4L80-E/4L85-E - Automatic Transmission

DTC P071A

Diagnostic Instructions

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



DTC Descriptors
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 0 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.


Conditions for Running the DTC

* No Transmission Mode Switch Signal DTC P1762.
* The ignition voltage is between 8 and 18 volts.
* The engine speed is between 500 and 6,500 RPM for 5 seconds.



Conditions for Setting the DTC

The tow mode switch remains closed, low voltage, for 600 seconds (10 minutes).


Action Taken When the DTC Sets

DTC P071A is a type C DTC.


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 (6.6L) (Electrical Diagrams)Automatic Transmission Controls Schematics (4.8L and 6.0L) (Electrical Diagrams)
Connector End View Reference

Component Connector End Views
Electrical Information Reference

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

DTC Type Reference

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

Control Module References for scan tool information
Special Tools

J 35616 GM-Approved Terminal Test Kit


Circuit/System Verification

Turn ON the ignition, with the engine OFF. Cycle the tow/haul switch while observing the Tow/Haul Mode parameter on the scan tool.


Circuit/System Testing

1. Remove the connector from the tow/haul switch. Using the J 35616 GM-approved terminal test kit, install a fused jumper wire from the tow/haul switch signal and ground circuits of the tow/haul switch connector.

• If the scan tool Tow/Haul Mode indicates a status change, replace the switch.

2. Using the DMM and the J 35616 , measure the voltage at the tow/haul switch signal circuit of the tow/haul switch connector.

• If Ignition Voltage is measured, test the ground circuit of the tow/haul switch for an open or shorted condition.

3. Using the DMM and the J 35616 , measure the voltage at the tow/haul switch signal circuit of the body control module (BCM).

• If Ignition Voltage is measured, test the signal circuit of the tow/haul switch for an open or shorted condition.


• If Ignition Voltage is not measured, replace the BCM.




Component Testing

Verify the Tow/Haul Mode parameter on scan tool while cycling the tow/haul switch.


Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.


Tow/Haul Switch Replacement (4L60-E/4L65-E/4L70-E - Automatic Transmission)