This is for my nieghbor's '80 300SD. His tach isn't working and we already tried the cigarette butt trick, but with no success. We also tried putting in a "new" tach from an instrument cluster that he bought to fix another problem. Anyways, his tach still wouldn't work. Does anyone know how the rpm sensor works and has anyone changed one before?

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member