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Q: alternator issue on 2002 BMW M3

I replaced alt 5 weeks ago. About 5 days ago the battery light came on. I noticed that if I rev car up to 4-5k rpm the light goes out. I replaced alt again and light went out, but after 10 miles or so the light came back on and car died. I was able to get it home and I put back in alternator #2 and I am able to make battery light go out as long as the engine is run up to 3k rpm or higher. Is there a relay or fuse etc that could be causing this. I have not made any modification or anything to the car. Thank you I appreciate the help.
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Where are you getting the Alternators? Are they from Bosch? Are they really good quality? Make sure that the big RED wire from your Alternator to your Battery is in good shape. Check the Voltage at the back of the Alternator and then at your Battery, they should be the same.
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