Charging Problems

[sepehr]

My battery does not charge, as would be expected of a dead alternator:

Measuring at the battery, I get 12.4 volts when I fully charge the battery. The potential difference between the alternator positive lead and ground is the same as the battery: Starts at 12.4 and slowly drops.

The battery light never comes on, the ABS light is the first one to flare up (ABS computer shutting off due to low voltage?).

The real question is: is it actually my alternator thats dead, or is there some electronic device (relay?) between the alternator and battery +ve terminal that has kicked the bucket?

Is there something between the alternator -ve terminal and the ground cable?

Thanks in advance, any help would be apprecialted.

I sort of feel like I'm driving an electric car right now; I'm having to charge the battery before I go anywhere, and recharge before I come back .

Car is a 91 318is.

[DaveSmed]

Judging by your measurements, you have access to a multimeter. Measure the voltage between D+ and ground, and as strange as it sounds, the actual metal case of the alternator and chassis ground. Let me know what you find.

[netcsk]

It sounds to me like your alternator is not getting ground (due to your issue being the same as one I had). Check the continuity with your engine off between the casing on your alternator and a well known ground (like the big ground spot right next to the alternator). If you're not getting good ground, run a new ground from the casing to a good ground. You can put the ground anywhere on the metal casing.

Mine was exactly your problem. The light wasn't coming on, the abs light would come on sometimes, and it wasn't charging at all.