Fan clutch, fan clutch, fan clutch! swap clutches to verify. Years ago we would test a clutch by blowing compressed air against normal rotation thereby stopping the clutch with engine running. Once engaged, the air could no longer hold it. Check temperature on dash when you can no longer hold the fan with air.

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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.