Parking interlock controller function - GF27.10-P-5000AHO

TRANSMISSION 724.2 in MODEL 212 

Phantom view of automatic transmission from left rear (shown with engaged gear range "P") 

Fig 1: Park Pawl Gear, Detent, Pawl Cone, Pilot Bushing & Spring

Courtesy of MERCEDES-BENZ USA

Sectional view of electrohydraulic park pawl actuation 

Fig 2: Sectional View Of Electrohydraulic Park Pawl Actuation

Courtesy of MERCEDES-BENZ USA

Function requirements for park pawl control, general 

• Circuit 87 (connected circuit 15) ON
• Combustion engine RUNNING
• Electric transmission oil pump (M42) ON (with Internal combustion engine at standstill)

Park pawl control, general 

The park pawl acts as an additional locking measure with the parking brake, and it helps to prevent the vehicle from unintentionally rolling away.

The park pawl essentially consists of the parking lock mechanism and the electrohydraulic components, which are located in the rear area of the transmission housing.

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

The park pawl is engaged and disengaged purely electrohydraulically, either by operating the DIRECT SELECT lever or depending on various factors such as, e.g. the driver door being opened, or when the drivetrain is operational.

Function sequence for park pawl control 

The function sequence is described in the following steps:

• Function sequence for actuating, holding and locking park pawl shift position "Emergency P" 
• Function sequence for actuating, holding and locking park pawl shift position "P" 
• Function sequence for actuating park pawl shift position "P" when park pawl lift solenoids malfunction 

Electrohydraulic park pawl actuation, shown with shift position "Emergency P" locked 

Fig 3: Electrohydraulic Park Pawl Actuation - Shown With Shift Position "Emergency P" Locked

Courtesy of MERCEDES-BENZ USA

Function sequence for actuating, holding and locking park pawl shift position "Emergency P" 

If the driver shifts from gear range "P" to "R, N or D", the park pawl is shifted towards the "Emergency P" shift position through switching pressure into the pressure chamber (4b), opposite the spring force exerted by the pretension spring (6).

If a sufficiently high level of working pressure is available, the park pawl is held by hydraulic pressure alone in the "Emergency P" position. In the axial direction there is not contact between the piston rod (10) and the notched lever (9).

Only after the pressure is reduced (spring force of pretension spring (6) > hydraulic pressure) or if there is a complete pressure drop is the piston rod (10) moved by the force exerted by the pretension spring (6) up against the locking piece, whereby the "Emergency P" position is maintained by the "mechanical lock".

In the "Emergency P" shift position, the park pawl cone (12) is in front of the park detent (14) and the park pawl gear (16) can turn freely.

Park pawl lift solenoid

The park pawl lift solenoid is pulse actuated to minimize the power loss in the fully integrated transmission control unit. The solenoid is actuated by the fully integrated transmission control unit (N73) or electronic ignition lock control unit, if the on-board electrical system voltage is interrupted.

The time period over which the fully integrated transmission control unit energizes the park pawl lift solenoid, is generally measured such that the piston rod (10) reaches the new position before it is deenergized again.

Electrohydraulic park pawl actuation, shown with shift position "P" locked 

Fig 4: Electrohydraulic Park Pawl Actuation - Shown With Shift Position "P" Locked

Courtesy of MERCEDES-BENZ USA

Function sequence for actuating, holding and locking park pawl shift position "P" 

If the driver shifts on the DIRECT SELECT lever (S16/13) into gear range "P", the energization of the park pawl switchover valve (Y3/8y11) overridden and the pressure in the pressure chamber (4b) immediately reduced. At the same time, the fully integrated transmission control unit energizes the park pawl lift solenoids.

This then lifts the notched lever (9) out of the "Emergency P" locking contour (2), opposite the spring force exerted by the catch spring (8), thereby preventing the "mechanical locking" of the piston rod (10). The spring force of the pretension spring (6) and the pressure switched into the pressure chamber (4a) compress the piston rod (10), which is coupled to the link rod (7), towards the park pawl gear (16) and the park pawl cone (12) is moved between the pilot bushing (11) and the park detent (14).

The increasing profile of the park pawl cone (12) raises the park detent (14) and presses it against the park pawl gear (16).

If the tooth of the park detent (14) on a stationary vehicle does not reach a tooth space, but reaches a tooth of the park pawl gear (16), then the park pawl cone (12) is preloaded by the spring (7) of the link rod and is positioned ready to engage. If the parking lock gear (16) continues to turn, the park detent (14) engages in the next tooth space.

If the park pawl is in position "P", the fully integrated transmission control unit de-energizes the park pawl lift solenoids.

When deenergized, the park pawl lift solenoids fall back into their initial position and they no longer apply any pressure to the catch detent (9). This is pressed by the spring force of the catch spring (8) into the "P" locking contour (1) and then locked by the piston rod (10).

Function sequence for actuating park pawl shift position "P" when park pawl lift solenoids malfunction 

If a park pawl lift solenoid cannot be actuated or if it is defective, switching pressure (increased pressure application > 10bar) into the pressure chamber (4a), the notched lever's (9) mechanical lock can then be overcome. This "overpressure" is made possible by a slight leveling of the "Emergency P" locking contour (2).

Disengaging the park pawl from position "P", when the park pawl lift solenoids malfunction through overpressure, is not intended, and therefore it cannot be performed/actuated.

Hydraulic valve (5a)

The hydraulic valve (5a) ensures that there is no vacuum in the pressure chamber (4a), when the piston (3) is moved to the "P" position by spring force only (Emergency-P function).

This serves to prevent hydraulic fluid from being sucked in from the oil duct, which flows into the pressure chamber (4a) and thereby negatively influencing the piston dynamics.