Commander 4WD V8-4.7L VIN N (2006)
Tachometer: Description and Operation
TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument clusters. The tachometer is located to the right of center in the instrument cluster, between the
speedometer and the coolant temperature gauge. The tachometer consists of a movable gauge needle or pointer controlled by the instrument cluster
circuitry and a fixed 210 degree scale on the gauge dial face that reads left-to-right from 0 to 7. The text RPM X 1000 imprinted on the cluster overlay
directly above the hub of the tachometer needle identifies that each number on the tachometer scale is to be multiplied by 1000 rpm.
The tachometer graphics are white against a black field except for several red graduations at the high end of the gauge scale, 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 text and graphics still appear white and the red graphics still appear red, but an inner secondary scale-like graphic
appears 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 tachometer is serviced as a unit with the instrument cluster.
OPERATION
The tachometer gives an indication to the vehicle operator of the engine 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 tachometer 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:
-
Engine Speed Message - Each time the cluster receives an engine speed message from the PCM it will calculate the correct engine speed reading
and position the gauge needle at that relative speed position on the gauge scale. The cluster will receive a new engine speed message and
reposition the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continually be repositioned at the relative engine
speed position on the gauge scale until the engine stops running, or until the ignition switch is turned to the OFF position, whichever occurs first.
-
Communication Error - If the cluster fails to receive an engine 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.
-
Actuator Test - Each time the cluster is put through the actuator test, the tachometer 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 crankshaft position sensor to determine the engine speed. The PCM then sends the proper engine speed messages to
the instrument cluster. For proper diagnosis of the crankshaft position sensor, the PCM, the CAN data bus, or the electronic message inputs to the
instrument cluster that control the tachometer, a diagnostic scan tool is required.
