Speedometer - Operation

OPERATION

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 over the Controller Area Network (CAN) data bus from the Controller Anti-lock Brake (CAB) (also known as the Antilock Brake System/ABS or Electronic Stability Program/ESP controller).

The speedometer 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:

- Vehicle Speed Message - Each time the cluster receives an electronic vehicle speed message from the CAB it will calculate the correct vehicle 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 vehicle speed positions on the gauge scale until the vehicle stops moving, or until the ignition switch is turned to the OFF position, whichever occurs first.
- Digital Speedometer - The vehicle operator can choose to have a digital speedometer information displayed instead of the ambient temperature within the cluster multi-function indicator, but the digital speedometer and ambient temperature indications cannot be displayed at the same time. On vehicles without the optional steering wheel buttons, the digital speedometer or ambient temperature indicator display option is selected using the multi-function indicator M (Mode) switch on the instrument cluster circuit board. This switch is actuated manually by depressing the push button that extends through the left edge of the cluster lens outboard of the analog speedometer. Actuating this switch for more than one second will toggle the display between the ambient temperature indicator and the digital speedometer modes. On vehicles with the optional steering wheel buttons, the digital speedometer or ambient temperature indicator display option is selected using the multi-function indicator MENU switches on the steering wheel.
- Communication Error - If the cluster fails to receive a vehicle 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 CAB continually monitors the four wheel speed sensors to determine the vehicle road speed. The CAB then sends the proper vehicle speed messages to the ElectroMechanical Instrument Cluster (EMIC) (also known as the Cab Compartment Node/CCN or KOMBI). The CAB will store a Diagnostic Trouble Code (DTC) for any malfunctions detected.

For proper diagnosis of the wheel speed sensors, the CAB, the EMIC, the CAN data bus or the electronic communication related to analog or digital speedometer operation, a diagnostic scan tool is required. Refer to the appropriate diagnostic information.