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The 1985 300SD has an early "EGR computer" through which the tach signal passes. The EGR brain is usually very reliable but it gets its power from an "over-voltage protection relay" which has a built-in 10 Amp fuse. Either the fuse or the entire relay may have failed. I believe the OVP relay in your car is in the relay box with the fuses, under the hood. [Source: Mercedes- Benz 1985 "Introduction into service" manual.]
This car does not have a "tach amp" on the inner fender.
There is also an RPM sensor and various wires and connectors. I would check the OVP and its fuse first.
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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