Active clutch (Freelander 2)

NOTE: 3rd generation variant shown.

Pos.	spare part no	Name
1	-	Control valves - exhaust side
2	-	Active clutch module
3	-	Control valve / axial solenoid valve
4	-	Oil pressure and temperature sensor
5	-	input shaft
6	-	Electro-hydraulic pump
7	-	Oil filter
8	-	Receiver
9	-	Control valve - inlet side
10	-	Combined control and bypass valve
11	-	ring-shaped piston
12	-	Internal connecting hub with slots for external hub.
13	-	Wet clutch inner and outer discs

The active clutch provides the benefits of a permanent four-wheel drive system, along with the efficiency and economy of a non-permanent system. The clutch, located between the differential and the propeller shaft, is an independent unit that performs mechanical, hydraulic and electronic functions to distribute the drive force between the front and rear axles, with transparent automatic control.

The active clutch performs the following functions:

• Electronic control of torque transmission.
• Quick engagement, taking into account the required traction force.
• Quick shutdown to ensure that its operation will not distort wheel speed signals and interfere with the stability control system; this is especially important on very low friction surfaces.
• Pre-engagement from standstill to minimize wheel spin.
• There are no opposing forces when maneuvering and when parking the car.
• No sensitivity to brake tests on the chassis dyno.

4th Generation Active Clutch - vehicles from model year 2009:

• The 4th generation clutch does not have the differential speed driven pump used on the 3rd generation clutch, but has a higher capacity electric axial pump and a high pressure accumulator.
• Throttle servo valve and pressure sensor on the 3rd generation clutch have been replaced by a pressure reducing servo valve.
• The torque limiter pressure reducing valve on the 3rd generation clutch is replaced by an electronic control valve and control software.

The 4th generation active clutch has the following improvements over the previous 3rd generation clutch:

• Reduced base torque at high differential speeds.
• Torque activation is now based on differential speed.
• Precise torque limiter control.
• The energy stored in the pressure accumulator results in a reduction in the maximum current in the electric pump and a faster response.

Control module - 3rd and 4th generation couplings

The control module, mounted on the active clutch housing, forms a single unit with the control valve / axial solenoid valve. By analyzing information from other modules and vehicle sensors, the control module regulates the axial solenoid valve by controlling the pressure of the hydraulic fluid acting on the clutch discs. Some of the modules and sensors that the control module communicates with are listed below:

• Elements connected with wires:
  • Control valve / axial solenoid valve
  • Electro-hydraulic pump
  • Oil pressure and temperature sensor
• High speed CAN bus
  • Engine control module
  • Anti-Lock Brake/Traction Control Module
  • Traction control switch
  • Vehicle yaw sensor
  • Steering angle sensor

The axial solenoid valve continuously adjusts the output parameters of the control valve using a PWM signal (pulse width modulated). The pressure of the working fluid acting on the clutch discs determines the amount of torque supplied to the rear axle.


The active clutch has built-in oil pressure and temperature sensors so that the control module can accurately control torque transmission under all operating and environmental conditions. Using these signals, the control module applies strategies to protect the clutch from overheating; in extreme cases, to protect the clutch from damage, the clutch will disengage if the temperature of the hydraulic fluid exceeds 105°C. Normal operation of the clutch resumes when the temperature drops below 101°C.

The control module has a built-in diagnostic system that continuously monitors the active clutch system and its input and output signals. If the control module detects a malfunction, a DTC is generated (diagnostic trouble code). The DTC is accessed using the diagnostic system prescribed by Land Rover.

Electro-hydraulic pump

When driving on very low friction surfaces, such as wet grass, snow or ice, initial wheel spin and poor traction may occur. With an active clutch, the wheels can turn nearly 60 degrees before torque can be transmitted through the clutch.

On the 3rd generation clutches, to counteract this phenomenon, Land Rover has developed a unique high pressure precharge feature that activates the hydraulic circuit as soon as the engine is started. In fact, the electrically driven hydraulic pump is designed to maintain a potential torque of 500 Nm in the clutch. (This torque potential for the 4th generation clutch has been increased to 1500 Nm).

Vehicles equipped with Terrain Response have the added benefit of varying the preload level for optimum traction on various surfaces. The precharge level changes depending on the Terrain Response mode, for example:

• When the Terrian Response system is operating in the "Special Programs Disabled" mode, which is equivalent to the normal operation of vehicles without the Terrian Response system, when the vehicle is moving in a straight line with starting from a standstill, the 3rd generation clutch is programmed to transmit torque to the rear axle at a level 500 Nm, and the 4th generation clutch - to transmit torque at the level of 1500 Nm. This strategy minimizes the loss of traction at start-up, regardless of the nature of the road/terrain. As the vehicle accelerates, the pressure in the clutch is reduced to reduce fuel consumption.
• By being able to track the steering angle, the clutch can be programmed so that no torque is transmitted through it. This prevents the clutch from locking up when maneuvering the vehicle at low speeds and at sharp steering angles.
• In Grass/Gravel/Snow mode, the clutch is programmed to maintain pre-charge until much higher speeds are reached. The same conditions apply even when driving at low speeds and at sharp steering angles, because on low friction surfaces, traction takes precedence over clutch lockup.

For more information, see the chapter: Ride and Handling Optimization (204-06 Ride and Handling Optimization, Description and Operation).

Mechanical hydraulic pump - 3rd generation couplings

The cardan shaft is connected to the front clutch disc assembly (primary), the rear clutch plate assembly is connected to the differential drive gear (secondary). The swash plate with 6 hydraulic rollers is also connected to the differential drive gear. When there is no difference in speed between the input and output sides of the clutch, the rollers are inactive.


However, when the front and rear axles begin to rotate at different speeds, the wobble plate rotates relative to the rollers, which creates hydraulic pressure. This pressure serves to force the opposite clutch discs into alignment, resulting in increased torque transmission to the rear axle. As the speed difference between the axles increases, the hydraulic pressure further pushes the clutch discs against each other to increase torque transfer to the rear axle.

The control valve/axial solenoid valve controls the amount of pressure acting on the clutch discs and therefore the amount of torque transmitted to the rear axle. Tight manufacturing tolerances and exceptionally low component wear guarantee precise torque control throughout the life of the vehicle.

Mechanical Hydraulic Pump - 4th Generation Couplings

The 4th generation clutch does not use a swashplate to mechanically increase hydraulic pressure, but instead uses a new hydraulic pump to generate hydraulic pressure and bring the clutch discs together Removing the swashplate increases the total surface area of the clutch discs, which in turn reduces requested hydraulic pressure. The pressure required to provide a torque of 1500 Nm has been reduced from 100 bar for the 3rd generation coupling to 40 bar for the 4th generation couplings.

The inlet and outlet position on the 4th generation coupler remains the same as on the 3rd generation coupler as described above.

Bypass valve

On very low friction surfaces, a driveline braking torque can be generated, for example:

• reverse torque when braking motors; or
• forced movement of the cardan shaft by the front wheels.

This can affect rear wheel speed, preventing true rear wheel traction from being determined because the wheel speed signal is distorted. To eliminate this possibility, an immediate opening of the clutch is provided when the directional stability system is activated. For this purpose, a bypass valve is used, which immediately reduces the pressure in the system to the nominal level.

To balance the base pressure of 4 bar (see below for details), the 3rd generation clutch uses a large belleville spring to spread the clutch discs to prevent transmission of torque through the clutch. Even at 0°C, torque transmission drops from 300 Nm to zero within 10 ms. However, the 4th generation clutch does not require a Belleville spring to spread the clutch discs because the clutch does not have enough base pressure to bring the discs together.

Receiver

The greater the distance that the clutch discs must be moved to make contact, the longer it takes to displace the hydraulic fluid needed to build pressure and transmit torque. To counteract this, a reservoir is provided in the 3rd generation clutch. It maintains a nominal pressure of 4 bar in the hydraulic circuit. While this pressure is not sufficient to transmit significant torque through the clutch, it forces the discs very closely together, requiring very little fluid displacement to ensure full engagement and maximum torque transfer. Full torque transmission can be achieved in 150ms.

On the 4th generation clutch, an activation time of 150ms is achieved by using a belleville spring which acts to compress the discs together (without initiating transmission of significant torque through the clutch). Since the pump is not constantly used to generate the base pressure (as it was on the 3rd generation clutch), provides an improvement in fuel economy.

Wet clutch disc kit

The clutch disc set consists of 7 pairs of discs, the inner discs are made of hardened steel, the outer discs are made of sintered steel. The clutch discs work in the transmission fluid.

Torque transmission through the clutch disc set is limited to 1500 Nm. Due to this, low gears retain the front-wheel drive element for stable traction. In high gears, the clutch is theoretically capable of transmitting all the drive force to the rear axle, although extreme conditions must be present for this to happen.