Look on the back of the alternator and you will see a kinda round device about the diameter of a quarter with two screws holding it's mounting plate on. You need to replace that part. It is the brushes. I think they run around $11 or so. When you pull it off you should be able to read the p/n off of the back. Call that number into a local Bosch distributor and they can get a replacement for you. Easy diy'er type of job.

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Jim