Instrument Cluster: Gauges: Speedometer

WARNING: This page is about a different car, the 2008 Dodge Magnum, 2008 Dodge Charger, and 2008 Chrysler 300. 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 Front Control Module (FCM). The FCM is provided the vehicle speed information across the Controller Area Network (CAN) data bus by either the Powertrain Control Module (PCM), the ESP module or the ABS module.

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

Vehicle Speed Message - Each time the cluster receives a vehicle speed message from the FCM, 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 vehicle speed message and reposition the gauge pointer accordingly about every 50 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 speedometer 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.
Actuator Test - Each time the cluster is put through the actuator test, the speedometer needle will be 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 vehicle speed sensor to determine the vehicle road speed. The PCM then sends the proper vehicle speed messages to the instrument cluster. For further diagnosis of the speedometer or the instrument cluster circuitry that controls the gauge, see DIAGNOSIS AND TESTING . The speedometer is serviced as a unit with the instrument cluster.