maksik7

I am pretty sure that my alternator is draining my battery (Dies overnight). However before I replace the alternator can anybody tell me how to test if it's bad. My suspicion is that its drawing curent even when the key is in the off position.

BTW I can't start my car, my starter is bad right now.

idrag2

that all depends on what kind of tool you have, if you have no tools just unhook it at night and see if the drain goes away. Or you could do a voltage drop with dvom

Vader

Disconnect the battery negative.

Disconnect the alternator output lead.

Meter the resistance of the output stud to the alternator case (ground) in both directions.

The initial resistance reading in each direction may be relatively low as the capacitor in teh regulator charges, but after about 5-10 seconds, the resistance should be greater than 20 megohms in either direction. If you get a resistance of 1.5 k ohms or less consistently, you may have a failing diode in the rectifier bridge.

91FirebirdFmla

Strangley enough, the first hint that my alternator had gone bad was a funny smell in the car which I then traced back to the alternator.