Emergency-P Function - GF27.10-P-3029RFH

Transmission 724.2 in model 205, 253 

Sectional view of electrohydraulic park pawl actuation 

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Vehicle stationary
• Terminal 15 ON

The electronic ignition lock control unit (N73) transmits the status of circuit 15 via the chassis FlexRay (Flex E), the powertrain control unit (N127) and the drive train CAN (CAN C1) to the fully integrated transmission control unit (Y3/8n4).

Emergency-P, general 

In the event of critical faults in the communication with the fully integrated transmission control unit (e.g. undervoltage in the on-board electrical system or open circuit), the park pawl is automatically engaged.

The electronic ignition lock control unit actuates the park pawl lift solenoid in the fully integrated transmission control controller unit (Y3/8). The voltage for this is provided by the additional battery (G1/7). The park pawl lift solenoid lifts the catch detent (9) via the piston rod (10). The park pawl switchover valve (Y3/8y11) is de-energized. Therefore, the transmission fluid escapes from the pressure chamber ("Not P") (4b). The compression spring (6) now moves the piston rod (10) in the direction of the park detent. The compression spring (6) supports itself on the piston housing (5) when doing this. Because of the increasing profile of the park pawl cone (12), the park detent is raised and pressed against the park pawl gear. If the tooth of the park detent does not end up in a tooth space when the vehicle is stationary, but on a tooth of the park pawl gear, the park pawl cone (12) is pre-compressed by the spring of the rod (7) and is in the ready position. If the parking lock gear continues to turn, the park detent engages in the next tooth space.

The park pawl lift solenoid is actuated in pulses to minimize noise in the vehicle. With Emergency-P, actuation takes place via the electronic ignition lock control unit.