Parking Lock Control, Function - GF27.10-P-5000AND
Transmission 725.0 (except 725.017/066/096) in model 217, 222
Phantom view of automatic transmission from left rear (shown with drive range "P" engaged)
Sectional view of electrohydraulic park pawl actuation
Function requirements for park pawl control, general
- Circuit 15 ON
- Internal combustion engine running
- Electric auxiliary oil pump (M42) ON (with combustion engine OFF)
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 by purely electrohydraulic means, either using the DIRECT SELECT lever or depending on various factors such as opening the driver door while the drive train is operational or by removing the transmitter key (A8/1).
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 with shift position "Not P" locked
Function sequence for actuation, retention and locking of park pawl shift position "Not P"
If the driver shifts from gear range "P" to "R", "N" or "D1", the park pawl is pushed by pressure being applied to the pressure chamber "Not P" (11p), against the spring force of the compression spring (11l), in the direction of shift position "Not P". This takes place after the notch 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 "Not P" purely by the hydraulic pressure. There is no contact between the piston rod (11f) and notch lever (11d) in an axial direction. The piston rod (11f) only moves against the detent as a result of the force of the compression spring (11l) if the pressure is reduced (spring force of compression spring (11l) > hydraulic pressure) or there is a complete drop in pressure, whereby the "Not P" state is held by the "mechanical lock". In shift position "Not P", the park pawl cone (11j) is in front of the park detent (9a) and the park pawl gear (9) can turn freely. The park detent spring (9d) holds the park detent (9a) away from the park pawl gear (9).
Park pawl lift solenoid
The park pawl lift solenoid is actuated in pulses in order to minimize the power loss in the fully integrated transmission control unit (Y3/8n4). Normally, the solenoid is actuated by the fully integrated transmission control unit. If the on-board electrical system voltage is interrupted, actuation is performed by means of the electronic ignition lock control unit (N73). The time period during which the fully integrated transmission control unit energizes the park pawl lift solenoid, is usually such that the piston rod (11f) can reach the new position before energization ends.
Electrohydraulic park pawl actuation, shown with shift position "P" locked
Function sequence for actuation and locking of park pawl shift position "P"
If the driver shifts to drive range "P" on the DIRECT SELECT lever, the pressure in the "Not P" pressure chamber (11p) is released. At the same time, the fully integrated transmission control unit energizes the park pawl lift solenoid. The solenoid lifts the notch lever (11d) against the spring force of the catch spring (11a) out of the locking contour "Not P" (11e) and thus prevents "mechanical locking" of the piston rod (11f). Through the spring force of the compression spring (11l), the piston rod (11f), which is coupled to the bar (11k), is pushed towards the park pawl gear (9) and the park pawl cone (11j) is pushed between the pilot bushing (9b) and park detent (9a). The increasing profile of the park pawl cone (11j) raises the park detent (9a) and presses it against the park pawl gear (9). If the tooth of the park detent (9a) on a stationary vehicle does not reach a tooth space, but reaches a tooth of the park pawl gear (9), then the park pawl cone (11j) is preloaded by the spring of the bar (11k) and is positioned ready to engage. If the parking lock gear (9) continues to turn, the park detent (9a) engages in the next tooth space. If the park pawl is at position "P", the fully integrated transmission control 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 notch lever (11d). The lever is pushed by the spring force of the catch spring (11a) into the locking contour "P" (11c) and thus locks 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 is defective, the mechanical lock of the notch lever (11d) is released by means of the notch lever release pin (11g). The notch lever release pin (11g) is actuated hydraulically by pressure being applied to pressure chamber "P" (11h). This causes the notch lever (11d) to be lifted out of the locking contour "Not P" (11e) of the piston rod (11f) and the park pawl is engaged by the spring force of the compression spring (11l).
Emergency engaging and emergency release of park pawl If the park pawl lift solenoid is intact, the electric auxiliary oil pump opens the park pawl hydraulically (Not P) with the combustion engine OFF; engaging occurs mechanically by means of a spring. If the park pawl lift solenoid is defective, the actuator is released by means of the notch lever release pin (11g), but only out of position "Not P" in direction "P". If the lift solenoid is defective, it is no longer possible to leave position "P" in the "Not P" direction, i.e. emergency release is no longer available.
| Overview of system components for automatic transmission (AT) | GF27.10-P-9992AND |