Tachometer - Operation

OPERATION

The tachometer gives an indication to the vehicle operator of the engine speed. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Controller Area Network (CAN) data bus.

The tachometer is an air core magnetic unit that receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the ON or START positions. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch turns to the OFF position. The instrument cluster 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 calculates the correct engine speed reading and position the gauge needle at that relative speed position on the gauge scale. The cluster receives a new message and reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle continually repositions at the relative engine speed position on the gauge scale until the engine stops running, or until the ignition switch turns to the OFF position, whichever occurs first.
- Communication Error - If the cluster fails to receive an engine speed message, it holds the gauge needle at the last indication for about three seconds, or until the ignition switch turns to the OFF position, whichever occurs first. After three seconds, the gauge needle return's to the left end of the gauge scale.
- Self Test - Each time the cluster is put through the self test, the tachometer needle is swept to several calibration points on the gauge scale in a prescribed sequence in order to confirm the functionality of the gauge and the cluster control circuitry.

The PCM continually monitors the crankshaft position sensor to determine the engine speed. The PCM then sends the proper engine speed messages to the instrument cluster, also known as the Cab Compartment Node (CCN). For further diagnosis of the tachometer or the instrument cluster circuitry that controls the gauge, Testing and Inspection.

Use a diagnostic scan tool for proper diagnosis of the crankshaft position sensor, the PCM, the CCN, the CAN data bus or the electronic communication related to tachometer operation. Refer to the appropriate diagnostic information.