Land Rover Discovery Owners & Service Manuals

Land Rover Discovery: Rear Drive Unit (RDU)/ Operation

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.

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