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 CircuitCircuit/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 .
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics
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
- 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.
- Using the DMM and the J 35616 , measure the voltage at the tow/haul switch signal circuit of the tow/haul switch connector.
- Using the DMM and the J 35616 , measure the voltage at the tow/haul switch signal circuit of the body control module (BCM).
| ⇒ | If the scan tool Tow/Haul Mode indicates a status change, replace the switch. |
| ⇒ | If Ignition Voltage is measured, test the ground circuit of the tow/haul switch for an open or shorted condition. |
| ⇒ | 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
