For 1990-2009 cars

### 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 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. 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 engine speed is greater than 500 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

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

#### Description and Operation

Transmission General Description

#### 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 connector from the tow/haul switch. Using the J 35616 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 kit , 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 kit , 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 repair.