Torque Converter Lockup Clutch Control, Function - GF27.20-P-3002ANG

Transmission 725.0 (except 725.017) in model 205 

Function requirements, general 

• Engine running

Control of torque converter lockup clutch, general 

The torque converter lockup clutch solenoid (Y3/8y20) is actuated by the fully integrated transmission control unit (Y3/8n4) and influences the torque converter lockup clutch regulating valve. The housing of the torque converter is flowed through by a lubricating oil volume flow. A lube oil pressure is thereby set that acts on the torque converter lockup clutch piston and opens this. The lubrication oil pressure is therefore fed back hydraulically to the torque converter lockup clutch regulating valve and is taken into account when the piston pressure is adjusted. The lubrication oil volume flow ensures optimum cooling of the torque converter.

The control of the torque converter lockup clutch is described in the following conditions:

• Torque converter lock-up clutch open 
• Torque converter lock-up clutch OPEN SLIP 
• Torque converter lock-up clutch SLIP 
• Torque converter lock-up clutch SLIP OPEN 

The state transitions are controlled to match the engine load, vehicle speed and vehicle acceleration.

Torque converter lock-up clutch open 

The torque converter lockup clutch solenoid is in the control position. The working pressure acts upon the torque converter lockup clutch regulating valve. Pressure from the compensation line for the converter internal pressure compensation is applied to the control surfaces of the torque converter lockup clutch regulating valve. The converter internal pressure that has an opening effect in the torque converter on the torque converter lockup clutch is thereby compensated and the solenoid/pressure characteristic of the torque converter lockup clutch thus becomes independent of the lubrication pressure. The solenoid is not energized. No power is transmitted by the torque converter lockup clutch. The torque converter alone transfers the torque.

Torque converter lock-up clutch OPEN SLIP 

The torque converter lockup clutch solenoid is in the control position. In addition to the pressure from the compensation line for the converter internal pressure compensation, a force acts on the torque converter lockup clutch solenoid due to the energization of the solenoid. The pressure in the torque converter lockup clutch increases, whereupon it is filled.

Torque converter lock-up clutch SLIP 

The specified slip is stored in the fully integrated transmission control unit. The fully integrated transmission control unit calculates the control current and actuates the torque converter lockup clutch solenoid. The pressure on the piston of the torque converter lockup clutch changes, the torque and thus the slip at the torque converter lockup clutch are thereby controlled. The actual slip is determined by the comparison of engine speed with turbine speed. If necessary, a post-regulation is introduced by the fully integrated transmission control unit. The clutch pressure in the torque converter lockup clutch can be increased or reduced by post-regulation.

Torque converter lock-up clutch SLIP OPEN 

In this control state the clutch pressure in the torque converter lockup clutch is reduced in stages until it is eliminated.

Torque converter with centrifugal pendulum 

Via a regulated min. slip at low speeds, the fuel consumption can be reduced depending on the engine. The response time to requested driver load changes is also improved.

Limp-home mode 

The torque converter lockup clutch is not actuated in limp-home mode. Protection functions are used which constantly monitor the transmission oil temperature, the frictional energy and the functionality in the clutch.

Overview of system components for automatic transmission (AT)

GF27.10-P-9992ANG