Park Pawl, Detailed Information - GF27.10-P-1103D
Model All with transmission 725
Automatic transmission with park pawl
Electrohydraulic park pawl actuation
Overview
This document contains information on:
- General
- Function
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 consists of the parking lock mechanism and the electrohydraulic component parts located in the rear area of the transmission housing.
Function
The function sequence is described in the following steps:
- Park pawl position Not P
- Park pawl position P
- Actuate shift position P in case of malfunction of park pawl lift solenoids.
- Emergency-P
Diagram showing park pawl position Not P
Park pawl position Not 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 the pressure being applied in the Not-P pressure chamber (16) against the spring force of the compression spring (20). This occurs after the detent lever (12) has been lifted against the force of the catch spring (8) and the mechanical lock has been opened by actuation of the park pawl lift solenoid (9). If there is sufficiently high working pressure, the park pawl is held in the Not-P position by hydraulic pressure. There is no contact between piston rod (14) and detent lever (12) in the axial direction. The piston rod (14) only moves against the detent through the spring force of the compression spring (20) when the pressure is reduced (spring force of compression spring (20) > hydraulic pressure) or when there is a complete drop in pressure. The Not P state is held by the mechanical lock. In the Not-P shift position, the parking lock cone (4) is located in front of the park detent (2) and the parking lock gear (1) can be turned. The park detent spring (5) keeps the park detent (2) away from the parking lock gear (1).
Park pawl lift solenoid
The park pawl lift solenoid (9) is actuated in pulses in order to minimize power loss in the fully integrated transmission control unit. Actuation of the solenoid is performed by means of the fully integrated transmission control unit. If the onboard electrical system voltage is interrupted, actuation is performed by the electronic ignition lock control unit. The time period in which the fully integrated transmission control unit actuates the park pawl lift solenoid is usually calculated so that the piston rod (14) has reached its new position before actuation ends.
Diagram showing park pawl position P
Park pawl position P
Pressure is reduced in the Not-P pressure chamber (16) if the driver shifts into drive range P. At the same time, the fully integrated transmission control unit actuates the park pawl lift solenoid (9). The park pawl lift solenoid (9) lifts the detent lever (12) opposite the catch spring (8) out of the Not-P locking contour (13) and so prevents mechanical lock of the piston rod (14). The spring force of the compression spring (20) pushes the piston rod (14), which is coupled to the bar (19), in the direction of the parking lock gear (1). The parking lock cone (4) is pushed between the pilot bushing (9b) and the park detent (2). The increasing profile of the parking lock cone (4) raises the park detent (2) and pushes it against the parking lock gear (1). If the tooth of the park detent (2) does not go into a tooth space when the vehicle is stationary, then the parking lock cone (4) is pre-compressed by the spring of the bar (19). This means that the park detent (2) is in a ready position. If the parking lock gear (1) continues to turn, the park detent (2) engages in the next tooth gap.
If the park pawl is in position P, the fully integrated transmission control unit stops actuation of the park pawl lift solenoid (9). If the park pawl lift solenoid (9) is not actuated, the park pawl lift solenoid (9) returns to its default position and no longer applies any pressure to the detent lever (12). The detent lever (12) is pushed into the P locking contour (11) by means of the spring force of the catch spring (8) and so locks the piston rod (14).
Actuate shift position P in case of park pawl lift solenoid malfunction
If the park pawl lift solenoid (9) cannot be actuated, the mechanical lock of the detent lever (12) is released by the detent lever release pin (15). The detent lever release pin (15) is hydraulically operated by the application of pressure to the Not-P pressure chamber (16). In this way, the detent lever (12) is lifted out of the Not-P locking contour (13) of the piston rod (14) and the park pawl is engaged by the spring force of the compression spring (20).
Emergency engaging and emergency release of park pawl
If the park pawl lift solenoid is intact (9), 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 (20).
If the park pawl lift solenoid (9) is defective, the actuator is only released from position Not-P in the direction of P by means of the detent lever release pin (15). If the lift solenoid is defective, it is no longer possible to go from P position to Not-P position, meaning that there is no emergency release.
Emergency-P
If the detent lever of the parking lock actuator is no longer released by the park pawl lift solenoid (9) in position Not-P, the detent lever release pin (15) opens the detent lever (12) hydraulically.
The hydraulic pressure required for this is provided by the electric auxiliary oil pump when the combustion engine is off.
When the Emergency-P function is active, the lock of the piston rod (14) is canceled by the detent lever (12). If there is sufficient hydraulic pressure, the detent lever release pin (15) prevents pressure being applied to the park pawl lift solenoid (9) in the direction of Not-P. If there is insufficient hydraulic pressure, the lubrication pressure solenoid is energized via the park pawl lift solenoid (9). This means that the hydraulic cylinder is applied without pressure in the Not-P direction. The compression spring (20) is supported on the piston rod guide of the piston housing (6) and moves the piston rod (14) into shift position P.
Park pawl lift solenoid
The park pawl lift solenoid (9) is actuated in pulses to minimize noise in the vehicle. Normally, the solenoid is actuated by the fully integrated transmission control unit. It is actuated by the electronic ignition lock control unit if the onboard electrical system voltage is interrupted.