Hydraulic function, overview (2006-)

1. Cam disc
2. Rollers
3. Pressure valves
4. Pressure limiting valve
5. Control valve cover
6. Oil pressure and temperature sensor
7. Differential Electronic Module (DEM)
8. Control valve/axial solenoid
9. Accumulator
10. Oil screen
11. Electrical feed pump.
12. Oil filter
13. Suction valve
14. Piston pump
15. Operating piston
16. Wet plate unit
17. Bearing
18. Springs.
19. Relief and by-pass valve

The Active On demand Coupling (AOC) can be described as a hydraulic pump.

The basic pressure is generated by the electronic feed pump. This pressurizes the annular pistons with oil so that they are pressed against the cam disc via rollers. The annular piston generates a working pressure which is led to the operating piston via the pressure valve. This pressure creates a fixed connection between the input and output shafts. The maximum pressure is limited by the pressure limiting valve. The difference in speed between the input and output shafts is in proportion to the oil pressure fed to the annular pistons. A large difference between the input and output shafts results in high oil pressure to the annular pistons. If the input and output shafts are at the same speed, the oil pressure to the annular pistons is less. The oil pressure at the multi-plate clutch is regulated by the control valve. The axial solenoid controls the control valve. A closed control valve gives maximum pressure to the multi-plate clutch, resulting in maximum power gearbox. An open control valve gives minimum pressure to the multi-plate clutch, resulting in limited power gearbox.

The accumulator maintains the basic pressure in the system. The oil filter keeps the fluid clean by filtering dirt and small particles which could damage the system.

Pretensioning 

The function of the four-wheel drive system is based in there being a difference in speed between front and rear wheels. This difference in speed can affect traction performance in certain driving situations. The role of the pretensioning function is to improve traction performance, primarily in starting off from being stationary, without affecting other properties of the system to achieve this.

The pretensioning function has been achieved by replacing one of the AOC unit suction valves with a new combined relief and by-pass valve, extra oil ducts in the housing and adapted software in the differential electronic module (DEM).

As before, the electronic feed pump starts as soon as the engine has started. Running the pump at a higher speed than before generates increased oil flow. The increased oil flow combined with the new relief and by-pass valve results in a pressure increase that can work on the clutch discs. Greater engine torque can thereby be transferred to the rear axle. The pretensioning function is controlled by software so that it is only activated when necessary.