Description And Operation - All Wheel Drive (AWD) System

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) controls the actuation of both assemblies to shift the drivetrain into AWD when needed. The system follows a specific sequence to switch between modes for a smooth transition.

CHANGING FROM FWD TO AWD MODE:

The RDM motor is driven to apply the clutch and allow the driveshaft to spin up to speed and match the front axle speed. This allows the locking cam and fixed cam in the PTU to mesh without clashing.
The PTU motor is driven to push the locking cam into the fixed cam connected to the ring gear in the PTU. A spring helps drive the locking cam into the fixed cam. This connects the front axle to the RDM through the driveshaft.
AWD mode is engaged. This typically happens in less than two seconds. Once in AWD mode, the RDM clutch can be varied between not applied (free spinning) to fully applied (maximum torque) as needed. If the module 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:

The RDM motor is driven to disengage the torque transfer clutch, disconnecting the RDM from the driveshaft.
The PTU motor is driven to unlock the locking cam from the fixed cam, disconnecting it from the ring gear in the PTU. The dog ring in the locking cam has a brake ramp that brakes the ring gear in the PTU stopping the driveshaft from spinning.
FWD mode is engaged. When the system is in FWD mode the driveshaft is disconnected at the PTU and RDM. This reduces drag on the powertrain system to improve fuel efficiency.

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

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 at or near a stop, allowing torque to all wheels during take-off. When the vehicle exceeds a calibrated speed, engine load is decreased below a calibrated threshold and the road conditions allow, the DTCM will dis-engage 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/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 Auto mode. The Transmission is boosted for maximum traction in SAND and MUD modes and can start off from a stop in 2nd gear when in SNOW mode to minimize wheel slippage.
ROCK MODE: Only available on vehicles equipped with 4WD Low range. This mode allows for boosted steering capacity when needed.

When a system fault is detected, the DTCM defaults to AUTO mode of operation.

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.