Circuit/System Testing

The DTC can be set by more than one type of fault. It could be set by faulty wheel speed sensors, a front axle actuator not mechanically engaging, a worn transfer case clutch, or a motor brake not holding properly causing excessive motor engagement.

1. This system compares front wheel speeds to rear wheel speeds to determine if slip is occurring and clutch engagement is necessary to correct this condition if the front axle actuator has not mechanically engaged this slip correction can not happen and the DTC will set.
   • Place the vehicle on a hoist, turn the ignition on, engine off and shift the transfer case to 2WD. With a scan tool command the front axle actuator on using special functions in the scan tool. While one person is holding one of the front tires still, spin the other front tire if the front propshaft turns then the front axle is engaged. If it is engaged go to the next step. If not, tear down and inspect the front axle clutch fork for mechanical binding.
2. Using the scan tool, go into EBCM data display and find the front and rear.
   • Take the vehicle on a short test drive on a level and smooth surface. Compare the front wheel speeds to the rear wheel speeds. If the wheel speeds are erratic or they drop out, this would cause this DTC to set. Consider checking the EBCM for DTCs for the wheel speeds. If the wheel speeds are erratic, use the DTC diagnostics in antilock brakes to diagnose the condition. If this appears normal, to too the next step.
3. The next check will be to see if the motor brake is holding. If it does not, this will cause the motor to stay engaged for extended periods of time causing it to exceed the thermal model.
   • With a scan tool clear the DTC, shift the vehicle to 2WD, and monitor the transfer case motor amperage using the data display. Shift the vehicle to the 4 high mode if the amperage goes up while the motor is shifting and goes back to 0 amps in 5 seconds or less then the brake is holding. If the amperage stays high longer, this indicates the brake is not holding and the actuator will need to be replaced. If the brake holds, go to the next step.
4. Next will be checking the transfer case clutch integrity.
   • Place the vehicle for 4 high and take it for a test drive. Clear the DTC if possible. Monitor front and rear wheel speeds using the EBCM data display. Under heavy acceleration, monitor the front and rear wheel speeds. If the front wheel speeds are much lower, this indicates a weak clutch and a transfer case tear down and inspection will be required. If all of the above check all check out good replace the transfer case shift control module if the DTC continues to set and perform the motor learn procedure/Transfer Case High/Low Clutch Reset.