Diagnostic Instructions
DTC Descriptor
DTC P279B: Four Wheel Drive (4WD) Low Range Performance
Circuit/System Description
The engine control module (ECM) monitors the state of the 4WD 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 P279B is a Type C DTC.
Conditions for Clearing the DTC
| • | DTC P279B 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
Connector End View Reference
Component Connector End Views
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References for scan tool information.
Circuit/System Verification
- Start the engine.
- Let the engine run for 1 minute, then turn the key OFF for 30 seconds.
- Retest for DTC P279B.
Circuit/System Testing
- 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 matches the transfer case range switch state, continue with testing. |
- 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 after completing the diagnostic procedure.
Control Module References for ECM replacement, setup, and programming.
