Alternator brush wear

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.
Reply to
Paul Hovnanian P.E.
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was this a Transpo adjustable unit by any chance? I ordered a (pricey) headlight and relay harness kit for my 944 a while back and part of the deal was a Transpo regulator to kick the system voltage up a few tenths. The brushes wore to nubbins in about 6 mos. of daily use, and when I posted to the Pelican list the guy that sold me the stuff actually wrote me and was all apologetic - apparently there was a bad batch of Transpo regulators made that wore out amazingly fast, he'd accounted for all the ones he'd sold except one, and apparently that was mine. I put my old regulator back in (he did send me a new one, it's still in the glovebox) and it's been fine ever since.

just a thought,

nate

Reply to
N8N

Nope. The latest alternator is made by Valeo (sp?) and has a built-in, fixed regulator. It could be a bad regulator, adjustable or otherwise.

I'm just trying to cover all possibilities so as not to burn out another unit prematurely.

Reply to
Paul Hovnanian P.E.

There are all sorts of things that cause premature brush wear, but you didn't say how much you drove the car in the last 6 years. If you drove the car for a lot of miles or even had the engine running, that would help the brushes to wear down.

It's possible your springs were too stiff, causing undue pressure to be applied ... and this helped to wear the brushes and slip rings out. It's also possible that your slip rings were/are out of round ... or perhaps you even have worn bearings ... and this all contributed. Dirt rarely causes this situation ... but improper installation or the wrong parts can do it, too.

Some of these alternators have 32mm slip rings where others have 28. The regulator/brushes look the same but they aren't. To someone who doesn't know the difference or doesn't bother looking for the information, they wouldn't know and might have installed the wrong parts.

Reply to
Walter

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