Alternators rarely fail. The brushes on the voltage regulator do. Remove 2 screws that hold the regulator to the back of the alt. and pull it out. get new regulator and compare brush length. If 50% gone, replace regulator. Easy to do with the alternator on the car.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.