Battery light stays on when engine is off

[Saw Doc Fan]

Really weird problem just started, I came out this morning to start car and the battery light was lit-up on the dash, the car obviously wouldn't start, so jump leads out and took it for a run.
Got back home, turned off engine, locked doors, checked to see if battery light was lit - it wasn't.
Went out an hour later, and low and behold the battery light was lit on the dash again.
Started car up and let it tick-over for 5 mins, turned off and so far it is ok.
I need my car to get to work very early in the mornings, so could do with some advice as to what the problem is.
Thanks

[philhoward]

I'd put money on the alternator...although it could be something back-feeding voltage into the igntion circuit. I'd start with the alternator first.

[AndersDK]

I'm with Phil.

If a power/charge diode in the alternator has shortcircuited, then the battery voltage feeds backwards to the charge control module (on the alternator) and makes the charge lamp output a live +12V - instead of the usual chassis level when alternator is at rest (not charging).

This +12V is then fed to the charge lamp one side.
As the ignition switch is set to off - there is no +12V fed to the other side of the charge lamp. Instead all the consumers connected to the ignition switched +12V line makes a low ohmic conduction path to chassis on the other side of the charge lamp. Then you suddenly have +12V over the charge lamp - and it is lit - with key removed.

The battery will also discharge for the same reason. You may leave the charger on until next morning to ensure you can get to work.

Easy option : have your trusted garage replace the alternator while you are on work.
Diy option : get a breakers alternator and replace your alternator asap.

[Peter.N.]

Yes, me too. First port of call - alternator. You could try disconnecting the thin wire to the alternator and see if the light goes out - that will prove it.

[Mandrake]

Lets not forget the simple things first - faulty ignition switch contacts. (Unfortunately very common on Xantia's)

On a Xantia the battery charge light will come on in the ACC position - this is normal, so it could be something as simple as the ACC contacts being (intermitantly) closed even when the key is completely off.

The ACC position left overnight may be enough to run the battery down, especially if the radio is on.

I would have thought that if the alternator diode was shorted, the problem would be there all the time, a diode is either faulty or it isnt, it wont be intermitant.

When you come back to the car and see the light on the dash, what happens if you turn the key on and off a few times, does the light go back out again in the off position without starting the engine ? If so my bet is the ignition switch contacts are on the way out...

Edit: another possibility - do you have an after market turbo timer fitted ? I just remembered that on Mum's 1.9TD with a turbo timer fitted, that when the key is turned off and the turbo timer is active, the battery light is lit on the dash. (I havn't checked to see exactly where/how it is wired in to cause this...) A fault in a turbo timer could cause your symptoms too...

Regards,
Simon

[Saw Doc Fan]

I say it's only just started, but, it's been happening a couple of months now, (when I think back)...
Not all the time though, just now and again.
We've checked the alternator with volt meter and it reads 14, that's correct?
It's charging the battery fine, it's just when the light on the dash comes on it tends to drain battery.
I never leave radio in car, always turn heater fans off etc. before I switch ignition off.
When it does happen and I turn the key on and off a couple times it then goes off, but like I said, it sometimes comes back on even after a couple of hours been stood.
I reckon my alternator is ok, and could be faulty wiring in ignition, bad earth or something.
Would this be expensive to sort out?

[Saw Doc Fan]

Forgot to say, Mandrake, I don't have an after market turbo timer fitted, it's the original one.

[SteveG]

This sounds like it is almost certainly the charge diode in the alternator failing. On most alternators this is part of the diode/brushes pack and can be replaced in a few minutes ... but I haven't looked at a Xantia unit so it's bound to be different

Just because the alternator pumps out 14V doesn't mean it's fully working. A dead (or dying) diode won't prevent it from charging the battery but will provide a leakage path to discharge the battery when the engine is off.

HTH

[Saw Doc Fan]

Hopefully (fingers crossed) the problem is now solved, put a new alternator on yesterday. If problem occurs again will then have to replace ignition barrel, bu so far so good.