3

Output shaft 2
643_048

2nd gear

R

4

1
5

K2 K1

2

6

3

7
643_049

32

3rd gear

R

4

1
5

K2 K1

2

6

7

3

643_050

4th gear

R

4

1
5

K2 K1

2

6

3

7
643_051

33

5th gear

Pinion shaft

Gearbox unit 1
Gearbox unit 2
Input shaft 1
Input shaft 2

R

1
4

Output shaft 1

5

K2 K1

PTO shaft for front final drive

2

7

6

3

Output shaft 2
643_052

6th gear

R

1
4

5

K2 K1

2

6

3

7
643_053

34

7th gear

R

4

1
5

K2 K1

2

7

6

3

643_054

Reverse gear

R

4

1
5

K2 K1

2

6

3

7
643_055

35

Parking lock – mechanical function
Because there is never any drive gear engaged when the
engine is not running (both clutches K1 and K2 are not
engaged), the OBZ transmission requires a parking lock.
The parking lock consists of a parking lock gear and a locking
pawl that is actuated by a spring loaded, tapered sliding
sleeve.
Unique to the OBZ transmission, is that the parking lock gear
is not located on the gearbox output shaft, as is normally the
case, but instead is integrated in sliding collar R/4. See page
29.

Engaging the parking lock – P-ON
The parking lock is engaged by the force of the parking lock
spring. The parking lock spring forces the parking lock piston
together with arrester element and tapered sliding sleeve
between the bracing plate and the locking pawl. As a result,
the locking pawl is pressed against the parking lock gear. The
claw engages in the teeth of the parking lock gear and locks it
in position.

Parking lock piston
Locking element with shoulder and holder for sensor magnet

Parking lock spring

Striker block
Spring-loaded tapered
sliding sleeve
Pawl
Pivot mounted in gearbox
housing

Resetting spring

Parking Lock Solenoid
N486

Parking Lock Solenoid Valve
N573 – Parking lock engagement valve

643_078

Hydraulic pressure acts on
parking lock piston

Disengaging the parking lock – P-OFF
The parking lock is disengaged by the
hydraulic force of the parking lock piston.
The parking lock piston pulls the tapered
sliding sleeve back against the force of the
parking lock spring. The return spring
forces the locking pawl out of engagement
with the gear teeth and holds it in that
position.

643_079

36

1st step

Engaging the parking lock if pawl and gear do not mesh
1. If the claw of the locking pawl clashes with a tooth of the
parking lock gear, the tapered sliding sleeve is still forced between
the bracing plate and the locking pawl by the preloaded spring
force. The tapered sleeve is spring-loaded and presses the locking
pawl against the tooth of the parking lock gear.

Tapered sliding
sleeve – springloaded

2nd step

Claw and tooth
clashing

643_080

2. As soon as the vehicle rolls a little and the parking lock
gear rotates slightly, the locking pawl snaps into engagement
with the parking lock gear because the tapered sleeve is
preloaded by the force of the spring. The parking lock is
engaged.
643_081

Holding the parking lock in position P-OFF with Parking Lock Solenoid N486
A - Keeping parking lock disengaged
If the parking lock needs to remain disengaged after the
engine is switched off, the parking lock piston has to be held
in the P-OFF position by the Parking Lock Solenoid N486.

In that position, the 3 ratchet springs of N486 engage with
the locking tip of the arrester element and so lock the parking
lock piston in the P-OFF position.

To hold the lock in position P-OFF, N486 remains energized
after the engine is switched off. That means that the armature of N486 remains pulled in against the force of the spring
(Figure A).

More information on activating the transmission setting
P-OFF can be found on page 12.

Ratchet spring

A

Arrester element with
parking lock piston

B

Snap hook

Piston spring

Pistons

Locking element

B - Engaging the parking lock
N4

86

If N486 is de-energized, the armature of N486 moves out (to
the right) under the force of the armature spring. As a result,
the ratchet springs are forced apart and release the arrester
element of the parking lock piston. The parking lock piston is
thus unlocked and the parking lock spring forces the parking
lock piston into the P-ON position.

643_082
37

Parking lock – electro-hydraulic function
Normal position – Parking lock ON – Engine OFF
The following circuit diagram shows the system in pressurized and depressurized condition.
The spring in the parking lock piston ensures that the parking
lock is engaged.
A hydraulic circuit with spring-loaded self-holding valve (SHV)
and an additional holding solenoid (N486) for the P-OFF
position make sure that the parking lock cannot be unintentionally engaged when the vehicle is moving.

Parking lock gear

WhenParking Lock Solenoid Valve 2 N574 and Parking Lock
Solenoid N486 are de-energized, the parking lock always
remains engaged. In that circuit setting, the pressure line of
the parking lock piston has an open connection to the sump
via the self-holding valve (SHV) and is not pressurized.
The basic principle is:
When it has no oil pressure and power supply, the parking
lock is always engaged (parking lock PN).
Solenoid valve N574 is only briefly operated. The self-holding
valve retains whichever switching condition is initiated.

Parking lock piston

Parking lock module

Auxiliary hydraulic module

Key:
N486 Parking Lock Solenoid
– PHM – Parking lock holding solenoid
N573 Parking Lock Solenoid Valve
– PEV – Parking lock engagement valve
N574 Parking Lock Solenoid Valve 2
– PAV – Parking lock disengagement valve
SHV Self-holding valve
A1
A2
A3

SHV piston face A1
SHV piston face A2
SHV piston face A3

Primary pressure
supply
643_083

38

Parking lock ON – engine running
Even if the primary pressure is present when the engine is
running, the self-holding valve remains in its normal position
and keeps connection between the parking lock piston
pressure line and the sump open and, therefore, depressurized. When P-ON is active, N573 remains energized until the
engine is switched off.
That means the connection between the parking lock piston
and the sump is open. This prevents unintentional pressure
build up that would result from the disengagement of the
parking lock.

The following relationships apply to the piston faces:
1. Face A2 is smaller than face A1.
2. Faces A2 and A3 combined are larger than face A1.

A larger face (A) = Greater force
In the situation illustrated in Figure 643_084 – primary
pressure applied to A2 and A1 – the SHV remains in the lefthand position because the force acting on piston face A1 is
greater than that on A2.

The self-holding valve, SHV
The SHV has a spring-loaded valve with 3 different-sized
piston faces – faces A1, A2 and A3. Depending on the switching position of the solenoid valves N573 and N574, either the
primary pressure (equal pressure) or no pressure is acting on
those faces. Since the same pressure acts on all the piston
faces, different forces result and enable the valve to be
moved to the desired setting.

Parking lock module

Auxiliary hydraulic module

Primary pressure
supply

643_084
39

Disengaging the parking lock and holding the P-OFF setting
To disengage the parking lock, primary pressure is applied to
the packing lock piston. The hydraulic force is many times
greater than the force of the parking lock spring, so the
parking lock piston is forced back together with the tapered
sleeve. The return spring of the locking pawl lifts the pawl out
of engagement with the parking lock gear and so releases the
parking lock. See Figure 643_079 on page 36.

Parking lock module

Electro-hydraulic function
Step 1

Auxiliary hydraulic
module

Parking Lock Solenoid Valve N573 has to be de-energized. By
operating Parking Lock Solenoid Valve 2 N574 the control line
for piston A1 is connected to the sump and de-pressurized.
Initially, pressure is only applied to piston face A2, causing the
self-holding valve, SHV, to switch over to the right. At that point,
the primary pressure is connected through to the parking lock
piston via N573 and the parking lock is disengaged.
At the same time, the primary pressure is directed to piston
A3 via a control line. The primary pressure is then acting on
both piston faces A2 and A3. At the same time as the valves
N573 (OFF) and N574 (ON) are switched over, Parking Lock
Solenoid N486 is also energized.

Primary pressure
supply

643_085

Parking lock gear

Parking lock piston

Parking lock spring

Step 2
As soon as the position P-OFF is definitely detected (signal
from G747 plus short delay), N574 is de-energized. N574 is
then back in its normal position and applies primary pressure
to piston face A1. The switching condition of the SHV remains
unchanged, however, because the forces acting on A2 and A3
are greater than the force on A1 (self-holding function).
Primary pressure is acting on the parking lock piston and
holds the parking lock in the position P-OFF.

Parking lock module

Auxiliary hydraulic
module

The energized Parking Lock Solenoid N486 acts as an additional safeguard against unwanted engagement of the
parking lock while the vehicle is being driven.
If, for example, the ATF supply were to fail (for example,
because of the engine stopping) while the vehicle is moving,
the parking lock would be engaged. In such a scenario,
Parking Lock Solenoid N486 prevents the parking lock engaging and so protects the parking lock (and the transmission)
from damage. Refer to Figure 643_089.

Primary pressure supply

40

643_086

Holding the position P-OFF in transmission setting N.
To prevent the parking lock being engaged when the engine
is switched off, transmission setting N must be selected
while the engine is running. If the engine is switched off with
the transmission in N, N486 remains energized. The Parking
Lock Solenoid N486 then keeps the parking lock piston in the
P-OFF position.
More information on the subject can be found on page 12
and page 37.
Key:
N486 Parking Lock Solenoid
– PHM – Parking lock holding solenoid
N573 Parking Lock Solenoid Valve
– PEV – Parking lock engagement valve
N574 Parking Lock Solenoid Valve 2
– PAV – Parking lock disengagement valve
SHV Self-holding valve
A1
A2
A3

SHV piston face A1
SHV piston face A2
SHV piston face A3
643_089

Engaging the parking lock – parking lock ON

643_087

Parking lock spring

643_088

643_084

Step 1

Step 2

Step 3

To engage the parking lock, N486 must
first be switched off.

At the same time, N573 is energized.

As primary pressure is acting on piston
face A3, the self-holding valve, SHV,
switches over to the left into its normal
position.

That connects the pressure line for the
parking lock piston and the control line
for piston A3 to the sump, thereby
de-pressurizing them. The parking lock
is engaged by the force of the parking
lock spring.

The entire pressure line for the parking
lock piston is then connected through
to the sump and thus de-pressurized.
N573 is not switched off until the
engine is switched off. Refer to page
38.
41

In normal operation, the parking lock is electro-hydraulically
actuated. To enable electro-hydraulic disengagement of the
parking lock, the engine must be running to generate sufficient ATF pressure. And to hold the parking lock in the OFF
position, there must also be sufficient ATF pressure or an
adequate power supply to Parking Lock Solenoid N486.
The emergency release mechanism serves to disengage the
parking lock and hold it in the P-OFF position if the electrohydraulic function fails or if it is necessary to hold the P-OFF
position for an extended period.

The emergency release mechanism of the parking lock must
be actuated in the following situations:
›› In general, when recovering or maneuvering the vehicle.
›› If the parking lock cannot be released electro-hydraulically
due to a malfunction.
›› If the vehicle cannot be maneuvered or moved due to
insufficient onboard voltage.
›› If the engine is not running and it is necessary to maneuver or move the vehicle, for example, in the workshop.
›› For function testing after carrying out assembly work on
components of the emergency release mechanism. See
Note.

After removing and installing the transmission or carrying out assembly work on components of the emergency release
mechanism, a function test must be performed as described in the workshop manual.

42

43

Parking lock emergency release
The parking lock is emergency-released from inside the
vehicle by means of a cable pull. The special socket tool and
the screwdriver are included in the vehicle tool kit.

Operating the parking lock emergency release (position P-OFF)
Follow the instructions on the page opposite.

Warning! Before actuating the emergency release device of the
parking lock, the vehicle must be secured to prevent it from rolling
away.

Indications on the instrument cluster
When the parking lock emergency release device is actuated,
the yellow gearbox warning lamp and the gear drive position
indicator N light up in the instrument cluster. In addition,
information on the instrument cluster indicates that the