Battery Charge Light

[GizmoMX3]

I recently purchased my 4th mx3. It's been almost 6 years since my last one. The previous owner installed HID headlights. They make a huge difference at night. However, when my lights are on at idle and I press the brakes the battery charge light will come on. If I'm at idle with lights on and use a turn signal the battery charge light will flash with the signal. If I increase the rpm's the light will go away. Any suggestions on the cause? I'm thinking either a weak alternator or battery, but I'm terrible at troubleshooting electrical. Any input would be appreciated.

Nate

[Nd4SpdSe]

The alternator is getting weak and with the HID's there's just enough of a draw that it triggers the charge light. Bringing up the revs spins it fast enough to compensate, but it's a sign of it being on the verge of failure, but it could do that for weeks or months. With my Mx-3 the charge light would show up only under 3000rpms, so being stick I just keps the revs up, drove like that for the week, by the time the weekend came, she had "fixed herself" after the long hour and a had drive to my parents place, and months later it did it again and fixed itself again. The 3rd time I had gotten that burning electronics smell and she was toast and had to be replaced, already had the spare alternator at my parents place waiting.

Anyway, if you throw on a multimeter on the posts of the battery, it'll give you the voltage it's putting out. Typically healthy charge voltage is 13.5-14.5. You gotta be around 12v or under, but I'm not sure at what voltage is had to be for it to trigger the Charge light, but with your info it would be good to know by reading the voltage with your lights on and with your lights off. I would suspect you gotta be 11 if not 10v when the light comes on.

[RobMinhas]

+1, it's probably your alternator. Not sure how it is where you are but in Canada you can get an alternator bench tested for free. If you have the B6 4 banger and you need to change the alternator I highly recommend swapping in a GS one as it's rated at 90AMPs versus the B6's 60AMPs.

[Daninski]

Is the under drive pulley stock? If not that could be the problem.

[GizmoMX3]

The under drive pulley is stock as far as I know. One thing I forgot to mention is that the previous owner said he never had a problem with the charge light until he had the battery replaced. I'll put a multimeter on it this weekend and post the readings.