We have had two alternator failures at almost exactly 205,000 miles - Our '88 745GLE needed brushes, our '90 744 Turbo had a squealing bearing.

A genuine "mechanic" in Michigan sanded down some larger brushes to fix the '88 when my daughter broke down. Scarey story - When the battery went dead at the highway rest stop, my daughter could not use her cell phone because it was dead and she had counted on plugging it into the car! She was 1500 miles from home - AAA was terrific.

I put a used alternator in the '90, the alternator had 110,000 miles on it and the parts supplier guaranteed it for one year. They told me that they had tons of good, used Volvo alternators. I think that it will last to 200,000.

We still drive both cars.

If you go used, get a 100 amp unit from a turbo, in my case it was the same price as an 80 amp unit. ($30 US around here, $60 for a dirty rebuild (new parts, no cleaning of reused parts), $220 for a new one.
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3 8s & 2 7s 725,000 miles total