Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Four-wheel Drive Systems - Vehicles With- Active Driveline - 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.


The RDU is driven by the driveshaft which is connected to the PTU. As the PTU synchroniser is engaged, the driveshaft begins to rotate the stationary components within the RDU. Pressure is then applied to the clutch plates and torque is the transferred to the halfshafts.

The drive flange is connected to a pinion shaft and drive gear, which drives a crown wheel to transfer the direction of drive through 90 degrees through the crown wheel shaft to the outer disc carriers.

The casing has an oil drain plug on its underside, and an oil fill/level plug on the left side.

RDU Lubrication

RDU Lubrication

  1. Primary lubrication circuit
  2. Secondary lubrication circuit

Lubrication of the bearings and clutch packs in the RDU is by a 'splash' lubrication system. When the oil level is correct, part of the crown wheel drive gear is immersed in the oil. As the drive gear rotates, the oil is carried on the drive gear teeth and thrown as a mist to the top of the RDU casing.

The oil collects in a cascade gallery above the drive gear and can then flow through channels in the casing to the left and right covers. Galleries in the covers allow the oil to fall onto the inner disc carriers to lubricate the ball bearings. The oil flows through the bearings to the left and right clutch packs. Centrifugal force, forces the oil through the clutch packs lubricating and cooling the plates and is thrown back to the channels from the cascade gallery to be recirculated through the cover galleries (secondary lubrication circuit) and into the bottom of the RDU casing.

RDU Clutches

The two independent, wet multiplate RDU clutches are operated by hydraulic pressure from the RDU pump and supplied to one or both clutches via two solenoid operated valves in the AWD valve block.

Hydraulic pressure is applied to the piston which compresses the clutch pack. The outer friction discs lock to the inner plain discs and transfer drive from the outer disc carrier to the inner disc carrier. The AWDCM can also vary the pressure applied to allow controlled slip of the clutches and can control each clutch independently.

RDU Pump

The RDU pump is electrically operated and controlled by the All Wheel Drive Control module (AWDCM).

A three phase electric motor drives a pump which is located in the AWD valve block.

The three windings of the motor are controlled by the AWDCM. Three Hall sensors in the motor provide positional and speed feedback to the AWDCM.

The AWDCM can drive the three phases of the motor to the optimum speed and torque to provide the required maximum pump output pressure of up to 40 bar (580 lbf/in²) to the AWD valve block. Using this motor control, and the PWM modulation of the solenoids to control the pressure control valves, the AWDCM can provide the required hydraulic pressure for RDU clutch application.

All Wheel Drive (AWD) Valve Block

The four pressure control valves located in the AWD valve block are solenoid operated and controlled by the AWDCM. The valves are marked P1 for the right side clutch, P2 for the left side clutch, P3 to connect the PTU and P4 to disconnect the PTU.

The solenoids are PWM controlled by the AWDCM.

The pressure control valves are used to electronically reduce the pressure supplied to the PTU synchroniser and the RDU clutches to give precise control.

When the solenoid coil is de-energised, the pressure control valve is closed by spring force and no pressure is applied to the synchroniser or the clutches. Any fluid pressure remaining in the system is decayed back to the AWD valve block fluid reservoir.

When a current is applied to the solenoid coil, the pressure control valve spool will begin to move to connect the pressure inlet from the pump to the synchroniser or clutch. The pressure applied is controlled by the pump output and the current applied to the solenoid coil. As the coil current signal increases or reduces the pressure applied to the synchroniser or clutch will change with respect to the signal. When the coil current is at its maximum, the full pressure will be applied.

The AWDCM can control each solenoid individually and simultaneously, giving infinite control over the operation of the synchroniser and the RDU clutch operation.


Active Driveline Hydraulic Circuit Diagram

Active Driveline Hydraulic Circuit Diagram

  1. Reservoir
  2. Suction filter
  3. Electric actuator
  4. RDU pump
  5. Pressure Control Valve (PCV) P1
  6. PCV P2
  7. PCV P3
  8. PCV P4
  9. PTU synchroniser piston
  10. Bleed orifice
  11. Left RDU clutch piston
  12. Right RDU clutch piston

The Active Driveline system is controlled by the AWDCM. Using inputs from other vehicle systems, the AWDCM can automatically disconnect and connect and provide AWD to assist vehicle traction and dynamics. In normal driving conditions, the AWDCM will disconnect the Active Driveline to provide FWD to improve economy and emissions.

The AWDCM is connected to the following vehicle system control modules and MultiCAN buses:

Anti-lock Brake System (ABS) control module - High Speed CAN Powertrain Systems bus

  • Wheel speeds, ABS status, Dynamic Stability Control (DSC) status, Hill Descent Control (HDC) status and vehicle speed inputs

Restraints Control Module (RCM) - High Speed CAN Chassis Systems bus

  • Lateral acceleration inputs

Steering Angle Sensor Module (SASM) - High Speed CAN Powertrain

Systems bus

  • Steering angle input

Electric Park Brake Control Module (EPBCM) - High Speed CAN

Powertrain Systems bus

  • Electric park brake applied or released input

Central Junction Box (CJB) - High Speed CAN Powertrain Systems bus

  • Outputs for Touch Screen (TS) images and Instrument Cluster (IC) displays

Terrain Response (TR) Switchpack - High Speed CAN Powertrain Systems bus

  • Selected TR mode selection input

Engine Control Module (ECM) - High Speed CAN Powertrain Systems bus

  • Engine torque, accelerator pedal position, ambient temperature, RDU Clutch temperature calculation

Transmission Control Switch (TCS) - High Speed CAN Powertrain Systems bus

  • Gear position

Transmission Control Module (TCM) - High Speed CAN Powertrain Systems bus.

  • Transmission status (automatic transmission only)

The AWDCM is connected by hardwired connection to the RDU pump and the AWD valve block. The AWDCM provides a PWM output to the motor and receives three inputs from the motor position sensors.

Low side driver outputs from the AWDCM control the pressure control valve solenoids for control and operation of each of the pressure control valves.

The AWDCM will automatically connect the PTU synchroniser and operate the RDU clutches at speeds below 20 km/h (12 mph) to provide improved traction when the vehicle is pulling away from a standstill in a straight line.

The system disconnects the driveline at speeds above 35 km/h (22 mph) to improve efficiency.


AWD is always provided when reverse gear is selected.

The AWDCM evaluates the received inputs and can predict when AWD will be required. Typical inputs for the evaluation are received from the Accelerator Pedal Position (APP) sensor, engine torque request from TCM (transmission control module), steering angle from the Steering Angle Sensor Module (SASM) and lateral acceleration from the RCM (restraints control module).

Loss of traction is detected from inputs from the ABS (anti-lock brake system) control module. Wheel spin, ABS and Dynamic Stability Control (DSC) activity and yaw signals.

Evaluation of these inputs will cause the AWDCM to connect the Active Driveline.

The AWDCM will automatically connect the PTU synchroniser and operate the RDU clutches to provide improved traction when the vehicle is pulling away from a standstill in a straight line, driving forward at speeds below 35 km/h (22 mph) and when in reverse gear.

The AWDCM will also operate the Active Driveline in AWD when the driver has selected a Terrain Response Auto2 mode that requires AWD. AWD is also activated when Hill Descent Control (HDC) is requested.

When ambient temperatures are below -10ºC (-50ºF), the AWDCM will keep the Active Driveline connected. When the vehicle is stationary and the engine is stopped, the Active Driveline will remain connected.

The RDU clutches can be fully or partially applied on each driveshaft individually to provide active torque biasing. The AWDCM can determine the clutch torque required for each halfshaft based on inputs for acceleration, yaw and oversteer or understeer. This allows the RDU to be locked to perform in a similar manner to a Limited Slip Differential.

Active Torque Biasing

Active Torque Biasing

  1. Accelerator pedal depressed - torque bias to outer wheel to reduce under steer
  2. Accelerator pedal released - increased locking torque applied to both wheels to reduce over steer

Torque biasing uses the RDU and the vehicle brakes to constantly balance the distribution of engine torque between the four wheels during cornering, resulting in improved grip and steering, and a reduced level of understeer.

The system also uses the ABS control module, monitoring the vehicle 100 times per second. As the car accelerates through a corner, the system uses yaw sensors to detect understeer, increasing engine torque along with subtle levels of braking to correct the vehicle attitude. Transferring engine torque to the outside wheels which have more grip maintains traction and steering control.

The torque biasing is also active in off-road conditions, with different settings according to the Terrain Response mode. It can be very effective in conditions such as sand, where it helps the vehicle turn in and avoid excessive understeer.



A = Hardwired; AM = High Speed CAN Chassis Systems Bus; AN = High Speed CAN Powertrain Systems Bus


  1. All Wheel Drive Control Module (AWDCM)
  2. Terrain Response (TR) switchpack
  3. Gateway Module (GWM)
  4. Engine Control Module (ECM)
  5. Steering Angle Sensor Module (SASM)
  6. Anti-lock Brake System (ABS) control module
  7. Transmission Control Module (TCM)
  8. Rear Drive Unit (RDU) pump actuator
  9. All Wheel Drive (AWD) valve block
  10. Ground
  11. Fused permanent 12V power supply from Central Junction Box (CJB)
  12. Ignition relay
  13. 12V power supply from Battery Junction Box (BJB)


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