Power Transfer Unit (PTU) Operation

The PTU has four (4) distinct modes of operation: Connected, Connecting, Disconnected, and Disconnecting. Each mode is commanded by the AWD module. The PTU contains a reversible DC motor and two hall effect position sensors. The motor moves the shift fork which connects or disconnects the dog clutch collar. PTU Position sensor A monitors the position of the actuator cam. PTU Position sensor B monitors the position of the shift fork. Some PTU models are equipped with an oil temperature sensor

Connected Mode 

Connected Mode is the default mode for the PTU. The AWD module will command Connected mode at the beginning of each key cycle. In connected mode, the fork and dog clutch collar are positioned towards the RH side of the PTU. The dog clutch is engaged. The driveshaft is rotating at an overdrive ratio compared to the front axle shafts. Torque is available at the RDU.

Connecting Mode 

To connect the PTU, the two halves of the Dog clutch must be within 40 RPM of each other. The AWD module calculates the speed differential based on transmission OSS, RDU Driveshaft Speed, and PTU gear ratio. The actuator motor will energize and rotate the gear reduction drive and the actuator cam against the actuator fork. When the dog clutch teeth line up, the cam pushes the actuator fork into position and the clutch engages. It can take approximately 100 - 150ms for the dog clutch to engage after the command is sent.

Disconnected Mode 

The AWD module will command disconnected mode based on vehicle conditions. The purpose of the disconnected mode is to reduce drag losses from spinning the driveshaft when conditions indicate AWD will probably not be needed in the near future. In disconnected mode, the fork and dog clutch collar are positioned towards the LH or transmission side of the PTU. The dog clutch is disengaged.

Disconnecting Mode 

To disengage the PTU, no torque can be routed through the dog clutch. That is, the RDU must be completely disengaged. Depending on vehicle conditions, the AWD module may keep the PTU engaged for the remainder of the key cycle. The actuator motor will energize and rotate the gear reduction drive and pull the actuator cam away from the actuator fork. When the dog clutch collar has moved far enough to the LH or transmission side, the dog clutch teeth disengage.