To verify a properly tightened alternator belt, try to rotate the alt pulley by hand against its normal rotation. If it rotates at all, the belt is loose. And more than likely the belt is now glazed and wont grip well because its been loose.

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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.