Commander 4WD V8-4.7L VIN N (2006)
Overdrive Indicator Lamp: Description and Operation
A tow/haul indicator is standard equipment on all gasoline engine instrument clusters. However, on vehicles not equipped with the optional overdrive
automatic transmission, this indicator is electronically disabled. This indicator is located near the bottom of the cluster overlay, between the speedometer
and the tachometer.
The tow/haul indicator consists of a stencil-like cutout of the text TOW/HAUL in the opaque layer of the instrument cluster overlay. The dark outer layer
of the overlay prevents the indicator from being clearly visible when it is not illuminated. An amber Light Emitting Diode (LED) behind the cutout in the
opaque layer of the overlay causes the indicator to appear in amber through the translucent outer layer of the overlay when it is illuminated from behind
by the LED, which is soldered onto the instrument cluster electronic circuit board.
When the exterior lighting is turned ON, the illumination intensity of the tow/haul indicator is dimmable, which is adjusted along with the cluster
illumination lighting using the panel lamps dimmer control sleeve on the left multifunction switch control stalk.
The tow/haul indicator is serviced as a unit with the instrument cluster.
The tow/haul indicator gives an indication to the vehicle operator when the OFF position of the tow/haul switch has been selected, disabling the
electronically controlled overdrive feature of the automatic transmission. This indicator is controlled by a transistor on 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 tow/haul indicator Light Emitting Diode (LED) is completely controlled by the instrument cluster logic circuit, and that logic will only allow this
indicator to operate when the instrument cluster receives a battery current input on the fused ignition switch output (run-start) circuit. Therefore, the LED
will always be off when the ignition switch is in any position except ON or START. The LED only illuminates when it is provided a path to ground by
the instrument cluster transistor. The instrument cluster will turn on the tow/haul indicator for the following reasons:
Bulb Test - Each time the ignition switch is turned to the ON position, the cluster illuminates the tow/haul indicator for about three seconds as a
Tow/Haul Lamp-On Message - Each time the cluster receives a tow/haul lamp-on message from the PCM indicating that the OFF position of the
tow/haul switch has been selected, the indicator will be illuminated. The indicator remains illuminated until the cluster receives a tow/haul
lamp-off message from the PCM, or until the ignition switch is turned to the OFF position, whichever occurs first.
Actuator Test - Each time the cluster is put through the actuator test, the tow/haul indicator will be turned on, then off again during the LED
portion of the test to confirm the functionality of the LED and the cluster control circuitry.
The PCM continually monitors the tow/haul switch to determine the proper outputs to the automatic transmission. The PCM then sends the proper
lamp-on and lamp-off messages to the instrument cluster. For proper diagnosis of the overdrive control system, the PCM, the CAN data bus, or the
electronic message inputs to the instrument cluster that control the tow/haul indicator, a diagnostic scan tool is required.