Your tachometer needs power ground and signal wire to function. Power and ground come from the ignition switch. The tach should zero when the ign is switched on, engine not running. If not, check power and ground supply to tach.
The signal comes from the voltage regulator or rectifier, whichever you have. The tach signal is really the raw output of the stator. It is usually routed thru the VR or rectifier. It will be found on a yellow/grey or grey wire.
