Hydraulics, Hydraulic System, Components and Function

Hydraulics, Hydraulic System, Components and Function

Hydraulic system

Hydraulic diagram (up to transfer case no.26 616)

Hydraulic diagram (as of transfer case no.26 617)

Function of hydraulic system The hydraulic system comprises the following components:
- Oil reservoir (2)
- Pressure oil pump (12)
- Service valve (15, production up to 05/91)
- 4MATIC valve control unit (A7/2)

The pressure oil pump (12) delivers oil from the oil reservoir (2) to the connection Po on the service valve (15).
In the operating position of the service valve, the oil flows to the connection of the pressure line P on the 4MATIC valve control unit (A712). The accumulator charging valve (a) holds the system pressure in the pressure reservoir (4) between approx. 25 and approx. 36 bar. The front axle drive train (AV), center differential lock (ZS) and rear axle differential lock (HS) valves (A7/2y1, 2, 3) are controlled by the electronics. Via them, the multi-disk clutches in the transfer case (1) and the ring cylinders on the rear axle differential (ASD) (5) are subjected to system pressure.
Where:
A7/2y1 Front axle drive train valve
A7/2y2 Center differential lock valve
A7/2y3 Rear axle differential lock valve

If the accumulator (4) is full, the delivered oil flows back to the service valve (15) via line N from the 4MATIC valve control unit (A7/2). On vehicles with level control, the level controller (6) is connected to line N.
The pressure limiting valve (ISa) in the service valve (15) ensures a residual pressure of approx. 5 bar in line N. In this way the contact pressure of the multi-disk clutch of the front axle drive train (AV) in the transfer case (1) of 1.3 bar can be prepared in any ready to operate condition. The further return from the service valve (15) to the oil reservoir (2) takes place via the line To. Oil from the 4MATIC valve control unit (A7/2) flows back via the return line T when:
- the pressure reduces in the multi-disk clutches,
- the pressure limiting valve (b) in the 4MATIC valve control unit (A712) opens.
If the service valve (15) is in the test position, oil flows from connection Po immediately to the oil reservoir (2) via line T1. Simultaneously the system pressure is reduced via lines M and To.

Oil reservoir

The plastic oil reservoir (arrow) is located in the front left of the engine compartment.

The oil reservoir (2) has connection pieces for:
- the suction line of pressure oil pump (A), and
- the return line from the service valve (To) and
- the return line from the 4MATIC valve control unit (A712) (T). The returning oil is cleaned by the filter element (2h).

The sealing plug (2a) has an oil dipstick with "MIN" and "MAX" markings for the oil level check.

Pressure oil pump

The engine drives the pressure oil pump. This supplies the hydraulic system with pressure oil.

Service valve (up to transfer case no.26 616)

The service valve is attached at the front right wheelhouse. The 4MATIC can be rendered inoperative with the service valve. By relocating the lever into test position (A) the complete hydraulic system is switched to a depressurized condition.

As of transfer case no.26 617 the previous service valve for switching off the system has been replaced by an electrical normal operationi service selection switch (S7/3). The selection switch is located on the inside of the components compartment panel on the right. To switch off the system, the coding plug must be withdrawn and reinserted in "Test" position. The ASD/4MATIC warning lamp comes on and indicates that the system has been rendered inoperative.

Caution! To insert the coding plug in the "function" position, switch off ignition, otherwise the ASD/4MATIC warning lamp does not go out.

As of transfer case no.26 617 the accumulator pressure must be relieved at the pressure relief screw (16) to switch the hydraulic system to a depressurized condition.

In operating position (B) the pressure oil is supplied from connection (P0) through the rotary-disk valve (15b) and via the line (P) of the valve control unit.
The pressure limiting valve (15a) holds a pressure of approx. 5 bar in the line (N).
The connection (Mo) is connected to the accumulator of the 4MATIC valve control unit (A712) via the lines P and M. Thus the system pressure (pressure at accumulator) can be checked by pressure testing at connection (Mo).

In test position (A) the rotary-disk valve (15b) connects the connection (Po) to the return line (T1). The pressure oil delivered from the pressure oil pump flows back directly into the oil reservoir via the service valve thus interrupting the pressure oil supply for the level control system. Simultaneously the pressure line (P) is connected to the return line (To) to the oil reservoir via a groove on the rotary-disk valve (15b). In the test position, the rotary-disk valve opens the check valve (15c), the contents of the accumulator flow through a ring gap on the rotary-disk valve (15b) into the return line (To) via the return line (M). Thus the pressure in the complete hydraulic system is reduced.

4MATIC valve control unit (A712)

The 4MATIC valve control unit (A712) is located on the rear right of the frame floor under a plastic cover (arrow).

The connection points for the lines are identified by cast letters on the 4MATIC valve control unit (A7/2).
Where:
AV - Front axle drive train
ZS - Center differential lock
HS - Rear axle differential lock
M - Measuring connection (service valve)
N - Return via service valve/level controller
T - Return to oil reservoir
P - Pressure oil

Design of 4MATIC valve control unit (up to transfer case no.26 616)

Design of 4MATIC valve control unit (as of transfer case no.26 617)

The measuring connection (M) which was previously located on the service valve is now on the 4MATIC valve control unit (A7/2).

Service valve (up to transfer case no.26 616)

As of transfer case no.26 617 the accumulator pressure must be relieved at the pressure relief screw (16) to switch the hydraulic system to a depressurized condition.

The 4MATIC valve control unit (A712) is located on the rear right of the frame floor under a plastic cover (arrow).

Design of 4MATIC valve control unit (as of transfer case no.26 617)

The measuring connection (M) which was previously located on the service valve is now on the 4MATIC valve control unit (A7/2).

Accumulator (4)

System pressure is stored in the accumulator (4). The accumulator volume is approx. 350 cm3. There is a pressure of approx. 21 bar in the gas compartment filled with nitrogen (b). At a system pressure (25 to 36 bar) this results in a working volume of approx. 40 cm3 in the oil compartment (a).

Oil pressure switch (A7/2s1)

If the pressure drops below 5 bar, the oil pressure switch (A7I2s1) switches ground to 4MATIC control unit (N3013), which switches off the 4MATIC. This is indicated when the 4MATIC warning lamp (Al e24) comes on.

Solenoid valves - AV, ZS, HS (A7/2y 1,2,3)

The 3 solenoid valves are located in the 4MATIC valve control unit (A7/2).
They act as 2-way valves. If there is no voltage at the solenoid valve (A7/2y), the hydraulic line from the respective multi-disk clutch (AV, ZS and HS) is connected to the oil reservoir by the return line (T). The oil pressure in the clutch is reduced.
If the solenoid valve is supplied with voltage when the system engages, the magnet core (a) is moved against the force of the spring (b). The system pressure opens the ball valve (c) and acts on the multi-disk clutch, and simultaneously the return line (T) is sealed by the ball valve. If the solenoid valve is switched off, the spring force presses the magnet core (a) back into its initial position, thus the ball valve opens the return line (T) and seals the supply line from the accumulator (P).

Accumulator charging valve (a)

The accumulator charging valve (a) controls the pressure in the accumulator between approx. 25 and 36 bar.

Pressure limiting valve (b)

The pressure limiting valve (b) prevents the accumulator pressure rising above 38 bar.

Oil flow limiting valve and pressure regulator (production up to 05/91)

The multi-disk stack of the multi-disk clutch for the front axle drive train (AV) is pre-loaded with the contact pressure of 1.3 bar. In this way a rapid and gentle engagement of shift stage 1 is achieved.

Pressure-dependent orifice (production up to 05/91)

The pressure-dependent orifice allows a rapid but at the same time gentle power transmission through the multi-disk clutch of the central locking differential.
It modulates the shift pressure as a function of system pressure.
Above 22 bar: rapid pressure reduction.
Below 22 bar: slow pressure reduction.

Check Valve

The check valve fulfils a safety function. If no pressure builds up in the multi-disk clutch of the front axle drive train (AV), the check valve prevents a pressure build up in the multi-disk clutch of the center differential lock (ZS). This prevents an interruption of the power flow and overspeeding of the engine.