Grand Cherokee 4WD V8-4.7L Flex Fuel (2009)
Overdrive Indicator Lamp: Description and Operation
Tow/Haul Indicator - Operation
OPERATION
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:
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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
bulb test.
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Tow/Haul Indicator Lamp-On Message - Each time the cluster receives an electronic tow/haul indicator 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 lamp-OFF message from the PCM, or until the ignition switch is turned to the OFF position, whichever occurs first.
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Self Test - Each time the cluster is put through the self test, the tow/haul indicator will be turned ON, then OFF again during the bulb check
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 or lamp-OFF message to the instrument cluster, also known as the Cab Compartment Node (CCN).
For proper diagnosis of the transmission control system, the PCM, the CCN, the CAN data bus or the electronic communication related to tow/haul
indicator operation a diagnostic scan tool is required. Refer to the appropriate diagnostic information.
