the application of that knowledge..,(Really! It took me a couple of hours)

I have never witnessed this light come on at all until I grounded the case, It then shone brightly with the key in the run position. This indicates that the white wire to the ignition is OK. Correct? Maybe?

The bulb is new, with the key in the on position, the light should shine if the yellow/brown (Brown/yellow?) wire at the alternator is attached and good. right?

If the yellow/brown is unattached there is no difference in potential to the bulb so no light right?

If the yellow/brown is grounded other than to the alternator, the bulb should shine brightly. correct? If the yellow brown IS grounded, and the bulb does not shine, I've got a bad wire or connector?

If the alternator is working, ie making juice, it feeds it back via yellow brown and lowers the difference in potential at the bulb, thus dimming and eventually extinguishing the bulb at the appropriate juice making RPM? Right?

Sooo, what happens when the alternator goes south? Does the bulb just fail to light?
Stay on constantly? Or does it depend on the situation?
T. D. Howard

Yes :o)

Basically your description of when the light glows and does not glow is correct. But if the brown/yellow (or the white for that matter) is disconnected it doesn't light primarily because there is no path for current to flow.

When you first turn on the ignition it glows because it has 12v from the white ignition wire and 'sees' a ground on the brown/yellow either via the alternator electronics.

When the engine is running and the alternator is charging the light goes out because it has 12v (or more correctly about 14.5v) both sides i.e. both from the white and the alternator.

When you switch off the ignition but the engine is still spinning the warning light again glows, and flickers and dies as the engine and hence the alt comes to a halt. This is because the alternator is still producing its 14.5 (and falling) volts but now the bulb can 'see' a ground via the other components connected to the white. This last bit may not happen if you have done the POs kludge of putting a diode in series with the warning light to make the engine switch off when turning off the ignition rather than fixing the real problem.

If the alternator has failed you may not see the bulb glow when you turn on the ignition. Or it will do but stay on even when the engine is started and revved. Or it will dim when started and revved but glow dimly again as more electrical load is put on the alt i.e. headlights. And I'm sure there can be other fault conditions as well.

If yours doesn't light when everything is connected up, but does so when you ground the case of the bulb-holder, then yes the white must be OK so either the brown/yellow itself is open-circuit or whatever it is connected to in the alternator is.
Paul Hunt

When my alternator packed in, the light came on as normal with key turned but engine off. Once the motor was started, the light didn't go out but kept glowing, dimly at first, then brighter, with the engine running. This is because the battery is providing potential to one side, and the alt isn't keeping it's end up, if you see what I mean.

Paul Hunt has a bit on the function of the light on http://mgb-stuff.org.uk/ click on "spanners" and follow the links. I think he calls it the "idiot light".

Mike
Mike Howlett

"Idiot light" is an expression I saw being used by our American cousins. There seems to be some difference of opinion as to whether it means "even idiots who can't read a gauge should be able to understand a warning light" or "only an idiot ignores the light".
Paul Hunt

Paul, It's the first explanation.
Ray

T.D. It sounds like your wiring is OK. Is the alternator charging and keeping the battery up? Is it a Lucas alternator?
Dana Wilson

Since the original question I have removed the alternator, and taken to a local auto parts to have it tested. DEAD. Not just a small problem but very dead. One of the capacitors had a hole rusted in it, The regulator pack's wires were all crisp and brittle, and the rectifier was shorted throughout. YES it is a LUCAS alternator. Plus the bearings are noisy and the brushes were shot. Quickly purchased a rebuilt one on e-bay and expect delivery soon. It's been 24 months since I acquired the car and I'm chomping at the bit to get her on the road at last!
T. D. Howard

This thread was discussed between 15/04/2004 and 16/04/2004

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