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The 1985 W123/OM617 models are unique in that the tach signal goes through that #80 controller, located behind the right front side panel in the footwell. As was previously mentioned, there's a "door" cut in the plastic panel. If the engine is otherwise operating well, the controller is probably OK. The culprit may be the tach sensor (#82) or the wiring somewhere between it and the controller or between the controller and the instrument cluster.
Check for damaged wires and then try replacing the sensor. I've never had to do this particular fix (knock on wood!) and don't remember where the sensor lives -- someone will tell us -- don't believe it is in the same place as the older cars.
Jeremy
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"Buster" in the '95
Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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