I believe that in your car's model year 1984, EGR is turned on and off depending on engine speed. The tach signal goes through the EGR module first, then to the tachometer. No EGR, no tach.

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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