Instrument Cluster Description and Operation: Tachometer

The instrument panel cluster (IPC) receives a discrete input from the engine control module (ECM) indicating vehicle speed. Each engine revolution is represents 2 input pulses to the ECM. The instantaneous engine speed is calculated by the ECM using the time difference between the 2 input pulses. The tachometer will default to 0 RPM if:

• The ignition is in OFF, ACCESSORY, or CRANK.
• The time duration since the last pulse is greater than or equal to one second.