Thread: 10-SI alternator checks good but discharges when in car.

Tweet

Originally Posted by mr-natural

Voltage drops a bit below static when car is running - maybe down to 12.4 or thereabouts
but I don't run it very long after starting. Just long enough to see if anything has changed. How about this - has anyone had problems with an accurate diagnosis on an alternator from a parts store?

Yes and most of them suck. As stated above you have a bad alternator, probably a shorted diode. They normally show open when bad, but that doesn't mean that it can't be shorted which, IMHO, causes your ammeter to indicate a very heavy discharge.