Help with battery light

[fmxa]

Here's an odd one - when the car is running no idiot lights on and everything is normal. When I turn the car off, the battery light comes on and stays on (no key in ignition etc). I'm suspecting bad diode in the alternator, anyone seen this before and have any ideas as to the cause?

Paul

Edited January 18, 2009 by fmxa

[mr.oogieboogie]

my s4 was doing this the alternator was toasted and feeding back to the battery but it waqs the oil light that was on on mine, so maybe a relay.

[toyroom]

Well, as it happens, I can tell you that exactly the same happened to my '97 V8 last month! See the post "flat battery" to which I added the punchline yesterday. I think that what is happening with your car is that the rectifier in the alternator is half broken. In other words, half of the rectifier is no longer a pair of one way diodes, but in fact a pair of two way conductors. This means that when the ignition is off, the battery is connected to the bulb via the rectifier and hence the bulb lights when the ignition is off. When you look at the circuit diagram (exciter wire to alternato comes via the bulb), the only way that the bulb can light is if the alternator end of its wiring is shorted out and the only logical way for this to happen is if the diodes are working as conductors. I would try disconnecting the thin red wire from the alternator (disconnect the red battery terminal first!) and then reconnect the battery. If the bulb fails to light with the battery connected and the ignition off, but lights when you touch the thin red wire back to its terminal then there must be a short in the alternator. The next thing that will happen is that the bulb will blow due to overheating and you will have TDOJ (the devil's own job) of replacing the damn thing. If you do, the bulbs are NLA but the factory tell me you can use a 3W bulb if you need to. After the bulb blows, you will have no charging at all and your car will die about 80 daylight to 20 headlight hours later.

I would recommend getting the alternator replaced as soon as possible and disconnect it and the battery in the meantime and put your battery on charge. At least that way you may avoid busting the bulb and will definitely avoid being stranded.

As a matter of interest, have you noticed any decline in your car's performance since this started. Only mine is going like it's got afterburners since it was fixed.

Alternator is

[fmxa]

Just to close the loop on this - as Peter & John said, it was the alternator.