1995 Beaver Where does the tach get its signal from.

it's a cummins check on the fuel pump side. if your bed raises for engine access it is easy access, if you have a vom unplug the sensor and check resistance while you wiggle the wires, most sensors have between 1000 to 1800 ohms resistance the reading should be steady while wiggling the wires, if that checks out you could also check the wiring in the rear of the tach, if the wiring checks out and the odometer reads correct but the needle is unstable it probably has a bad tach.