Description And Operation - All Wheel Drive (AWD) System: Notes

GENERAL OPERATION:  This system is a base Front Wheel Drive (FWD) system. The All Wheel Drive (AWD) system consists of the Power Transfer Unit (PTU) and Rear Driveline Module (RDM). The Drivetrain Control Module (DTCM) communicates with the smart actuator mounted on the PTU over a private CAN bus. The 1-Speed system has one fork that shifts between FWD/AWD modes. The 2-Speed PTU has two forks. One shifts between FWD/AWD and the second fork is for Low Range/AWD Neutral. The DTCM sends messages to the smart actuator requesting modes of operation. The smart actuator controls the operation of the shift fork actuator(s). The DTCM directly controls the actuation of the hydraulic pump in the RDM. The hydraulic pump builds and applies pressure on the torque transfer clutch in the RDM to apply torque to the wheels. The DTCM uses a Pulse Width Modulated (PWM) control to vary the pressure applied to the clutch as needed. On the KL 2-Speed systems the RDM also has a smart actuator that communicates with the DTCM over a private CAN bus. The DTCM sends messages requesting 4WD High/Low range and axle locker operation. The smart actuator controls the operation of the shift fork to select the requested position. The system follows a specific sequence to switch between FWD and AWD modes for a smooth transition.

CHANGING FROM FWD TO AWD MODE: 

1. The DTCM operates the hydraulic pump inside the RDM to apply the torque transfer clutch. This allows the propeller shaft to spin up to speed and match the front axle speed allowing the shift fork in the PTU to engage without clashing.
2. The smart actuator on the PTU is sent a message to engage AWD mode. The smart actuator shifts the fork to move a collar engaging the splines on concentric shafts inside the PTU. This connects the front axle to the RDM through the propeller shaft.
3. AWD mode is engaged. This typically happens in less than two seconds. Once in AWD mode, the torque transfer clutch inside the RDM can be varied between not applied to fully applied (maximum torque) as needed. If the DTCM detects that there may be conditions present that could require torque to the rear wheels the PTU will stay engaged, but the torque transfer clutch inside the RDM may be disengaged. Having the PTU engaged and the RDM at ready allows for a faster reaction to conditions that require torque to the rear wheels when needed by not having to re-engage the PTU.

CHANGING FROM AWD TO FWD MODE: 

1. The hydraulic pump is de-energized to disengage the torque transfer clutch, disconnecting the RDM from the propeller shaft.
2. The smart actuator on the PTU is sent a message to disengage AWD mode. The smart actuator shifts the forks to move the collar, disengaging the concentric shafts inside the PTU. This disconnects the front axle to the propeller shaft.
3. FWD mode is engaged. When the system is in FWD mode the propeller shaft is disconnected at the PTU and RDM. This propeller shaft stops spinning which reduces drag on the powertrain system to improve fuel efficiency.

4WD LOW RANGE AND AXLE LOCKER (2-SPEED SYSTEM ONLY):  The 4WD Low Range option can come with or without an axle locker.

1. To engage 4WD Low the brake must be applied, the transmission in neutral, and the vehicle speed less than 3 mph (5 km\h).
2. The DTCM sends a message requesting 4WD Low range to the smart actuators on the PTU and RDM. The smart actuators control engagement of 4WD Low and the RDM axle locker (if equipped). For more details on the 4WD low and axle locker features, read the PTU and RDM component functional descriptions.

There are several modes of operation that can be selected by the vehicle operator on the KL models:

• AUTO MODE:  The DTCM can operate the vehicle in either FWD and AWD modes. The vehicle will engage the PTU and RDM shifting the vehicle into AWD mode when the vehicle speed is below 18 mph, allowing torque to all wheels. When the vehicle exceeds a calibrated speed, approximately 25 mph, engine load is decreased below a calibrated threshold and the road conditions allow, the DTCM will disengage the PTU and RDM shifting the vehicle into FWD mode for maximum fuel efficiency. If the vehicle speed drops below the threshold, or the road conditions compromise vehicle stability, the DTCM will re-engage the PTU.
• SNOW/SPORT/SAND/MUD MODES:  In each of these modes the PTU is constantly engaged providing continuous AWD capability. The RDM is applied to provide torque to the rear wheels as needed. The Transmission operates differently than when in Auto mode. The Transmission is boosted for maximum traction in SAND, SPORT and MUD modes and can start off from a stop in 2nd gear when in SNOW mode to minimize wheel slippage. The sensitivity of the Electronic Stability Control (ESC) system is reduced to limit the intervention of the traction and yaw control.
• ROCK MODE:  Only available on vehicles equipped with 4WD Low range. This mode allows for boosted steering capacity when needed.

KL models - When a system fault is detected, the DTCM defaults to AUTO mode of operation.  When a smart actuator detects a fault it sends a fault flag to the DTCM. The system faults are stored in the DTCM memory.

The DTCM monitors several signals from other modules to determine the many operating variables of the system. The main contributing modules are the Powertrain Control Module (PCM), Transmission Control Module (TCM), Brake System Control Module (BSCM) and Electric Power Steering (EPS) Module. The DTCM can anticipate operation based on environmental conditions and driving style. Some contributing inputs include:

• Gear selection, Electronic Stability Control (ESC) switch input, wiper settings and ambient temperature input.
• Environmental influences such as vehicle speed, surface grade, rough roads, launch characteristics and related ESC events.
• Dynamic factors including wheel slip, detection of understeer/oversteer, steering angle rate of change, and acceleration/deceleration rates.

If the DTCM detects poor road conditions or poor environmental conditions based on the inputs it can engage the PTU in AWD mode to allow faster reaction time if torque to the rear wheels is needed.