Allison - Automatic Transmission

DTC P071A

Circuit Description

Tow/haul mode enables the operator to achieve enhanced shift performance when towing or hauling a load. When tow/haul mode is selected, the tow/haul switch input signal to the body control module (BCM) is momentarily toggled to zero volts. This signals the transmission control module (TCM) to extend the length of time between upshifts and increase transmission line pressure. Cycling the tow/haul switch again disables tow/haul mode and returns the transmission to a normal shift pattern.


DTC Descriptor

This diagnostic procedure supports the following DTC:


DTC P071A Transmission Tow Mode Switch Circuit


Conditions for Running the DTC

* No Transmission Mode Switch Signal DTC P1762.
* The engine speed is between 200-7500 RPM for 5 seconds.
* The system voltage is 9-18 volts.



Conditions for Setting the DTC

The Tow Mode switch remains closed, low voltage, for 600 seconds.


Action Taken When the DTC Sets

* The TCM does not illuminate the malfunction indicator lamp (MIL).
* The TCM records the operating conditions when the Conditions for Setting the DTC are met. The TCM stores this information as Failure Records.
* The TCM stores DTC P071A in TCM history.



Conditions for Clearing the MIL/DTC

* A scan tool can clear the DTC.
* The TCM clears the DTC from TCM history if the vehicle completes 40 warm-up cycles without an emission-related diagnostic fault occurring.
* The TCM cancels the DTC default actions when the fault no longer exists and the DTC passes.



Diagnostic Aids

* The tow/haul switch is hard wired to the BCM where it is then broadcasted on Class 2 Serial Data link to the TCM.
* Observe tow/haul status on the scan tool to see if the status changes.
* Inspect the wiring for poor electrical connections at the BCM. Inspect for the following conditions:

- A bent terminal
- A backed out terminal
- A damaged terminal
- Poor terminal tension
- A chafed wire
- A broken wire inside the insulation

* When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.



Test Description

The numbers below refer to the step numbers on the diagnostic table.
2. This step tests for a faulty tow/haul switch.
3. This step tests for voltage input from the BCM to the tow/haul switch.
6. This step tests for ground integrity.