RAM 2500 Truck 4WD V8-5.7L VIN D (2005)
Overdrive Indicator Lamp: Description and Operation
A tow/haul indicator is standard equipment on all instrument clusters. However, on vehicles not equipped with the optional overdrive automatic
transmission, this indicator is electronically disabled. The tow/haul indicator consists of the text "TOW/HAUL", which appears in the lower portion of
the odometer/trip odometer indicator Vacuum Fluorescent Display (VFD) unit.
The odometer/trip odometer VFD is soldered onto the cluster electronic circuit board and is visible through a window with a smoked clear lens located
on the lower edge of the tachometer gauge dial face of the cluster overlay. The dark lens over the VFD prevents the indicator from being clearly visible
when it is not illuminated. The text "TOW/ HAUL" appears in an amber color and at the same lighting level as the odometer/trip odometer information
when it is illuminated by the instrument cluster electronic circuit board. The tow/haul indicator is serviced as a unit with the VFD in 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 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 tow/haul indicator 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 indicator will always be off when
the ignition switch is in any position except On or Start. The indicator only illuminates when it is switched to ground by the instrument cluster circuitry.
The instrument cluster will turn on the tow/haul indicator for the following reasons:
-
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 tow/haul 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 VFD
portion of the test to confirm the functionality of the VFD 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
tow/haul lamp-on and lamp-off messages to the instrument cluster.
For proper diagnosis of the overdrive control system, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control
the tow/haul indicator, a diagnostic scan tool is required.
