Land Rover Discovery: Active on-demand Coupling

1. Clutch assembly
2. Pressure relief/air bleed valve
3. Casing
4. Driveshaft flange
5. Electric motor and pump assembly
6. Active on-demand coupling control module
7. Pump housing
8. Electrical connectors

The active on-demand coupling is an electronically controllable All Wheel Drive (AWD) coupling. An active on-demand coupling control module is located externally on the casing of the coupling. The control module uses control information from the Anti-lock Brake System (ABS) control module on the high speed Controller Area Network (CAN) chassis systems bus and the Engine Control Module (ECM) on the high speed CAN powertrain systems bus to operate a high pressure hydraulic pump to control a multiplate clutch within the coupling.

The active on-demand coupling is attached to the front of the rear differential with four bolts.

The rear differential is connected via a wet multi-plate clutch within the active on-demand coupling which makes it possible to vary the torque distribution to the rear differential. The active on-demand coupling is independent of the differential speed between the front and rear differentials, full locking torque to the rear differential, if needed, is available at any given time and speed.

Active On-Demand Coupling Section View

1. Clutch drum
2. Steel clutch plates
3. Friction clutch plates
4. Input shaft
5. Piston
6. Pressure relief/air bleed valve
7. Radial ball bearing
8. Oil seal
9. Dust seal
10. Nut
11. Driveshaft flange
12. Electric motor
13. Axial needle roller bearings

The hydraulic pump 12V electric motor is controlled by the active ondemand coupling control module. The motor is started simultaneously with the vehicle engine to ensure hydraulic pressure is available when the vehicle starts to move. The pump is a six piston axial pump with an integral centrifugal overflow valve. The hydraulic pump generates hydraulic pressure which compresses the clutch plates. Increasing the pressure increases the clutch torque to a maximum of 1160Nm.

The output pressure is determined by the rotational speed of the motor and the centrifugal overflow valve. The active on-demand control module governs the hydraulic pump motor speed by controlling the current supplied to the motor. Three weighted levers and spring loaded ball valves form the overflow valve. As the pump rotational speed increases, the levers are moved outward by centrifugal force. As the levers move, they allow the ball valves to lift from their seats, allowing hydraulic pressure to bleed through the three ball valves and return to the fluid reservoir in the casing.

Pressurized hydraulic fluid from the pump is directed via a gallery to the clutch piston.

The clutch comprises the outer drum, a set of friction and steel clutch plates and the input shaft. Torque output from the Power Transfer Unit (PTU) is passed from the driveshaft to the driveshaft flange on the active on-demand coupling. Rotation of the driveshaft flange is passed to the input shaft.

When hydraulic pressure is applied to the clutch plates by the piston, the plates lock and torque is passed through the plates to the outer drum which rotates. The outer drum has a splined center bore which mates with the rear differential input shaft. Torque is passed from the outer drum to the rear differential. The outer drum is located on axial needle roller bearings.

The piston is located in the casing. Two seals are fitted to the piston and seal against the casing. A gallery behind the piston distributes hydraulic pressure evenly on the piston full area. When hydraulic pressure is applied, the piston moves to compress the clutch plates. The active on-demand control module operates the electric motor and the hydraulic motor and applies the required pressure to the plates via the piston. The control module can allow slippage of the clutch plates for partial lock-up, or apply full pressure for full lock-up.

A mechanical overflow valve is located in the casing to prevent overpressure.

When the pressure at the piston full area reaches a predetermined value, the overflow valve is lifted off its seat against spring pressure and hydraulic fluid is allowed to bleed back to the reservoir within the casing.

The input shaft is mounted radially on radial ball bearings. A thread on the outer end of the input shaft provides for the attachment of the driveshaft flange which is secured with a nut. The clutch end of the input shaft is fitted with needle roller bearings, which support the rear differential input shaft.

OPERATION

REAR DIFFERENTIAL

Rotational input from the active on-demand coupling clutch drum is passed to the rear differential pinion shaft and pinion gear. The angles of the pinion gear to the crown wheel drive gear moves the rotational direction through 90 degrees. The transferred rotational motion is passed to the crown wheel drive gear, which in turn rotates the differential carrier. The pinion shaft, which is secured to the casing, also rotates at the same speed as the casing.

The pinion gears, which are mounted on the crosspin shaft, also rotate with the casing. The pinion gears transfer rotational motion to the left and right hand sun gears, rotating the left and right halfshafts

When the vehicle is moving in a forward direction, the torque applied through the differential to each sun gear is equal. In this condition both halfshafts rotate at the same speed. The pinion gears do not rotate and effectively lock the sun gears to the differential casing.

If the vehicle is turning, the outer wheel will be forced to rotate faster than the inner wheel by having a greater distance to travel. The differential senses the torque difference between the sun gears. The pinion gears rotate on their axes to allow the outer wheel to rotate faster than the inner one.

ACTIVE ON-DEMAND COUPLING

When starting the vehicle, the electric motor is started, operating the hydraulic pump to swiftly to provide the system with pressurized hydraulic fluid and making the system ready for operation. The hydraulic pump sets the pressure to the clutch piston which compresses the clutch plates. The level of pressure set depends on the torque level needed and on the driving situation. In traction/high slip conditions, a high pressure is delivered providing partial or full all wheel drive. In tight curves (for example parking) or at high speeds, a much lower pressure is delivered providing partial or full front wheel drive.

CONTROL DIAGRAM

NOTE:

A = Hardwired; AM = High speed CAN chassis systems; AN = High speed CAN powertrain systems

1. Active on-demand coupling control module
2. Terrain Response (TR) switchpack
3. Gateway Module (GWM)
4. Engine Control Module (ECM)
5. Anti-lock Brake System (ABS) control module
6. Transmission Control Module (TCM)
7. Electric motor and hydraulic pump assembly
8. Ground
9. Fuse 30A - located in Rear Junction Box (RJB)
10. Ignition relay - located in Central Junction Box (CJB)
11. Fuse 5A - located in CJB

Rear DifferentialRear Drive Axle- Differential - Vehicles without- Active Driveline/  Diagnosis and Testing

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