Land Rover Discovery: Rear Drive Unit (RDU)

1. Drive flange
2. Mounting bracket
3. RDU pump
4. All Wheel Drive (AWD) valve block oil filler plug
5. AWD Valve block
6. PTU synchroniser piston pressure line P4
7. PTU synchroniser piston pressure line P3 including pressure damper
8. Left RDU clutch pressure line P2
9. Right RDU clutch pressure line P1
10. RDU oil level plug
11. Left inner disc carrier
12. Breathers
13. AWD Control Module (AWDCM)
14. Right inner disc carrier

Rear Drive Unit (RDU) Sectional View

1. AWD valve block
2. RDU pump
3. Pre-load nut
4. Drive flange
5. Oil seal
6. Mounting bracket
7. AWD Control Module (AWDCM)
8. Taper roller bearing
9. Pinion shaft and drive gear
10. Collapsible spacer
11. Right clutch pressure connection P1
12. Ball bearing
13. Right inner disc carrier
14. Oil seal
15. Right cover
16. Clutch pack
17. Outer disc carrier
18. Casing
19. Crown wheel shaft and drive gear
20. Intermediate housing
21. Left outer disc carrier
22. Left cover
23. Left inner disc carrier
24. Left clutch pressure connection P2
25. Hydraulic fluid reservoir

The RDU comprises a main casing, an intermediate casing and left and right covers. Within the casing is a crown wheel drive gear and shaft, a pinion drive gear and shaft, two wet, multiplate clutch packs and two hydraulically actuated pistons.

Externally mounted on the casing is an AWD valve block which contains four solenoid operated pressure control valves and an electrically operated hydraulic pump. The All Wheel Drive Control Module (AWDCM) is located externally on the casing.

The casing is filled with oil for lubrication of the pinion and crown wheel drive gears and bearing and also lubrication and cooling for the multiplate clutch packs. A fill/level plug is located on the left cover and a drain plug is fitted to the underside of the casing.

The intermediate casing allows for the internal components to be assembled within the main casing and also houses the left multiplate clutch pack assembly.

The crown wheel drive gear is mounted longitudinally across the RDU on a crown wheel shaft. The drive gear is a low-offset hypoid bevel gear configured for minimal power loss across the speed range. Both ends of the crown wheel shaft have splines which mate with corresponding splines in the outer disc carrier of the multiplate clutch pack.

The crown wheel shaft is supported on opposing taper roller bearings which are pressed into the casing and the intermediate casing.

The pinion drive gear and shaft is locked in the casing at 90 degrees to the crown wheel drive gear. The pinion drive gear shaft is supported in the housing on opposing taper roller bearings. The pinion drive gear shaft is retained in the casing with a pre-load nut. The outer end of the pinion gear is splined and mates with the drive flange. The drive flange is secured on the pinion gear by the pre-load nut.

A collapsible spacer is located between the taper roller bearings. The collapsible spacer holds the taper roller bearings in alignment and also collapses under pressure applied by the pre-load nut. This allows the preload nut to be tightened to a predetermined torque, which collapses the spacer, setting the correct taper roller bearing pre-load and therefore the correct meshing of the teeth of the pinion drive gear and the crown wheel drive gear.

RDU Clutch

RDU Clutch Exploded View

NOTE: Left clutch assembly shown, right clutch assembly similar

1. Taper roller bearing
2. Roller bearing
3. Outer disc carrier
4. Inner disc carrier
5. Inlay
6. Outer friction disc
7. Inner plain disc
8. Pressure disc
9. Roller bearing
10. Circlip
11. Ball bearing
12. Circlip
13. Spring washer
14. Ring
15. Shim
16. Piston
17. Left cover

The clutch assemblies comprise an outer disc carrier which locates on splines on the crown wheel drive shaft, an inner disc carrier which is located in a ball bearing in the left cover, six inner plain metal discs, six outer friction discs and a piston which is located in the left cover.

The outer disc carrier has a splined bore which locates on the crown wheel drive shaft. Machined teeth on the inside face of the carrier provide positive location for the outer friction discs.

The inner disc carrier is located inside the outer disc carrier and supported on a ball bearing in the left or right cover. External machined teeth provide positive location for the inner plain discs. An internal bore in the inner disc carrier is splined to accept the halfshaft.

A piston with two seals is fitted to a machined bore in the left or right cover.

The piston receives hydraulic pressure from the RDU pump via the AWD valve block and applies pressure to lock the inner and outer clutch discs together. A needle roller bearing is located between the piston and the pressure disc which allows the piston to remain stationary as the clutch discs and carriers rotate.

The outer disc carrier is permanently driven, when the PTU synchroniser is engaged, via the splines on the crown wheel shaft. When pressure is applied to the piston, it presses the inner and outer discs to together, locking the inner and outer carriers together and allowing drive from the crown wheel shaft to be passed to the halfshaft via the inner disc carrier.

All Wheel Drive (AWD) Valve Block

AWD Valve Block

1. Electrical connector
2. RDU pump and actuator
3. AWD valve block

AWD Valve Block Components

1. Solenoid P4
2. Solenoid P3
3. Solenoid P2
4. Solenoid P1
5. Electrical contact element
6. Valve body
7. Oil filler plug
8. Pressure adaptor
9. RDU pump
10. Pump housing
11. Seal
12. Electric actuator
13. Electrical connector
14. Suction filter
15. Spool sleeve (4 off)
16. Spool (4 off)

The AWD valve block is located at the front left side of the RDU, adjacent to the drive flange. The valve block is sealed to the RDU casing and secured with screws.

The body of the valve block and the casting voids in the casing form the hydraulic fluid reservoir. A sealed plug is located at the top of the valve block body for hydraulic fluid filling. A second sealed plug is located at the base of the valve body for hydraulic fluid draining.

Four machined bores house the spool sleeves and spool valves for the operation of Pressure Control valves (PCV) P1, P2, P3 and P4.

The PCV's control the following functions:

• P1 - Right RDU clutch
• P2 - Left RDU clutch
• P3 - PTU synchroniser piston - engage synchroniser
• P4 - PTU synchroniser - disengage synchroniser

Control Solenoid and Spool Valve

1. Control solenoid
2. Spool
3. Spring
4. Spool sleeve
5. Sleeve spring

Four variable force control solenoids are secured in the valve block with screws. The solenoids are PWM (pulse width modulation) controlled by the AWDCM and provide measured operation of the spool valves to supply the required hydraulic pressure for RDU clutch and PTU synchroniser operation.

The spool valves comprise a spool sleeve and spool valve. The spool valve is operated by the control solenoid which moves the spool within the sleeve.

Movement of the spool opens and closes ports on the sleeve supplying pressure to the applicable function.

A electrical contact element with an electrical connector is located on the top of the valve block. The electrical connector has a contact element which connects with each of the four solenoids in the valve body. The contact element is secured in the valve body with two screws and sealed with an O ring seal.

Service Pressure Lock Pins

1. PCV port P4
2. PCV port P3
3. Pressure lock pins

Spool valves P3 and P4 are fitted with an anti-spill valve device that can be operated externally via the pressure lock pins. The anti-spill valves block the ports and enable the hydraulic pipes and RDU to be removed or serviced, without loss of hydraulic fluid from the reservoir. To open the anti-spill valves once servicing is complete, the bleeding procedure must be run. Pressure is applied to both P3 and P4 ports forcing the spill valves to open. This can be verified by checking that the pins are now in the fully extended position.

During normal operation, the pins should be visible, protruding from the valve block body.

The RDU pump is located in the pump housing and secured with two screws. The pump drive shaft engages on the electric actuator output shaft.

The pump housing is attached to the valve body, secured with three screws and sealed with an O ring seal. The pump output port connects with a pressure adaptor within the valve body. Pressure from the pump flows through the pressure adaptor and into drillings in the valve body to supply each of the four spool valves.

RDU Pump and Actuator

RDU Pump

1. Oil seal
2. Screw (2 off)
3. Seal
4. Pump body
5. RDU pump
6. Electrical actuator
7. Compensation seal
8. Compensation plate
9. Driving gear input shaft
10. Driving gear and driven gear
11. Compensation plate
12. Compensation seal
13. Seal
14. Nut (2 off)
15. Pump cover
16. Dowel (2 off)

The RDU pump is located in the pump housing and is secured with two screws. The pump is a rotary gear pump with a flow of 1 cm³ (0.061 in³) per revolution and maximum output pressure of 42 bar (609 lbf/in²).

The pump fluid inlet port is fitted with a suction filter which is located at the bottom of the fluid reservoir within the valve block.

A pressure adaptor connects the pump outlet port to drillings in the valve body.

The driving gear input shaft is engaged with the output shaft of the electrical actuator, which directly drives the driving gear.

As the driving and driven gears rotate they separate on the inlet side of the pump. This creates a void and a depression which is filled by the hydraulic fluid. The hydraulic fluid is carried by the gears to the outlet side of the pump, where the meshing of the gears displaces the fluid. The mechanical clearances of the gears and the pump body are small allowing for the generation of high output pressure.

The electric actuator is a brushless DC (direct current) motor. The actuator has a fixed stator and a rotating permanent magnet rotor. The actuator has a maximum rotational speed of 5000 Revolutions Per Minute (RPM) and a maximum torque output of 0.9 Nm. Maximum current consumption is 30 Amperes.

The AWDCM provides an AC (alternating current)PWM output to operate the actuator in the correct direction and speed. The output is dependent on inputs received by the AWDCM from other system modules.

Three Hall sensors within the motor provide position and rotation speed feedback signals to the AWDCM.

Power Transfer Unit (PTU)All Wheel Drive Control Module (AWDCM)

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