Battery light on with no key, off with key!?

This morning my wife's Disco (92 200Tdi) battery was flat as it could be. I dumped by Defender battery in to get her to work, all seemed ok. When she got to work and turned the ignition off, the battery warning light came on (with the key removed!), she left it like that all day, started the engine first time and drove home. Now I have a Disco with the warning light on all the time the ignition is off, and when I turn the key to the point where the dash lights go on the battery light goes out! the light stays out when the engine is running and the battery seems to charge (or at least its not flat yet and seems to have plenty of power). REALLY lost on this one, is it a new alternator? Any clues? Expecting an early start swapping batteries tomorrow. ?

It is a fault in the regulator, the diodes (or one of them at least) that are supposed to prevent the current flowing from the battery connection, to the rotor, have gone short circuit, so your rotor is always live, rather than just being live when the ignition is on. This means that the charge light will be getting a backfeed from the rotor. When the ignition is off it is earthing itself through the rest of the ignition circuit. When you switch the ignition on, the battery light has live on both sides of it, so it goes out. It will still charge ok, but as you have noticed, it will flatten the battery if left.

On or around 19 Jul 2005 11:32:26 -0700, "VeryYellow" enlightened us thusly:

alternator fault. If you have a local branch of lucas service or similar electrical specialists, it'd be worth taking it in - might just need a regulator or a diode pack or something.

failing that, exchange alternator I'm afraid, or a second-hand one.