AC power at battery? - Ford Forums

rob567 · 02-25-2009, 12:23

In my ongoing saga of the car not starting I have determined that it is not the starter after all so I will start a new thread.

Turns out that I have something that is causing key off battery drain. The car is stock so there are no after market modifications.

When I checked the battery again with the car running I accidentally put the multimeter on AC and I noticed that I am getting 25V AC current at the battery terminals. I thought that a car was strictly a DC electrical system.

Would this indicate that the alternator is gone and the diodes are allowing electricity to travel in both directions?

70mstang · 02-27-2009, 19:17

could be the alternator if it's a later model mustang, i'd suspect the regulator, but i'm no electrical expert by any means

1meanGT · 03-08-2009, 20:40

Quote:

Originally Posted by rob567

In my ongoing saga of the car not starting I have determined that it is not the starter after all so I will start a new thread.

Turns out that I have something that is causing key off battery drain. The car is stock so there are no after market modifications.

When I checked the battery again with the car running I accidentally put the multimeter on AC and I noticed that I am getting 25V AC current at the battery terminals. I thought that a car was strictly a DC electrical system.

Would this indicate that the alternator is gone and the diodes are allowing electricity to travel in both directions?

I would agree with your thoughts of diode failure. Some alternators can have this part swapped out if you are on a budget. I had an alternator that the diodes failed on and they blew the fuse in my radio, the headlights went really bright till they blew, the dash lights looked like from the movie Chistine, and within 5 minutes, blew the battery through the hood of the car. The car still kept running for 6 more miles all the way home. It was not a pretty mess to clean up.