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I have a new Bosch CS-130 style alternator with a couple hundred miles on it.

Since I installed it, it's been a little noisy, but I attributed that to the brushes breaking in to mate with the slip rings.

Unfortunately, it got worse recently and has turned to a squeak. Since I don't drive the car much, the new alternator is already out of warranty (1 year) even though I've hardly used it.


Upon close inspection, I found that the alternator squeaks during one specific part in its rotation. The squeak is coming from the brushes, not a bearing.

I disassembled the alternator and found that that the front slip ring has a strange color on one spot. It looks like excess epoxy was dripped onto the ring during assembly, and this caused one of the brushes to wear it unevenly.

The bad section looks like this:

Bad slip ring wear

The rest of the slip ring is worn evenly:

Good slip ring wear


How do I repair the slip ring surface so the brush can break in properly? Can I just turn it on a lathe? What about the surface finish?

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up vote 4 down vote accepted

I found a repair/rebuild guide for the CS130 alternator. Page 5 contains the following:

SLIP RING SERVICING It the slip rings are dirty, they may be cleaned and finished with 400 grain or finer polishing cloth. Spin the rotor and hold the polishing cloth against the slip rings until they are clean.

CAUTION: The rotor must be rotated in order that the slip rings are cleaned evenly. Cleaning the slip rings by hand with-out spinning the rotor may result in flat spots on the slip rings, causing brush noise. Slip rings that are rough or out of round should be trued in a lathe to .002 inch maximum indicator reading. Remove only enough material to make the rings smooth and round. Finish with 400 grain or finer polishing cloth and blow away all dust.

So it looks like reworking the slip rings consists of chucking the rotor in a lathe and turning the slip rings down just enough until they are round within 0.002", then finish to 400 grain or finer.

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I'm guessing you're like me and didn't keep the receipt, otherwise I'd say to return it as a defective alternator! – jp2code Sep 26 '12 at 13:56
@jp2code Unfortunately, I bought it over a year ago so the retailer return period has ended along with the manufacturer's warranty. – Steven T. Snyder Sep 26 '12 at 15:26

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