4
Left-hand clutch and pump unit
6
Right-hand clutch and pump unit
Layout
In two-wheel drive operation, power is transmitted to the rear axle.
The torque is transmitted continuously to the intermediate shaft via the right-angle drive.
The intermediate shaft drives the left-hand clutch and pump unit.
In all-wheel drive mode the halfshafts of the front wheels are connected non-positively to the transmission output shaft via the clutch and pump units.
Function during normal straight-ahead driving
Under normal driving conditions, only the rear axle is driven. In this operating state, the intermediate shaft and the front-axle halfshafts rotate at identical speed. The relative movement of the two shafts is zero.
Thus, the pump unit is not able to generate sufficient fluid pressure. The multi-plate clutch cannot establish a non-positive connection between intermediate shaft and halfshaft. All of the drive torque is directed to the rear axle.
Function during cornering
During cornering, the front and rear wheels rotate at slightly different speeds, due to the differing curve radii at the front and rear axles. This results in the intermediate shaft and the halfshafts rotating at different speeds. The relative movement of the two shafts is greater than zero.
The pump unit generates a low fluid pressure, which acts on the multi-plate clutch. However, a non-positive connection between the intermediate shaft and the halfshafts cannot be established.
Function with slipping rear wheels
If there is a loss of traction at the rear axle, there is a large difference in speed between front and rear axle (slip). This results in the intermediate shaft and the halfshafts rotating at widely differing speeds. The relative movement of the two shafts generates fluid pressure in the pump unit.
This fluid pressure generates an axial force on the plate set. The clutch and pump unit makes a non-positive connection.
The torque is distributed to the front axle and the rear axle.
Torque distribution between front and rear axles
B
Speed difference – intermediate shaft and halfshafts
Drive
Torque – front axle
The torque distribution between front and rear axle depends upon the speed difference between the intermediate shaft and the halfshafts.
If a specified value (40 rpm) is exceeded, then the torque is transmitted to the front as well as the rear axle (point F).
With increasing speed difference, the torque can be transmitted almost completely to the front axle (point G).
In this case, the rear wheels rotate at high speed. However, due to the high slip, they are not capable of transmitting the torque to the road surface.
Therefore, on poor road surfaces, the torque is transmitted to the wheels which have the better adhesion.
Power Flow
1
Clutch and pump unit ring gear
2
Output drive pinion, right-angle drive output shaft
5
Input drive pinion, right-angle drive mainshaft
The torque is transmitted to the right-angle drive via direct take-off from the 3rd gear wheel.
The idler gear prevents reversal of the direction of rotation.
Power is transmitted to the ring gear of the clutch and pump unit via the output shaft drive pinion.
Transmission control during braking
2
Pressure switch, all-wheel drive warning indicator
In order to ensure driving stability, all-wheel drive mode is interrupted during braking.
Solenoid valves are located in the clutch and pump unit housings (shown on the left in the illustration), which control the prevailing pressure there.
Through their actuation, the pressure can be reduced abruptly. The disengaging multi-plate clutches then interrupt the non-positive connection between the intermediate shaft and halfshafts.
The all-wheel drive warning indicator is activated by a pressure switch.
Solenoid valve intervention
The right-angle drive consists of idler gear, main shaft and output shaft. It transmits the engine torque from the 3rd gear of the manual transmission to the ring gear of the left-hand clutch and pump unit. This produces a power deflection through 90 degrees.
The transmission ratio of the right-angle drive enables a speed ratio between the front and rear axle of 1:1.
The idler gear, mainshaft and output shaft are only available in service as a pre-assembled unit with the front transmission housing..
2
Clutch and pump unit ring gear
The front and rear axles are non-positively connected via the clutch and pump units.
The ring gear connects the left-hand clutch and pump unit to the right-angle drive. The intermediate shaft transmits the applied torque to the right-hand clutch and pump unit.
The clutch and pump unit can only be replaced completely during servicing.
The function of the left and right-hand clutch and pump units is identical.
The clutch and pump unit can only be replaced completely during servicing.
2
Clutch and pump unit housing
The plate set, which is immersed in fluid, consists of inner and outer plates.
The inner plates and the stub axle are each provided with splines, which permit the transmission of torque.
The teeth of the outer plates engage in the internal grooves of the clutch and pump unit housing.
If the oil pressure acts via the working piston on the plate set then a non-positive connection is formed between the outer and inner plates.
When the plate set is pressed together, a non-positive connection is established between the housing of the clutch and pumping unit and the stub axle.
The transmission fluid of the clutch and pump unit is filtered in order to protect the plate set from contamination.
2
Clutch and pump unit housing
The rotor pump is a positive displacement pump.
It generates the required operating pressure in the clutch and pump unit.
In the event of differences in rotational speed between the input shaft and the intermediate shaft, the relative movement between the sub axle and the housing of the clutch and pump unit provides a rotational movement of the internal rotor. This causes a continuous displacement of a volume of oil and thus builds up oil pressure.
3
Pressure-differential valve
The working piston controls the engaging and disengaging of the plate set. The piston housing and the working piston form a single unit, in which the working piston can move axially.
The oil pressure directed into the piston housing exerts an axial force on the working piston.
The axial force is therefore proportional to the oil pressure of the rotor pump and thus depends upon the difference in speed between the front and rear axles.
This speed difference permits smooth engaging and disengaging of the plate set.
The pressure-differential valve provides the connection between the high-pressure and low-pressure sides.
If the speed difference between the intermediate shaft and the halfshaft is not more than 40 rpm, the spring-loaded differential pressure valve remains open.
The fluid pressure generated on the high-pressure side escapes to the low-pressure side. All-wheel drive mode is prevented.
If a larger speed difference occurs, the pressure-differential valve closes. On the high-pressure side, a corresponding oil pressure is built up which acts via the working piston on the plate set.
5
Wiring harness connector
The solenoid valve opens or closes a bypass bore in the housing.
If the brake pedal is actuated in all-wheel drive mode, the solenoid valve opens. This reduces the pressure in the working piston.
The non-positive connection to the front axle is interrupted as a result.
The solenoid valves can be renewed during servicing.
