Commander 4WD V8-4.7L VIN N (2006)
Speedometer Head: Description and Operation
SPEEDOMETER
DESCRIPTION
A speedometer is standard equipment on all instrument clusters. The speedometer is located to the left of center in the instrument cluster, between the
tachometer and the fuel gauge. The speedometer consists of a movable gauge needle or pointer controlled by the instrument cluster circuitry and a fixed
210 degree primary scale on the gauge dial face that reads left-to-right either from 0 to 140 mph, or from 0 to 230 km/h, 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 above the hub of the speedometer needle abbreviates the unit of measure for the primary scale (MPH or km/h), and text inside the
low end of the secondary scale indicates the unit of measure for that scale.
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 primary scale graphics still appear
white, and the white secondary scale graphics appear blue-green. The red 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 Powertrain Control Module (PCM) over the Controller Area
Network (CAN) data bus.
The speedometer is driven by an electronic stepper motor 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 a 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 vehicle speed message and
reposition the gauge pointer accordingly about every 88 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 proper diagnosis of the vehicle speed sensor, the PCM, the CAN data bus, or the electronic message inputs to the instrument
cluster that control the speedometer, a diagnostic scan tool is required.
