Any ideas about what might cause excessive brush and slip ring erosion in an alternator?
I put a new unit in my '79 Porsche 928 about 6 years ago. A few months ago, it began to exhibit intermittent operation. I took it apart and opened it up. The brushes were worn down to 3 or 4 mm i height and the slip rings were badly grooved. The main and field source diodes all tested OK as did all the windings. I happened to have the brushes from the original alternator, so I replaced them and polished up the slip ring surfaces with steel wool. It ran well upon installation. For about three months.
The alternator just cut out again, so I took it out and now the slip rings have been worn through. There is no telling how little of the slip ring there was left when I did the brush replacement, so they may not have had long to go. There is still significant brush material left. The rest of the alternator (diodes, windings, etc.) still test OK.
Any ideas about what might cause this kind of brush/slip ring wear?
One thing I considered was that the alternator is mounted quite low on a
928, practically at the lowest part of the engine. Its possible that some dirt may have worked its way into the cooling air duct and caused this, but things didn't look too dirty when I opened it up.