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 P2771: Four Wheel Drive (4WD) Low Switch Circuit

### Circuit/System Description

The four wheel drive (4WD) low circuit consists of the engine control module (ECM), a transfer case control module and the circuit wiring. The transfer case control module controls the 4WD low signal circuit. When the operator selects 4WD low, the transfer case control module grounds the signal circuit, and the 4WD low signal voltage on the circuit changes from ignition voltage to zero volts. The ECM then compensates for transfer case gear reduction in the transmission output shaft speed (OSS) sensor signal. The ECM uses the transmission OSS sensor signal to adjust shift points, line pressure and torque converter clutch (TCC) scheduling.

### Conditions for Running the DTC

• | No throttle position (TP) sensor DTCs P0122, or P0123.

• | No OSS assembly DTCs P0722 or P0723.

• | No cam or crank sensor DTCs.

• | The system voltage is 9-18 volts.

• | The engine speed is 1,000-5,500 RPM for at least 30 seconds.

• | The vehicle speed is at least 40 km/h (25 mph).

• | The TP angle is 5-40 percent.

• | The gear range is not PARK, NEUTRAL or REVERSE.

• | The TFT is -20 to +130°C (-4 to +266°F).

### Conditions for Setting the DTC

#### Condition 1

• | The 4WD low switch is not in 4WD low, open/12 volts.

• | The Transfer Case Ratio is 2.64-2.9 for 2 seconds.

• | Both conditions must be met twice.

#### Condition 2

• | The 4WD low switch is in 4WD low.

• | The Transfer Case Ratio is 0.8-1.75 for 2 seconds.

### Action Taken When the DTC Sets

• | DTC P2771 is a Type B DTC.

• | The ECM commands a normal shift pattern, not a 4WD low shift pattern.

### Conditions for Clearing the DTC

DTC P2771 is a Type B DTC.

### Diagnostic Aids

A malfunctioning speed sensor could cause DTC P2771 to set.

• | Verify that the input speed sensor (ISS) and the OSS are functioning properly.

• | In 3rd gear at approximately 64 km/h (40 mph), command TCC ON. If functioning properly the OSS will track ISS and the engine RPM. If not refer to DTC P0716 for the ISS and DTC P0723 for the OSS.

• | If the speed sensors pass testing, refer to Testing for Intermittent Conditions and Poor Connections .

### Reference Information

#### Schematic Reference

Automatic Transmission Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Transmission General Information

#### 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

Transmission Control Module Scan Tool Information

#### Special Tools

J 35616 GM-Approved Terminal Test Kit

### Circuit/System Verification

Important: It may be necessary to drive the vehicle slightly forward to engage 4LO on the transfer case.

Verify the scan tool displays Disabled when you select 4HI and Enabled when you select 4LO.

### Circuit/System Testing

1. Remove connector from the transfer case control module. With ignition ON and engine OFF, observe with the scan tool the 4WD Low parameter.

⇒ | If the scan tool indicates Enabled, test the 4WD Low signal circuit for a short to ground.

2. Using the J 35616 GM-approved terminal test kit, install a fused jumper between the 4WD low signal circuit and ground at the transfer case control module connector.

⇒ | If the scan tool 4WD Low parameter indicates Enabled when grounded, replace the transfer case control module.

3. Test the 4WD Low signal circuit for an open.

⇒ | Fix the open in the 4WD Low circuit.

4. Replace the ECM.

### Component Testing

Verify the 4WD Low parameter on scan tool while cycling the 4WD Low switch.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

Control Module References for replacement, setup, and programming