Sorry, I stand corrected, there is precharge in GEN3, but the following still does not answer the original reverse question.

Extract from Workshop Manual 'REAR DRIVE AXLE / DIFFERENTIAL

The active on-demand coupling provides the benefits of a permanent 4x4 system with the efficiency and economical benefits of a part-time system. Located between the differential and driveshaft, the coupling is a self-contained unit combining mechanical, hydraulic and electronic functions to distribute drive between the front and rear axles with transparent automatic control.

The active on-demand coupling provides the following functions:
Electronic management of torque transfer. Rapid engagement in response to traction demands. Rapid disengagement to ensure its operation cannot corrupt wheel speed signals and compromise stability control system operation; this is especially important on very low friction surfaces. Pre-engagement from rest to minimize the risk of wheel-spin. No opposing forces when maneuvring or parking the vehicle.
Not sensitive to brake testing on a chassis dynamometer.

Generation 4 Active On-Demand Coupling - Vehicles from 2009MY
The generation 4 coupling does not have the differential speed driven pump used on the previous generation 3 coupling, but is fitted with a bigger capacity electrically operated axial pump and a high pressure accumulator. The proportional throttle valve and pressure sensor on the generation 3 coupling is replaced with a proportional pressure reducing valve. The torque limiter pressure reducing valve on the generation 3 coupling is replaced by an electronic control valve and controlling software.

The generation 4 active on-demand coupling gives the following improvements over the previous generation 3 coupling:
Reduced base torque at high differential speeds Torque activation is now independent of differential speed Accurate torque limiter control Energy stored in high pressure accumulator giving a low maximum current consumption of the electric pump and faster response.

Control Module - Generation 3 and 4 Couplings
The control module, attached to the casing of the active on-demand coupling, forms a single unit with the control valve/axial solenoid. By analyzing information from other vehicle modules and sensors the control module regulates the axial solenoid to control the hydraulic fluid pressure supplied to the clutch plates. Some of the modules and sensors the control module communicates with are listed below:
Hardwired: - Control valve / axial solenoid - Electric hydraulic pump - Oil pressure and temperature sensor

High speed CAN (controller area network): - Engine control module - Anti-lock brake system / traction control module - Traction response switch - Yaw rate sensor - Steering wheel rotation sensor

The axial solenoid constantly adjusts the control valve output using a Pulse Width Modulation (PWM) signal. The fluid pressure delivered to the clutch plates determines the amount of torque that is delivered to the rear axle.

The active on-demand coupling has integrated oil pressure and temperature sensors to enable the control module to accurately manage the torque transfer under all environmental and operating conditions. Using these signals the control module will use strategies to protect the coupling from overheating; in extreme cases to protect the coupling from damage the coupling will disengage if the temperature of the hydraulic fluid exceeds 105°C. The coupling will return to normal functionality when the temperature falls below 101°C.

The control module has an integrated diagnostics system, which constantly monitors the active on-demand coupling system as well as its input and output signals. If the control module detects a fault a Diagnostic Trouble Code (DTC) is stored. The DTC is accessed using the Land Rover approved diagnostic system.

Electric Hydraulic Pump

When negotiating very low friction surfaces such as wet grass, snow or ice; initial wheel-spin can cut into the surface and reduce grip. With a re-active on-demand coupling, almost 60 degrees of wheel rotation would occur before torque could be transmitted through the coupling.

On Generation 3 couplings, to counteract this Land Rover developed a unique high-pressure pre-charge facility which energizes the hydraulic circuit as soon as the engine is started. Essentially an electrically operated hydraulic pump was designed to maintain a potential of 500 Nm (369 lb ft) of torque pressure within the coupling. (This pre-charge torque capacity has been increased to 1500Nm (1106 lb ft) for the Generation 4 coupling.)

Vehicles fitted with Terrain Response also add further benefits by varying the level of pre-charge to deliver optimum traction over a range of different terrain surfaces. The level of pre-charge is varied depending on the particular terrain response mode, for example:
Terrain response in 'Special Programs Off' mode as common with vehicles without terrain response, the coupling is programmed to transmit 500 Nm 369 lb ft of torque on Generation 3 couplings and 1500 Nm (1106 lb ft) of torque on Generation 4 couplings to the rear axle when the vehicle moves from rest in a straight line. This strategy minimizes traction loss from a standing-start regardless of the terrain. When the vehicle accelerates the pressure in the coupling is decreased to improve fuel economy. The ability to sense the steering angle allows the coupling to be programmed to provide no torque transfer through the coupling. This prevents the coupling locking when the vehicle is maneuvring at low speeds and acute steering angles. In 'Grass / Gravel / Snow' mode the coupling is programmed to maintain its pre-charge state until much higher speeds are obtained. The same applies even if the vehicle is traveling at low speeds and acute steering angles, as traction takes precedence over coupling lock-up on low-friction surfaces.

Mechanical Hydraulic Pump - Generation 3 Couplings - Vehicles up to 2009MY
The driveshaft is attached to the coupling's front clutch plate assembly (input), with the rear clutch plate assembly connected to the differential pinion (output). A swash-plate with 6 hydraulic rollers is also attached to the differential pinion. When there is no speed difference between the coupling's input and output, the rollers do not function.

However, when the front and rear axles start to rotate at different speeds, the swash-plate rotates relative to the rollers which generates the hydraulic pressure. This pressure is used to force the opposing clutch plates together, increasing the transmission of torque to the rear axle. As the difference in axle speed increases the hydraulic pressure pushes the clutch plates further together to increase the torque to the rear axle.

A control valve/axial solenoid controls the amount of pressure applied to the clutch plates, and hence the amount of torque transmitted to the rear wheels. Close manufacturing tolerances and exceptionally low component wear ensure torque control remains accurate throughout the vehicle's life.

Mechanical Hydraulic Pump - Generation 4 Couplings - Vehicles from 2009MY

The Generation 4 coupling does not use the swash plate to mechanically raise hydraulic pressure; instead, a new hydraulic pump is used to generate hydraulic pressure and force the clutch plates together. The removal of the swash plate allows a larger overall clutch plate surface area, which in turn reduces the hydraulic pressure requirement. The pressure required to achieve 1500Nm (1106 lb ft) has been reduced from 100 Bar for Generation 3 couplings to 40 Bar for Generation 4 couplings.

The positioning of the Generation 4 coupling's input and output remains the same as the Generation 3 coupling, as described above.

By-pass Valve

On very low friction surfaces, driveline drag torque can occur, for example:
reverse torque from engine braking, or forced movement of the driveshaft by the front wheels.

This can influence rear wheel speed, making it impossible to determine the true friction capability of the rear wheels, by distorting the wheel speed signal. To prevent this, the active on-demand coupling is designed to open immediately in response to a stability control event. This is achieved by a by-pass valve instantly reducing system pressure to nominal.

To balance the 4 bar base pressure (see below for details), the Generation 3 coupling uses a large Belleville spring to force the clutch plates clear of each other to prevent torque transmission through the coupling. Even at 0°C, torque transmission is reduced from 300 Nm to Zero within 10 ms. The Generation 4 coupling however, does not require a Bellville spring to separate the plates as the coupling does not have a significant base pressure pushing the plates together.

Accumulator

The further the clutch plates have to move in order to contact each other, the longer it takes to displace the hydraulic fluid necessary to build pressure and transmit torque. To counter this, the Generation 3 coupling incorporates an accumulator. This retains a nominal 4 bar pressure within the hydraulic circuit. Although this is not enough pressure to cause significant torque transmission through the coupling, it forces the plates very close together so that very little fluid displacement is required to achieve full engagement and maximum torque transfer. Full torque transmission can be achieved in 150 ms.

On Generation 4 couplings, the 150ms activation time is achieved through the use of a Bellville spring that acts to push the plates together (without causing significant torque transmission through the coupling). As the pump is not continuously used to create this base pressure (as it was with Generation 3 coupling), improvements in fuel economy have been achieved.

Wet Clutch Pack

The clutch pack is made up of 7 pairs of plates; the inner discs are produced from hardened steel with the outer discs manufactured from steel with a sintered face. The clutch plates operate in transmission fluid.

Torque transmission across the clutch pack is limited to 1500 Nm (1106 lb ft). This ensures the lower gears retain an element of front-wheel-drive for traction stability. Within the higher gears the coupling is theoretically capable of transmitting all the drive to the rear axle; although conditions would have to be extreme for this to occur.

PRINCIPLES OF OPERATION

Generation 3 Couplings - up to 2009MY
An internal electronically-controlled pump provides hydraulic pre-charge pressure within the coupling. The pre-charge pressure supplies the required operating pressure to the clutch plates to eliminate initial wheel-spin as the vehicle accelerates from standstill.

In conjunction with the pre-charge pressure a mechanical hydraulic pump operates within the clutch plates to supply the coupling's main hydraulic operating pressure. The mechanical pump is functioned by the 'input' and 'output' of the coupling:
input - driveshaft connection from the front axle, output - differential connection to the rear axle.

Any speed difference between the front and rear axles will start the operation of the mechanical hydraulic pump. The amount of hydraulic pressure applied to the clutch pack by the pump determines the gap between the clutch plates. For example, the greater the hydraulic pressure, the smaller the gap between the plates and subsequently the greater the torque transmitted through the coupling from the front axle to the rear axle.

This main hydraulic pressure is designed to transmit the torque for traction demands of off-road driving, and to provide lock-up as required.

Generation 4 Couplings - from 2009MY

The Generation 4 coupling still uses an electrically-controlled pump to provide hydraulic pre-charge to eliminate wheel-spin from standstill; however, the same pump is also used to provide the coupling's main hydraulic operating pressure.
The principles of transmitting torque through the clutch plates remains the same, as described above for the Generation 3 coupling. FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

Do you ever get the feeling that this is all too complicated and that what you really want is a lo-box and diff lock? 3 x FL1 2 manual + 1 auto
5 x FL2 4 manual + 1 auto
Now Discovery Sport P250 MHEV SE

Hell yeah, reading that explanation makes me feel that the car's 4wd ability is being decided on the move by a committee. There are hints in the article where it may not be locked on purpose so as not to hinder common things like reverse parking.
As I said, there was no need to use rescue tracks or jack anything up. If you let your tyres down to around 15psi you'll drive out of a lot sand bogs. That actually happened at Lake Eyre where a German tourist perished while walking the 60kms for help after bogging her rented 4wd. The local copper let some air out of the tyres and drove it straight out.

Even if you all you have was a bit of rope in the car & can't be towed, cut it into a few lengths & weave it through as many wheels as possible to create something like a snow chain. if the rope is destroyed in 10 metres so be it. it's only a few dollars down the drain. It is more than likely enough to get you out of trouble.

If your life is in danger cut out the seat belts & use them. Better than been dead! Procrastination, mankind's greatest labour saving device!

Acoustic insulation ARB TPMS 3xARB air compressors After cooler Air tank On-board OCD pressure air/water cleaning Additional 50L fuel Carpet in doors ABE 2x1kg Waeco 28L modified fridge Battery 4x26ah Solar 120w Victron MPPT 100/20 DC-DC 18amps 175amp jumper plug Awning 6x255/60R18

but then the highway coppers do you for not wearing seat belts and your life may as well be over anyway.....

dorsetfreelander wrote:

Do you ever get the feeling that this is all too complicated and that what you really want is a lo-box and diff lock?

Or you could just have "Full Time Synchronised All Wheel Drive" like my daily drive  2009 GS Auto Zermatt Silver - Sold June 21 after 10 years of ownership

2016 Subaru Outback SE 2.0 diesel SE Premium Lineartronic Sold 2024 after 8 years and 80k miles . Best Car I ever owned !

2023 Toyota Hilux invincible X 2.8 Auto .

I'm a member of the Subaru Club here and been out on trips with quite a few and been underwhelmed with the traction system in them sorry to say. I've owned 2 in the past, nice cars but I haven't witnessed the TC live up to the hype.