Gauge Pack, Instrument Cluster: Operation: Gauges: Tachometer

The tachometer gives an indication to the vehicle operator of the engine speed. This gauge is controlled by the IC circuit board based upon cluster programming and electronic engine speed  messages received by the Instrument Panel Cluster (IPC) from the PCM over the CAN data bus.

The tachometer is driven by an electronic stepper motor unit that receives battery current on the IPC electronic circuit board through the fused ignition (run) circuit whenever the status of the ignition switch is ON. The IPC is programmed to move the gauge needle back to the low end of the scale after the status of the ignition switch transitions to OFF. The IPC circuitry controls the gauge needle position and provides the following features:

• Engine Speed Message  - Each time the cluster receives an electronic engine speed  message from the PCM it will calculate the correct engine speed reading and position the gauge needle at that relative speed position on the gauge scale. The IPC will receive a new message and reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continually be repositioned at the relative engine speed position on the gauge scale until the engine stops running, or until the ignition switch status transitions to OFF, whichever occurs first.
• Communication Error  - If the IPC fails to receive an electronic engine speed  message, it will hold the gauge needle at the last indication for about three seconds, or until the ignition switch status transitions to OFF, whichever occurs first. After three seconds, the gauge needle will return to the left end of the gauge scale.

The PCM continually monitors the crankshaft position sensor to determine the engine speed. The PCM then sends the proper electronic engine speed  messages to the IPC.