GAUGE PACK, Instrument Cluster: Operation: Operation: Gauges: Tachometer

WARNING: This page is about a different car, the 2011 RAM Pickup and 2011 RAM Cab & Chassis. However, it is still accessible from the selected car via links, so may be relevant.

The tachometer gives an indication to the vehicle operator of the engine speed. This gauge is controlled by the Cab Compartment Node (CCN) based upon cluster programming and electronic messages received over the Controller Area Network (CAN) data bus. These electronic messages are received from the Powertrain Control Module (PCM) or Engine Control Module (ECM), depending on engine application.

The tachometer is a stepper motor design 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 is turned to the OFF position. The CCN electronic circuitry controls the gauge needle position and provides the following features:

Engine Speed Message - Each time the CCN receives an electronic engine speed message from the PCM or ECM it will calculate the correct engine speed reading and position the gauge needle at that relative speed position on the gauge scale. The CCN 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 is turned to the OFF position, whichever occurs first.
Communication Error - If the CCN fails to receive a 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 PCM continually monitors the crankshaft position sensor to determine the engine speed. The ECM continually monitors the engine speed sensor to determine the engine speed. The PCM or ECM, depending on engine application, then sends the proper engine speed messages to the CCN. For further diagnosis of the tachometer or the instrument cluster circuitry that controls the gauge, refer to Diagnosis and Testing .