SPEEDOMETER
DESCRIPTION
A speedometer is standard equipment on all instrument clusters. The speedometer is located next to the tachometer, just to the left of center in the instrument cluster. The speedometer consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry, and a fixed 255 degree primary scale on the gauge dial face that reads left-to-right either from 0 to 120 mph, or from 0 to 240 km/h, depending upon the market for which the vehicle is manufactured.
Most versions also have a secondary inner scale on the gauge dial face that provides the equivalent opposite measurement units from the primary scale. Text appearing on the cluster overlay just above the hub of the speedometer needle abbreviates the unit of measure for the primary scale (either MPH or km/h), and text for the unit of measure for the secondary scale is located adjacent to the low end of that scale.
The speedometer primary scale text is white against a black field, while the major and minor primary scale increments are black against a silver field. If equipped, the secondary scale text and increments are light gray against a black field. All text and increments are 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, all primary scale text and major increments appear blue-green. The primary scale minor increments and the secondary scale are not illuminated. The red gauge needle has internal optical illumination. Gauge illumination is provided by replaceable incandescent bulb and bulb holder units located on 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 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.