P2771
DTC P2771
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 P2771
- Four Wheel Drive (4WD) Low Switch Circuit
Diagnostic Fault Information
Circuit/System Description
The 4WD LO circuit is used to notify the powertrain control module (PCM)/engine control module (ECM) that the vehicle is in 4WD low range. The PCM/ECM supplies 12 volts to the 4WD Low Signal circuit. When the operator selects 4WD low, the transfer case shift control module (TCCM) grounds the signal circuit. The voltage on the 4WD low signal 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 greater than 5 km/h (3 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 transfer case shift control switch (TCSCS) indicates 4WD HIGH and the TCCM calculated transfer case range indicates a 4WD LOW transfer case ratio of 2.64-2.9, for 2 seconds.
OR
Condition 2
The TCSCS indicates 4WD LOW range and the TCCM calculated transfer case range indicates a 4WD HIGH transfer case ratio of 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 engine RPM. If not functioning properly 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 (Component Tests and General Diagnostics).
* EL-49742 harness is a good source for B+ and ground.
* A diagnostic voltage does not supply enough current to light a test light.
Reference Information
Schematic Reference
Automatic Transmission Controls Schematics (Electrical Diagrams)
Connector End View Reference
* Component Connector End Views (Connector End Views By Name)
* Inline Harness Connector End Views (Diagrams)
Description and Operation
Transmission General Information (Transmission General Information)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
Scan Tool Reference
Transmission Control Module Scan Tool Information (Transmission Control Module Scan Tool Information)
Special Tools
* EL-49742 Battery Voltage B+ and Ground Source Harness
* J 35616 GM-Approved Terminal Test Kit
Circuit/System Verification
Note: It may be necessary to drive the vehicle slightly forward to engage 4LO on the transfer case.
1. Key ON. Engine OFF.
2. Parking brake ON. Transmission in Neutral.
3. Using the transfer case data display functionality of the scan tool, verify that the Mode Switch Selected, Encoder Gear Position/Transfer Case Position and corresponding Indicator Light parameters all match when using the TCSCS to change modes.
• If the scan tool indicates different than stated, Refer to Symptoms - Transfer Case (Symptoms - Transfer Case).
4. Using the scan tool, verify that the, 4WD Low Active parameter, reads Yes/No when using the TCSCS to go In/Out of 4WD Low.
• If the parameter does not perform as specified, suspect a 4WD LO Signal circuit or control module condition.
Circuit/System Testing
Note: When test probing harness terminals, it is imperative to use the proper test probe from the most current version of the J 35616 GM terminal test kit. Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.
1. Key OFF. Disconnect the X1 connector at the TCCM.
2. Key ON, Engine OFF. Install a scan tool.
3. Using the BN probe J 35616-6, test for a diagnostic B+ (+0.0/-1.0V) between the 4WD LO Signal circuit, TCCM X1 harness pin B5, and a good ground.
• If the voltage is less than the specified value, test the affected circuit for an open/high resistance or short to ground. If all circuits and connections test normal, test the PCM/ECM for proper operation.
4. Using the BN probe J 35616-6, connect a test light between the 4WD LO Signal circuit, TCCM X1 harness pin B5, and a good ground. The test light should not illuminate.
• If the test light illuminates, test the affected circuit for a short to voltage. If all circuits and connections test normal, test the PCM/ECM for proper operation.
Component Testing
PCM/ECM/TCM
Note: When test probing harness terminals, it is imperative to use the proper test probe from the most current version of the J 35616 GM terminal test kit. Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.
1. Key OFF. Disconnect the X1 connector at the TCCM.
2. Key ON, Engine OFF. Install a scan tool.
3. Monitor the 4WD LO parameter on the scan tool. The 4WD LO parameter should read disabled.
• If the 4WD parameter reads enabled, test the 4WD LO circuit for a short to ground. If all circuits and connections test normal, replace the PCM/ECM.
4. Using a BN probe J 35616-6, connect a fused jumper between the 4WD LO Signal circuit, TCCM X1 harness pin B5, and a good ground. When grounded the 4WD LO parameter should read enabled.
• If the 4WD parameter reads disabled, test the 4WD LO circuit for an open/high resistance condition. If all circuits and connections test normal, replace the PCM/ECM.
5. Using the BN probe J 35616-6, test for greater the 1K ohm between the 4WD LO Signal circuit, TCCM X1 harness pin B5, and all other pins of the TCCM X1 connector.
• If the resistance is less than the stated value, test the affected circuits for a short together condition. If all circuits and connections test normal, replace the PCM/ECM.
6. If all circuits, connections and the PCM/ECM test normal, replace the TCCM.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.
Control Module References (Programming and Relearning) for replacement, setup, and programming