Allison - Automatic Transmission

DTC P071A

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

DTC Descriptor
DTC P071A
- Transmission Tow Mode Switch Circuit

Circuit/System 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. The BCM is constantly monitoring the status of the tow/haul switch. The BCM sends the status of the tow/haul switch out on the serial data line to other control modules. The transmission control module (TCM) monitors the status of the tow/haul switch. When the tow/haul request is received, the TCM changes the shift point schedule and when TCC is applied. Cycling the tow/haul switch again disables tow/haul mode and returns the transmission to a normal shift pattern.

Conditions for Running the DTC

* DTC P071A or P1762 is not set.
* The ignition voltage is between 9-18 V.
* The engine speed is greater than 200 RPM for 5 seconds.

Conditions for Setting the DTC

The Tow/Haul Mode switch remains active for greater than 20 seconds.

Action Taken When the DTC Sets

* DTC P071A is a Type C DTC.
* Tow/Haul mode is disabled.

Conditions for Clearing the DTC

DTC P071A is a Type C DTC.

Diagnostic Aids

If the electrical circuit tests are OK and the shift pattern is not occurring, there may be a mechanical/hydraulic condition that prevents operation. Refer to Symptoms - Automatic Transmission (Symptoms - Automatic Transmission).

Reference Information
Schematic Reference

Automatic Transmission Controls Schematics (Allison - Automatic Transmission)
Connector End View Reference

* Component Connector End Views (Connector End Views By Name)
* Inline Harness Connector End Views (Connector End Views By Number)

Description and Operation

* Transmission General Description (Transmission General Description)
* Transmission Component and System Description (Transmission Component and System Description)
* Electronic Component Description (Electronic Component Description)

Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference

Control Module References (Control Module References) for scan tool information

Circuit/System Verification

1. Ignition ON, cycle the tow/haul switch while observing the Tow/Haul Mode parameter on the scan tool TCM Transmission Data list. The Tow/Haul Mode parameter should change states between Active and Inactive with each cycle of the switch.
2. Operate the vehicle within the Conditions for Running the DTC to verify that the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Circuit/System Testing

1. Ignition OFF, transmission in Park, disconnect the automatic transmission shift lever harness connector.
2. Ignition OFF, all vehicle systems OFF, this may take up to 2 minutes, test for less than 5 ohms between the ground circuit terminal 2 and ground.

• If greater than the specified value, test the ground circuit for an open/high resistance.

3. Ignition ON, test for greater than 11 V between the tow/haul signal circuit terminal 1 and ground.

• If less than the specified value, test the ignition voltage circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the BCM.

4. Ignition OFF, test for less than 3 ohms between terminal 1 and terminal 2 on the automatic transmission shift lever component harness connector while depressing the Tow/Haul switch button, and an open circuit when the Tow/Haul switch button is released.

• If not the expected results, replace the automatic transmission shift lever.

5. If all circuits test normal, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Shift Lever Replacement (Shift Lever Replacement)
* Control Module References (Control Module References) for BCM replacement, setup, and programming.