my tach and oil pressure gauges stopped working (and my clock stopped keeping correct time) after I removed the console once. someone on the board told me to run a ground wire from the metal housing at the back of the gauges (he said to solder it on, but I just put it under a screw with an eye terminal) to a ground point in the dash area behind the console (I used a screw back there). It worked like a charm. disconnect the battery before you run the ground wire.

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car