The tach works off an rpm sensor on the front of the motor. The sensor very seldom goes bad. The AC signal generated by the sensor is interpreted and converted to a pulsed DC signal by a transducer screwed into the diagnostic port on the left fender. This is almost allways the problem and isn't very fixable without replacement as the electronics are sealed in plastic (molded in plastic).
The pulsed signal goes to the tach which is fairly trouble free. From fixing many bad ones I can verify their failures though. The failures are almost always the capacitors in the tach circuitry.