The tach in my '85 300sd was doing the same thing for nearly two years. It finally gave out completely. I replaced the speed sensor and it works fine now. The sensor is located on the top of the bell housing just slightly toward the drivers side of the engine. It is held in place with a single allen bolt. If I remember correctly the replacement is available on FastLane for about $50.00

Good Luck.

Bill

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Current Vehicles:
1990 Mercedes 300SE (147,000)
1985 Mercedes 300SD (203,000)
1983 Mercedes 240D (167,000)