Instrument Cluster: Speedometer: Operation: Speedometer

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) over the Programmable Communications Interface (PCI) data bus.

The speedometer 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 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 86 milliseconds. The gauge needle will continually be repositioned at the relative vehicle speed position on the gauge scale until the 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 six seconds, or until the ignition switch is turned to the OFF position, whichever occurs first. After six 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 information received from the Body Control Module (BCM) to determine the vehicle road speed. The PCM then sends the proper vehicle speed  messages to the ElectroMechanical Instrument Cluster (EMIC).

For proper diagnosis of the VSS, the BCM, the PCM, the EMIC, the PCI data bus or the electronic communication related to speedometer operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.