Did you verify the fuse is not simply corroded? My 1981 300CD clock kept perfect time up until I sold it. Check all the fuses before doing anything else!!

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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.