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6L45/6L50/6L80/6L90 - Automatic Transmission (NQG)




DTC P2771 (NQG)

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 Low circuit is used to notify the engine control module (ECM) that the vehicle is in 4WD low range. The ECM supplies 12 volts to the 4WD Low Signal circuit. When the operator selects 4WD Low, the Transfer Case Shift Control Switch grounds the signal circuit. The voltage on the 4WD low signal circuit changes from ignition voltage to zero volts. The ECM senses this voltage change and sends a message to the TCM that the transfer case is in 4WD Low. The TCM then compensates for transfer case gear reduction in the transmission output shaft speed (OSS) sensor signal. The TCM 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

The ECM calculated transfer case ratio is not consistent with the position of the transfer case shift control switch.

Condition 1

The transfer case shift control switch indicates 2WD High or 4WD High and the ECM calculated transfer case range indicates a 4WD Low transfer case ratio of 2.64-2.9, for 2 seconds.

OR

Condition 2

The Transfer Case Shift Control Switch indicates 4WD Low range and the ECM calculated transfer case range indicates a 2WD High OR 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 4WD High 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.

* Road test the vehicle while monitoring the transmission OSS parameter on the scan tool. The transmission OSS parameter should vary with transmission ISS and engine speed. As engine speed increases, transmission ISS and OSS RPM should increase. As engine speed decreases, transmission ISS and OSS RPM should decrease. 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 Testing for Intermittent Conditions and Poor Connections.

* EL 49742 - harness is a good source for B+ and ground.

* The transfer case drain plug is a good source for ground.

Reference Information

Schematic Reference

Automatic Transmission Controls Schematics 6L45/6L50/6L80/6L90 - Automatic Transmission

Connector End View Reference

Component Connector End Views 1-2 Shift Solenoid (SS) Valve (M30)

Description and Operation

Transmission General Description Transmission General Description

Electrical Information Reference

* Circuit Testing Circuit Testing

* Connector Repairs Connector Repairs

* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections

* Wiring Repairs Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References Control Module References for scan tool information

Special Tools

J 35616 - GM-Approved Terminal Test Kit

For equivalent regional tools, refer to Special Tools 6L45/6L50/6L80/6L90 - Automatic Transmission.

Circuit/System Verification

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

1. Ignition ON. Engine OFF.

2. Parking brake ON. Transmission in Neutral.

3. Using the scan tool, verify that the, 4WD Low Signal parameter, reads Enabled/Disabled when using the Transfer Case Shift Control Switch to go In/Out of 4WD Low.

• If the parameter does not perform as specified, suspect a 4WD Low 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. Ignition OFF. Disconnect the connector at the Transfer Case Shift Control Switch.

2. Ignition ON, Engine OFF. Transfer case NOT in 4WD Low. Install a scan tool.

3. Using the GY probe J 35616-2A, connect a test light between the 4WD Low Signal circuit, Transfer Case Shift Control Switch X1 harness pin B and a good ground, such as the ground terminal of the EL-49742 - harness, black terminal. 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 ECM for proper operation.

4. Using the GY probe J 35616-2A, test for a diagnostic B+ (+0.0/-1.0V) between the 4WD Low Signal circuit, Transfer Case Shift Control Switch harness pin B, 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 ECM for proper operation.

5. Using the GY probe J 35616-2A, test for a diagnostic B+ (+0.0/-1.0V) between the 4WD Low Signal circuit, Transfer Case Shift Control Switch harness pin B, the ground circuit Transfer Case Shift Control Switch pin A.

• If the voltage is less than the specified value, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, test the ECM for proper operation.

6. Go to Component Testing - Transfer Case Shift Control Switch.

Component Testing

Transfer Case Shift Control Switch

1. Ignition OFF. Disconnect the connector at the Transfer Case Shift Control Switch.

2. Using the GY probes J 35616-3, measure the resistance for each of the transfer case positions listed below between the 4WD Low Signal circuit, Transfer Case Shift Control Switch pin B, and the ground circuit Transfer Case Shift Control Switch pin A.

• If the resistance is outside the specified ranges, replace the Transfer Case Shift Control Switch.

3. Test the ECM for proper operation.





ECM

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. Ignition OFF. Disconnect the connector at the Transfer Case Shift Control Switch.

2. Ignition ON, Engine OFF. Install a scan tool.

3. Monitor the 4WD Low parameter on the scan tool. The 4WD Low parameter should read disabled.

• If the 4WD Low parameter reads enabled, test the 4WD Low circuit for a short to ground. If all circuits and connections test normal, replace the ECM.

4. Using a GY probe J 35616-3, connect a fused jumper between the 4WD Low Signal circuit, Transfer Case Shift Control Switch harness pin B, and a good ground. When grounded the 4WD Low parameter should read enabled.

• If the 4WD parameter reads disabled, test the 4WD Low circuit for an open/high resistance condition. If all circuits and connections test normal, replace the ECM.

5. Using the GY probe J 35616-3, test for greater than 1K ohms between the 4WD Low Signal circuit, TCSC harness pin B, and all other pins of the TCSC 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 ECM.

6. If all circuits, connections and the ECM test normal, test the Transfer Case Shift Control Switch for proper operation.

Repair Instructions

* Perform the Reset Transmission Adapts Reset Procedure following all transmission related repairs.

* Perform the Diagnostic Repair Verification Verification Tests after completing the repair.

* Refer to Control Module References Control Module References for Control Solenoid Valve Assembly replacement, programming, and setup.

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