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MG Midget and Sprite Technical - Alternator light on when ignition off

Just been out for a drive and as I was putting the car into the garage the alternator light flickered. Nothing unusual with that except with the ignition OFF and key removed the light stayed ON.

What's up? Have I got a short circuit or has the alternator fried in a way that I've never known one fry before? The power is off now on the circuit breaker and it's too late to run the engine to test the output?

Any ideas other than buying a new alternator?
Daniel Thirteen-Twelve

diode in alternator U/S

It will still charge but you will get a flat battery over night

Needs a new one really Daniel but a temporary "fix" would be to unplug the alternator overnight, plug back in next time you use the car

That diode usually stops leakage back through the alt

I rather think it's the zener inside the pack
bill sdgpm

This is a very common fault on an alternator and means it is poorly

It can be repaired easily if the alternator has a removable brush/reg pack as some do, otherwise a recon alternator is required.
Bob Turbo Midget England


Does you car have a battery isolator switch? That would be a temporary solution already.

Is the alternator charging the battery OK, e.g. with headlights on?

Anthony Cutler

Is the engine still running :-)
Brad 1380

It's not a Lucas unit but probably can be repaired for a lot less than the price of a new one. However, before I pulled the alternator tonight I re-applied power via the circuit breaker and voila it's back to normal though I didn't start the engine. I'm guessing a fault that goes away by itself comes back by itself.

Any more views on what exact piece of electrickery is on its last legs?
Daniel Thirteen-Twelve

the lamp runs between the ign and the alternator field coil (bear with me)

the alternator field coil is a electromagnet (the wires on the spinning bit) which needs to be powered before the alternator can generate any power.

the power is fed through the lamp, so when the ign is on the lamp will light

when the alternator is running it provides its own power, in this case it will switch the lamp off by raising the alternator side to 12v

the lamp will light if
1: the alternator is drawing current because the ign is on and the alternator is not spinning (start condition)
2: the alternator is generating power but the ignition is off (stopping condition)

the lamp will not light if:
1: the ignition is off and the alternator is not spinning
2: the ignition is on and the alternator is spinning

since the lamp is on and the engine is not then
1: the ignition switch is faulty and feeding power when off
2: the diode pack in the alternator is faulty and is feeding battery voltage to the lamp as if it had been generated

to test whch , wait until the lamp is on, then pop open the bonnet and put a volt meter on the brown wire with yelow stripe. if the voltage is high (12v ish) then the diode pack is bust, if the volts are low (6v ish) then the ignition switch is duff.
a better test is the other side of the bulb, same test and diag, but 12v or 0v will be seen (bulb is harder to reach!)

the diodes are cheap (1$ a piece) and most voltmeters have a diode test option (triangle and line), this should beep and/or show 0.6 when one way and not beep/and or show "1 " or other out of range reading the same as not connecting the wires.

a reading of 0 is a good sign of a fried diode, a ading of 1.2v is a sign that you are testing two diodes (perhaps a diffrent route than you expected to test ;)

i havn't opened a midget alternator, but alternators are fairly simple.

Diodes are usually black with a stripe on one end, make sure you replace them the right way around!
if the stripe has worn away then the stripe should go to the +ve end (brown wire or brown/yellow wire)
Will Munns

This thread was discussed between 06/07/2010 and 08/07/2010

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