In my case, I finally determined that the brush holder was grounding out. I know, it didn’t make sense to me either? The only thing I could figure was that there was enough carbon dust from brush wear on the brush holder to actually create a path to ground. I removed the brush holder, and with a little compressed air and electrical cleaner, I got the alternator working like new.
Here’s a couple of other things to check that can create problems with those brushes.....
1) The little clock springs that push down on the brushes have to loop over the brush holder and push down on the brushes. They can become elongated and push on the side of the brush holder “instead” of the brushes. A pair of needle nose pliers will squeeze them back into shape.
2) The brushes should be installed in the right direction. The braided wires coming off the brushes can actually hang up on the brush holder if they’re in the wrong position and prevent the brushes from contacting the slip rings. The wires should be facing each other when the brushes are installed correctly.
If you’re chasing alternator gremlins, I recommend that you pull the alternator stator and thoroughly check those brushes. (You just can’t see that inside brush with the stator in place).
There’s 6 wires and 3 bolts to remove, and the stator pry’s out of a boss. To put it back, tighten the 3 mounting bolts only finger tight, then tap the stator back in the boss with a screwdriver handle. This makes it easy to get in straight and not tear anything up.
