Only 9v coming from alternator

Mon Jul 04, 2011 10:58 pm

I have a 91 325i sport and have a charging problem with it.
Yesterday i drove the car 60 miles, turned it off and it was hard restarted. did a few small trips in it and came back turned it off and went to start it again. wouldnt start. Now the alternator has just been refurbed and everything replaced in it. (80 miles on it) I am getting 9v from it.

could it possibly be a bad earth and where would be the best place to start looking or is there any way of diagnosing it?

Any help greatly appreciated.

Mon Jul 04, 2011 11:04 pm

Sand off a small area of the alternators alloy case, and connect a voltmeter between this and the earth terminal on the RH suspension turret, with the engine running.
Any voltage recorded is a sign of an earthing problem.

Mon Jul 04, 2011 11:20 pm

^^Ahhh interesting.

So does this mean there is a difference in potential between the alternator body and the earth terminal you have described?I thought this only applied to ac circuits ?

Tue Jul 05, 2011 12:36 pm

Not only AC! What Brian describes is to see if the alternator is really grounded. An alternative would be for him to suggest all the earth wires are removed, cleaned and re attached, but that assumes the OP knows where they all are, easier to check voltage between a known earth and the alternator case, any thing shown will require the earthing looking at, if there is no voltage then we look at something else.