Speedometer Head: Description and Operation

SPEEDOMETER

DESCRIPTION
A speedometer is standard equipment on all instrument clusters. The speedometer is located in the center of the instrument cluster. The speedometer consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry and a fixed 210 degree primary outer scale on the gauge dial face that reads left-to-right either from "0" to "100" miles-per-hour, or from "0" to "180" kilometers-per-hour, depending upon the market for which the vehicle is manufactured. Each version also has a secondary inner scale on the gauge dial face that provides the equivalent opposite units from the primary scale. Text appearing on the cluster overlay near the left end of each scale abbreviates the unit of measure, either "mph" or "km/h".

The speedometer graphics are white against a black field, making them clearly visible within the instrument cluster in daylight. When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the exterior lamps turned On, the white graphics appear amber. The orange gauge needle is internally illuminated. Gauge illumination is provided by Light Emitting Diode (LED) units soldered onto the instrument cluster electronic circuit board. The speedometer is serviced as a unit with the instrument cluster.

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 from the Controller Anti-lock Brake (CAB) over the Controller Area Network (CAN) data bus. The speedometer is an air core magnetic 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 a 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 continue to be positioned at the actual vehicle speed position on the gauge scale until the ignition switch is turned to the Off position.
- 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.

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 instrument cluster. For proper diagnosis of the wheel speed sensors, the CAB, the CAN data bus, or the electronic message inputs to the instrument cluster that control the speedometer, a diagnostic scan tool is required. Refer to the appropriate diagnostic information.