Most gasoline engines get a tach signal from the spark ignition (at least thru 1995 in small block Chevy), but I don't know M-B gas. A 1984-85 OM617 diesel gets the tach signal from a pickup on the transmission bell-housing (driver's side). If a 1984 gas is the same and you kept the transmission, it "should work". These OM617 still had the front pickup on the crank damper but the cable dead-ends at the "diagnostic port" w/ no "tach amp" needed like earlier cars. Perhaps dealer mechanics hooked that to an engine monitor. It isn't needed for the car to work fine.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans