Component Functional Description - Rear Drive Module (RDM)

RDM FUNCTIONALITY:  The RDM assembly is filled with one fluid to lubricate the internal components and a separate hydraulic fluid to operate the clutches. The RDM hydraulic fluid is a fill for life fluid. The level should be full unless there is a leak at the hydraulic pump assembly or pressure sensor. The Drivetrain Control Module (DTCM) operates a hydraulic pump to apply hydraulic pressure to the torque transfer clutch when torque to the rear wheels is required. An H-Bridge inside the DTCM is used to vary the Pulse Width Modulation (PWM) to the pump motor. This allows the DTCM to vary the amount of pressure applied to the torque transfer clutch inside the RDM, varying the torque applied to the rear wheels as needed. The duty cycle can range between 0% when off to as high as 60-70% for full torque. The duty cycle can be affected by fluid temperature and the hydraulic pumps capability.  There is a pressure sensor in the RDM that allows the DTCM to determine the hydraulic pressure, The pressure signal is used to calculate the torque being applied to the wheels. The DTCM directly monitors the pressure sensor signal, hydraulic pump voltage and current. If no circuit faults are present, the DTCM monitors the current applied to the hydraulic pump and compares it to the pressure sensor signal input to determine if the pump is operating properly. The RDM system faults are stored in the DTCM memory. Some general operating parameters of the hydraulic pump are given in the Diagnostic Overview information. 

TWO SPEED SYSTEM (KL ONLY):  The RDM on the two speed systems also has a smart actuator mounted on the RDM assembly that controls the operation of a shift fork. The smart actuator is supplied with a Fused B+, Ignition Supply and Ground circuit. The Ignition Supply circuit is used to wake up the internal electronics of the smart actuator. The Fused B+ circuit provides power for the operation of the shift fork. The smart actuator communicates with the DTCM over a Private CAN bus circuit. The DTCM sends messages to the smart actuator commanding 4WD Low range and axle locker operation when selected.  The smart actuator moves the shift fork from High Range to Low Range. If equipped, the axle locker feature can only be engaged when in 4WD Low. To engage the axle locker the fork is shifted further to physically lock the axles. This usually takes some vehicle movement once the axle locker feature is selected to lock the axles, sometimes needing to turn the steering wheel back and forth. This may be necessary because the splines can get torque locked when there is no vehicle movement.

The DTCM calculates the temperature of the hydraulic fluid and torque transfer clutch based on a thermal model. If the DTCM determines that the clutch plates are overheated, the DTCM temporarily suspends torque transfer clutch application in the RDM to open the clutches and allow them to cool. If the DTCM calculates that there is an overheat condition the hydraulic pump will be commanded off, disengaging the clutch until the temperature returns to a normal threshold.