I have been working hard to find the source of some electrical problems.

This morning, after disconnecting all the fuses and links in the main box near the battery (except the main 120/140 amp link), and after completely disconnecting the BCU, and removing all of the fuses in the fuse box under the dash, I found that I had a very small -- 0.15 mA -- leak through the alternator. When I disconnected the alternator's wire from the output terminal, the leak went away. When I reconnected the wire, the leak returned.

Is this normal?

The dealer just installed a "new" re-manufactured alternator. The car has been driven 2.5 miles since then.

A leak of 0.15 milliamps (less than one milliamp!) is probably not a factor in draining my battery...but I can't understand why there would be any leak at all, and I wonder if it is a symptom of a problem that might be worse when the car is running.

Please let me have your thoughts.

Thanks.

 

The alternator is essentially a permanent magnet combined with a coil. The spinning magnet makes a variable magnetic flux, which in turn induces a current in the wire. Now, I think they have a diode, to prevent current from going backwards from the battery to the alternator when the car is off.

Theoretically, a diode allows zero reverse current, but in practice, it's a small number, less than 1mA. My guess would be that 0.15mA is just current going back through the diode.

 

the diode is a rectifier... it turns ac in from the alternator to dc..

 

Unless you have a REALLY expensive DMM, you can't even measure current that low accurately.