P279A

DTC P279A

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 Instructionsprovides an overview of each diagnostic category.

DTC Descriptor
DTC P279A
- Four Wheel Drive (4WD) High Range Performance

Circuit/System Description

The engine control module (ECM) monitors the state of the 4WD range signal message, if equipped, over the GMLAN communication bus. The ECM uses this message to determine motor torque required and transmission shifting. There are no external circuits involved.

Conditions for Running the DTC

The engine is running for greater than 5 seconds. Vehicle speed is less than 200 km/h (124 mph) for greater than 5 seconds.

Conditions for Setting the DTC

* The ECM uses two ways to determine if the transfer case is in 4 low. The first way is to compare the transmission output speed sensor to the transfer case speed sensor in order to calculate transfer case range. The second way is to use a LAN message sent by the transfer case shift control module. If the first way and second way of determining this does not match while in 4 high then this DTC is set.
* The ECM has detected 32 fail counts out of 400 samples.

Action Taken When the DTC Sets

DTC P279A is a Type C DTC.

Conditions for Clearing the DTC

* DTC P279A is a Type C DTC.
* When attempting to set ECM internal DTCs, run the engine for at least 1 minute, and then shut the engine off for 30 seconds. Following this procedure will allow for a complete ECM shutdown. Program the ECM with the latest software version and retest the DTC to see if the DTC resets.

Reference Information
Schematic Reference

Automatic Transmission Controls Schematics 2ML70 - Automatic Transmission
Connector End View Reference

Component Connector End Views Connector End Views By Name
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 Programming and Relearningfor scan tool information.

Circuit/System Verification

1. Start the engine.
2. Let the engine run for 1 minute, then turn the key OFF for 30 seconds.
3. Retest for DTC P279A.

Circuit/System Testing

1. Observe the 4WD Signal in the ECM Data List. The 4WD signal should display Enabled when you shift into 4WD and Disabled when you shift into 2WD.

• If the scan tool display does not match the transfer case range switch state, repair the transfer case system. Refer to Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle.
• If the scan tool matches the transfer case range switch state, continue with testing.

2. Program the ECM with the latest software version and retest for DTCs.

• If the DTC resets, replace the ECM.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.

Control Module References Programming and Relearningfor ECM replacement, setup, and programming.