Parking Lock Control, Function - GF27.10-D-5000TSA

Transmission 725.0 in model 907

Phantom view of automatic transmission seen from left rear (shown on vehicles with rear wheel drive and drive range "P"

Sectional view of electrohydraulic park pawl actuation

Function requirements for park pawl control, general

Circuit 150N
Internal combustion engine running.
Electric auxiliary oil pump (M42) ON (with internal combustion engine at standstill)

Park pawl control, general

The park pawl is used as an additional safety feature for the parking brake and its purpose is to secure the vehicle to prevent it from rolling away inadvertently.

The park pawl essentially consists of the parking lock mechanism and the electrohydraulic components located at the rear of the transmission housing.

There is no mechanical connection whatsoever between the park pawl components of the automatic transmission and the DIRECT SELECT lever (S16/13) ("Park-by-Wire").

The park pawl is engaged and disengaged purely by electrohydraulic means, either by operating the DIRECT SELECT lever or depending on various factors such as opening the driver's door whilst the drivetrain is operational.

Park pawl control, function sequence

The function sequence is described in the following steps:

Function sequence for actuation, retention and locking of park pawl shift position "Not P".
Function sequence for actuation and locking of park pawl shift position "P".
Function sequence for actuation of park pawl shift position "P" with fault at park pawl lift solenoid.

Electrohydraulic park pawl actuation (shown on vehicles with rear wheel drive and shift position "Not P" locked).

Function sequence for actuation, retention and locking of park pawl shift position "Emergency P"

If the driver shifts from drive range "P" to "R", "N" or "D1", the park pawl is pushed in the direction of shift position "Not P" by pressure being applied to the "Not P" pressure chamber (11 p), against the force of the compression spring (11l).

This takes place after the detent lever (11d) has been raised against the force of the catch spring (11a) and the mechanical lock has been opened as a result of energization of the park pawl lift solenoid. If a sufficiently high working pressure is available, the park pawl is held at position "Emergency P" purely by the hydraulic pressure. There is no contact between the piston rod (11f) and detent lever (11d) in an axial direction.

Only after pressure is reduced (force of compression > hydraulic pressure) or after a complete drop in pressure, does the piston rod (11 f) move through the force of the compression spring (11l) up against the latch, whereby the "Emergency P" position is then held by the "mechanical lock".

In shift position "Emergency P", the park pawl cone (11j) is in front of the park detent (9a) and the parking lock gear (9) can turn freely. The park detent spring (9d) holds the park detent (9a) away from the parking lock gear (9).

Park pawl lift solenoid

The park pawl lift solenoid is actuated in pulses in order to minimize power loss in the fully integrated transmission control unit (N15/15n4). The solenoid is actuated by the fully integrated transmission control (VGS) electric controller unit (N15/15). If the on-board electrical system voltage is interrupted, actuation is performed by the electronic ignition lock (EZS) control unit (N73/8). The time period during which the fully integrated transmission control (VGS) electric controller unit (N15/15) energizes the park pawl lift solenoids, is usually measured so that the piston rod (11f) can reach the new position before energization ends.

Electrohydraulic park pawl actuation (shown on vehicles with rear wheel drive and shift position "P" locked) y

Function sequence for actuation and locking of park pawl shift position "P"

If the driver shifts to drive range "P" at the DIRECT SELECT lever, the pressure in the "Not P" pressure chamber (11P) is reduced. At the same time, the fully integrated transmission control (VGS) electric controller unit energizes the park pawl lift solenoids.

The solenoid lifts the detent lever (11d) against the force of the catch spring (11a) out of the "Emergency P" locking contour (11 e), thereby preventing "mechanical locking" of the piston rod (11f). Through the spring force of the compression spring (111), the piston rod (11 f), which is coupled to the bar (11 k), is pushed toward the parking lock gear (9) and the park pawl cone (11j) is pushed between the pilot bushing (11o) and park detent (9a). The increasing profile of the park pawl cone (11j) raises the park detent (9a) and presses it against the parking lock gear (9).

If, for a stationary vehicle, the tooth of the park detent (9a) does not reach a tooth space, but rather is on a tooth of the parking lock gear (9), the parking lock cone (11 j) is preloaded by the spring of the rod (11k) and is in the ready-to-engage position. If the parking lock gear (9) continues to turn, the park detent (9a) engages into the next tooth space.

If the park pawl is at position "P", the fully integrated transmission control (VGS) electric controller unit ends energization of the park pawl lift solenoid. When deenergized, the park pawl lift solenoid falls back into its initial position and no longer applies any pressure to the detent lever (11 d).

The lever is pushed into locking contour "P" (11c) by the force of the catch spring (11a), thereby locking the piston rod (11f).

Function sequence for actuation of park pawl shift position "P" with fault at park pawl lift solenoid

If the park pawl lift solenoid cannot be actuated or are defective, the mechanical lock of the detent lever (11d) is released by the detent lever release pin (11g). The detent lever release pin (11g) is actuated hydraulically by pressure being applied to pressure chamber "P" (11h).

This causes the detent lever (11 d) to be lifted out of the locking contour "Not P" (11e) of the piston rod (11f) and the park pawl is engaged by the force of the compression spring (11l).

Emergency engaging and emergency release of park pawl

If the park pawl lift solenoids are intact, the electric auxiliary oil pump opens the park pawl hydraulically, "Not" P, when the combustion engine is off; engaging occurs mechanically by means of the compression spring (11l).

If the park pawl lift solenoids are defective, the actuator is released by the detent lever release pin (11g), but only from the "Not P" position in the direction of "P".

If the lift solenoid is defective, it is no longer possible to leave the "P" position in the direction of "Not P", meaning that emergency release is no longer available.

Overview of system components of automatic transmission (AT)

GF27.10-D-9992TSA