On my Range Rovers, tach is run by signal from the alternator. Not sure on the Benz. A bad alternator could be the cause of your starting failure.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro