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Ignition off, charge light comes on.


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It was a lovely sunny day yesterday so figured I'd take the old girl out for a thrash. All was well, until the alternator belt started to slip. I figured this was just general "oops the bolts come undone" syndrome and made a mental note to tighten it when I got to my mates.

Upon arrival at my friends house it became apparent there was more of an issue. Turning the ignition key off causes the charge light to come on. Turn the key on and the charge light goes out.

I put a multi-meter on the output from the alternator and this seems to still be charging although getting it above tickover and reading the display is a little difficult.

My opinion is a fubared alternator, opinions anyone?

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Sounds like your diagnosis of the alternator's control pack going wrong is correct. The only other likely alternative is that the warning light has a short somewhere, with it contacting the permanent live feed to the ignition switch, but that wouldn't explain why the light goes out when the switch is on and the engine not running - the light should still be on if the light was getting a feed from the permanent live.

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There is probably a back feed from a faulty regulator in the alternator, the circuit will be completed to earth via all the items that are fed by the ignition, most likely the coil.

Does pressing the brake pedal with the ignition off alter the brightness of the ign. light?

Does disconnectinmg the coil (LT) cause the light to go out?

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  • 3 weeks later...

A late post I know, this fault sounds like a shorted regulator diode.

If its a lucas alty or similar then there are 6 large rectifier diodes on a square heatsink within the rear casing, above this part of the same assy there are 3 small diodes usually marked 1n4004, one or more of these could be shorted, you can get them from maplin and resolder them in.

If you can get to the alty warning light with a meter then measure the voltage to it, if the polarity is back to front when the lights on and the ign off, ie what should be the ground is + then this backs up the above diagnosis.

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