If the tach senser on an MB is like othe pick up coils, and it probably is. an ohms ready probably won't diagnous it. changing the senser is the easiest test. or after put your ohm meter on ac volts (about10v setting) and connect to the two senser leads. crank the engine and look for any sign of voltage. If the senser is out of the car moving a metal object (screwdriver etc.) across the tip will simulate the engine cranking.

BenQ
2 87 fords, looking for am MB to replace one
1 88 ford
1 72 ford