Hello, Its a crossing zero input required from memory, atleast 8V p-p with 0V offset, 10V p-p is best,
The input to the tacho is usually about 1K to 5K so it should be close to your function gens maximum rating, however exceeding this will not blow it up,
V8 should be 200Hz @ 3000RPM
V6 should be 150Hz @ 3000RPM
V4 should be 100Hz @ 3000RPM
if you have a different engine this calibration may differ on your output signal, however these are the usual calibrations.
If its reading wrong, the main chip on the board will be a frequency to voltage converter working in current mode, there will be a reference RC pair, and there will be a low side resistor that scales the meter, either can be tweaked to calibrate it.
Throw me a PCB image and i can likely dig up better info on the thing, I see so many different makes and models that they have blurred a little. from memory the pinout is marked on the top side of the PCB.