Instrument Panel Cluster (IPC) Description and Operation: Tachometer
WARNING: This page is about a different car, the 2003 Chevrolet Corvette. However, it is still accessible from the selected car via links, so may be relevant.
The IPC displays the engine speed as determined by the PCM. The IPC receives a class 2 message from the PCM indicating the engine speed. The tachometer will default to 0 RPM if the PCM detects a malfunction in the engine speed sensor circuit.