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 over the Controller Area Network (CAN) data bus from the Engine Control Module (ECM).

The tachometer is driven by an electronic stepper motor unit that receives battery current on the instrument cluster electronic circuit board when the instrument cluster detects that the ignition switch is in the ON position. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch is turned 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 ECM it will calculate the correct engine speed reading and position the gauge needle at that relative speed position on the gauge scale. The gauge needle will continually be repositioned at the correct relative engine speed positions on the gauge scale until the engine stops running, or until the ignition switch is turned to the OFF position, whichever occurs first.
- Communication Error - If the cluster fails to receive an engine speed message, it will hold the gauge needle at the last indication for about three seconds, or until the ignition switch is turned to the OFF position, whichever occurs first. After three seconds, the gauge needle will return to the left end of the gauge scale.

The ECM continually monitors the crankshaft position sensor to determine the engine speed. The ECM then sends the proper engine speed messages to the ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN or KOMBI). The ECM will store a Diagnostic Trouble Code (DTC) for any malfunctions detected.

For proper diagnosis of the crankshaft position sensor, the ECM, the EMIC, the CAN data bus or the electronic communication related to tachometer operation, a diagnostic scan tool is required. Refer to the appropriate diagnostic information.