The wire that goes from the engine rpm sensor behind the below the oil filter to the tachometer is green in the 124 and 126 so likely green in the 201 as well.

Function of tach, AC compressor, cruise control and idle control of 60x engines relies on an engine rpm signal. If only the tach is affected, it can't be just the wire nor OVP relay. If everything is affected it can be sensor wire or OVP relay or both... or any number of other things.

Sixto
87 300D