The tach signal comes from the ECM based on the signals the ECM gets from the crankshaft sensor. The teeth on the crank gear cause electrical fluctuations, pulses, when they pass by the sensor. The fluctuation and time between the fluctuations is how the ECM knows piston position and engine speed.
From the FSM
“The tachometer drive circuit connects to engine revolution detecting circuit in engine control module.
When the engine revolution increases/decreases, the voltage of this circuit also increases/decreases, changing the magnetic force of tachometer drive coil.
Thus, the tachometer hand moves together with engine revolution change.”
Now, on to the problem you’re having…….
