Gauge Pack, Instrument Cluster: Operation: Speedometer

WARNING: This page is about a different car, the 2017 RAM 2500 HD. However, it is still accessible from the selected car via links, so may be relevant.

The speedometer gives an indication to the vehicle operator of the vehicle road 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) or Engine Control Module (ECM), depending on engine application, over the Controller Area Network (CAN) data bus.

The speedometer 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 instrument cluster circuitry controls the gauge needle position and provides the following features:

Vehicle Speed Message - Each time the cluster receives an electronic vehicle speed message from the PCM it will calculate the correct vehicle speed reading and position the gauge needle at that relative speed position on the gauge scale. The cluster will receive a new message and reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continually be positioned at the relative vehicle speed position on the gauge scale until the vehicle stops moving, or until the ignition switch is turned to the OFF position, whichever occurs first.
Communication Error - If the cluster 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 Vehicle Speed Sensor (VSS) or electronic wheel speed messages from the Controller Anti-lock Brake (CAB) to determine the vehicle road speed. The PCM then sends the proper vehicle speed messages to the instrument cluster, also known as the Cab Compartment Node (CCN). For further diagnosis of the speedometer or the instrument cluster circuitry that controls the gauge. Refer to DIAGNOSIS AND TESTING .