Goes up to what. If you stall the car does the light come on. You may have a regulator problem. Regards Peter

Sounds like a diode or diodes have failed and flattened the battery or the regulator has failed and left current passinf through the field windings. I assume the alternator is under warrantee. Regards Peter

Edited by Peter D on 28/07/2013 at 11:21

This takes me back. I once aquired a car through family connections that I knew the full history of, including that it would occasionally drop dead with a flat battery with no prior indication that there was anything wrong. It had been in and out of garages many times looking for that fault.

One day, it did it to me and I decided to change the alternator's electronics and brush pack[1] as a matter of course. On going to remove the old pack, I found that the spade connector from the pack to the windings came off rather easily. On closer inspection it became obvious that the terminal had been between the spade connector and its plastic sheath rather than correctly plugged into the connector, it had always been so and that it had almost certainly come out of the factory like that.

Iffy charging? Check the connections to the electronics first!

[1] All in one on that unit, held on the end with two setscrews. Very clever, very simple, very cheap, very easy to change. I've been mystified ever since as to why they're not all like that.