Diagnostic Overview - AWD System

POWER TRANSFER UNIT (PTU):  The smart actuator performs the following diagnostics on the shift motor(s) that control the shift fork(s).

• Shift motor voltage:  The voltage for the shift motor is monitored during operation. If the shift motor voltage is not correct during the different modes of operation, the smart actuator will report a circuit fault. Resistance in the Fused B+ circuit or ground path to the smart actuator can affect the motor voltage when it is commanded to operate and should not be overlooked when diagnosing a circuit fault.
• Shift fork position:  If a fork is not able to reach the commanded position the smart actuator will report one of the performance faults. A fork that cannot reach the commanded position could be due to a mechanical or electrical condition. A broken or damage fork, collar, or damaged splines on the shafts can cause binding or shifting issues. This condition could also cause an over temp condition if it is persistent. Resistance in the Fused B+ or ground can also cause a voltage drop in the system that may affect motor operation. This may or may not cause a circuit fault depending on the amount of resistance in the Fused B+ circuit. A faulty ground circuit can also cause a loss of communication with the smart actuator depending on the severity of resistance in the ground.
• Shift motor temperature:  If the shift motor temperature is above a calibrated threshold, the smart actuator will report the over temp fault. The motor temperature can exceed the calibrated threshold for several reasons. The system being heavily used in extreme conditions can set a fault without any actual system failure. Other possible issue could be an electrical issue with the shift fork actuator, a mechanically stuck or binding actuator that is overworking, low fluid level or other internal PTU issues causing the internal temperature to rise can cause the over temperature condition. The fluid in the unit lubricates the internal components and helps to cool the PTU.
• Loss of communication:  The Ignition Supply voltage is used to wake up the module and run the internal electronics. A poor ground circuit can also cause an actuator not to wake up. An intermittent issue with the ground may not bad enough at times to lose communication but cause the shift motor not to operate properly. Any opens or shorts in the Private CAN bus circuits between the DTCM and actuator will also cause a communication fault.

REAR DRIVE MODULE (RDM):  The Drivetrain Control Module (DTCM) monitors for opens and shorts in the hydraulic pump and circuits. The DTCM monitors the pressure sensor 5-volt reference and signal circuit voltage for circuit faults. The DTCM also performs functional checks on the system. First, the pressure sensor rationality diagnostic monitors the pressure signal when the hydraulic pump is commanded off. The pressure signal should be zero with the pump off. If pressure is detected above a calibrated threshold the pressure sensor rationality diagnostic fails. During operation of the hydraulic pump, the DTCM compares the commanded torque to the calculated torque. The calculated torque is based on the pressure sensor signal. This can be affected by a drifted sensor reading, mechanically failing hydraulic pump or low hydraulic fluid level. Listed below are some general operating parameters that can help when diagnosing the hydraulic pump motor: 

• Normal operation:  When the pump motor harness connector is plugged in, and the pump is not energized, the circuit voltage measured on both motor control circuits is typically around 2.8 volts. Also, there should be very little to no current flow present. The DTCM energizes the pump motor with a battery voltage Pulse Width Modulated (PWM) signal. The duty cycle can range between 0% when off to as high as 60-70% for full torque. As the duty cycle increases, the current draw to the motor will typically also increase. The duty cycle can be affected by fluid temperature and the hydraulic pump capability. 
• Approximate readings with the transmission in PARK (pump motor off):  - Both control circuits should read approximately 2.8 volts. There should be no current draw. The pressure sensor should be reading 0 kPa.
• Approximate readings with the transmission shifted into DRIVE, foot on brake (pump motor on):  The duty cycle will typically be between 8 - 12%. Motor B control circuit voltage should measure between approximately 0.8 - 1.2 volts. Motor A control circuit voltage should measure approximately 0.03 volts. Current draw between approximately 0.5 - 2.0 amps.
• Approximate readings during a full throttle acceleration (Max torque), normal road conditions:  Duty cycle can range from 40 - 70% with the motor B control circuit voltage approximately 4.5 - 7.0 volts. Motor A control circuit voltage should measure approximately 0.03 volts. Current draw can be as high as between approximately 10.0 - 15.0 amps.

Motor circuit voltage readings with a circuit failure issue: 

• If either circuit is shorted to voltage, both circuits will measure battery voltage with the harness connector plugged in.
• If motor A control circuit is shorted to ground, both circuits measure 0.0 volts with the harness connector plugged in.
• If motor B control circuit is shorted to ground, the motor A control circuit will measure 0.0 volts and the motor B control circuit will have measure approximately 1.3 volts with the harness connector plugged in.
• If either circuit is open, there will typically be approximately 3.6 volts on motor A control and 0.0 volts on motor B control circuit. NOTE: Both control circuits will have the same voltage readings at the pump motor harness connector when the connector is unplugged. 

If the vehicle has a RDM with a shift fork (two speed system), the smart actuator performs diagnostics in the same manner as the smart actuator on the PTU (described above).