That does indeed sound like a tach pickup. You say its wired into the harness? I am not familiar with W123 240D so I am not really qualified to answer what purpose it might serve. However I did add a tach to my '79 diesel, swapped the instrument cluster and tach amp from a turbo diesel - mounted it on the left fenderwell and fed 12V to it thru a small fuse. I connected these two with a short length of cable and hooked it up to the sender on the transplanted '85 engine and Voila! I now have a tach. Is it possible someone installed a later version engine in your car? Did 240D come with a tach? I don't know the answer to these questions. But I do not think the sensor is used with cruise, as that need driveshaft speed. The cruise control pickup is a small motor located on the back of the speedometer.
FWIW, the '85 300D does not have a tach amplifier in the engine compartment, it is integrated into the ECU (engine control unit) and supplies RPM info to the computer in that module. Perhaps your 240D has something like this? If it does its the first I heard of, and kinda neat if it does (let us know what you find out!).

Dieseldiehard
1971 220 (gas) 4-spd manual 104041 or is it 204041?
1979 300TD w/ ’85 turbo engine 288975
1983 300D 237300
1985 300TD 205437