Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Power Transfer Unit (PTU) / Operation


The Active Driveline Power Transfer Unit (PTU) input shaft is driven directly from the transmission differential. When the hydraulically operated triple cone synchroniser is in the disengaged position (B in the following illustration), the crown wheel shaft and the crown wheel is disconnected from the input shaft and no drive is transferred to the driveshaft or the Rear Drive Unit (RDU).

When the triple cone synchroniser is engaged (A in the following illustration), drive is transferred from the transmission differential, through the synchroniser to the crown wheel shaft. The crown wheel rotates and transmits the drive through 90 degrees to the pinion gear and the drive flange to rotate the driveshaft.

Triple Cone Synchroniser Operation

Triple Cone Synchroniser Operation

  1. Engaged
  2. Disengaged
  1. Input shaft
  2. Input shaft engagement ring
  3. Blocker ring
  4. Synchroniser Sleeve
  5. Synchroniser detent (3 off)

The synchroniser is operated by a hydraulic piston with pressure supplied from RDU pump via the hydraulic supply lines connecting the PTU to the All Wheel Drive (AWD) valve block. Solenoid operated spool valves in the AWD valve block direct hydraulic pressure to the piston in the PTU casing to engage or disengage the triple cone synchroniser. A spring loaded detent locks the synchroniser selector fork in either position, removing the requirement for hydraulic pressure to maintain to hold the synchroniser piston in the required position.

The All Wheel Drive Control Module (AWDCM) controls the operation of the active driveline system and the operation of the PTU synchroniser hydraulic operation.

The synchroniser process comprises a number of steps from disengagement to full engagement.

  1. When the transmission differential is rotating the input shaft, the engagement ring integral with the input shaft is also rotating. With the synchroniser disengaged no drive is transmitted to the crown wheel shaft and the crown wheel drive gear, the pinion drive gear and the drive flange remain stationary.
  2. When engagement is required, hydraulic pressure is applied to the piston and the synchroniser selector fork begins to move the synchroniser sleeve in the engagement direction. The selector fork moves the synchroniser sleeve in an axial direction and the synchroniser detents contact the blocker ring. This starts to align the blocker ring teeth with the teeth on the synchroniser sleeve.
  3. As the synchroniser selector fork continues to move axially, the friction caused by the detents between the synchroniser sleeve and the blocker ring reduces the speed of both components until they are rotating at the same speed. The teeth of the synchroniser sleeve now start to mesh with the teeth on the blocker ring.
  4. With the blocker ring and the synchroniser sleeve rotating at the same speed, the sleeve fully engages with the blocker ring teeth. However, there is still a speed difference between the input shaft engagement ring and the synchroniser sleeve. This speed difference is reduced as friction surfaces (cones) between the input shaft synchroniser sleeve and the blocker ring start to mesh.
  5. The synchroniser selector fork now moves the synchroniser sleeve axially into contact with the teeth of the input shaft engagement ring.

    The tapered teeth of both components now start to mesh and the synchroniser sleeve rotational speed is increased as it starts to engage with the engagement ring.

  6. When the synchroniser sleeve is fully engaged with the input shaft engagement ring, the rotational speed of the input shaft and the crown wheel shaft are the same and drive is passed via the crown and pinion drive gears to the drive flange.
  7. Disengagement is a reverse of the engagement process. When the synchroniser selector fork moves the synchroniser sleeve fully out of engagement with the engagement sleeve and the blocker ring, it touches the casing. Friction between the casing and the synchroniser sleeve brakes the crown wheel shaft and drive gear, the pinion drive gear and the drive flange to prevent them from rotating through oil drag between the components.

Synchroniser Piston Operation

Synchroniser Piston Operation

  1. Disengaged
  2. Engaged
  1. Detent
  2. Synchroniser sleeve
  3. Synchroniser selector fork
  4. Piston

The synchroniser piston is located in a bore in the PTU casing. Hydraulic pressure is supplied from the RDU pump via the AWD valve block.

Two hydraulic lines from the valve block supply hydraulic pressure to each side of the synchroniser piston to engage or disengage the synchroniser.

Once the piston is in the engaged or disengaged position, hydraulic pressure is not required to hold it in this position.

The piston has an orifice which allows the system to be self bleeding, allowing any air trapped to pass from one side of the piston, via the orifice, to the other side of the piston. The system allows a measured leakage through the orifice which allows the low pressure side of the piston to act as a return line.


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