Lock-Up Clutch Mechanism

The torque converter clutch is hydraulically controlled by an Electronic Pressure Regulating Solenoid (EPRS), which is controlled by the Transmission Control Module (TCM).

This allows the torque converter to have three states of operation as follows:

• Fully engaged
• Controlled slip variable engagement
• Fully disengaged.

The torque converter pressure valve reduces system pressure and guarantees the pressure needed for the torque converter. It also limits the maximum torque converter pressure to prevent the torque converter from expanding.

The solenoid valve is operated by Pulse Width Modulation (PWM) signals from the TCM to give full, partial or no lock-up of the torque converter.

The lock-up clutch is a hydro-mechanical device which eliminates torque converter slip, improving fuel consumption. The engagement and disengagement is controlled by the TCM to allow a certain amount of controlled 'slip'. This allows a small difference in the rotational speeds of the impeller and the turbine which results in improved shift quality. The lock-up clutch comprises a piston and a clutch friction plate.

In the unlocked condition, the oil pressure supplied to the piston chamber is reduced and the pressure in the turbine chamber is allowed to push the piston back. In this condition the clutch plate are released and torque converter slip is permitted.

In the locked condition, the torque converter clutch spool valves are actuated by the EPRS. Pressurized fluid is directed into the lock-up clutch piston. The piston moves with the pressure and pushes the clutch plates together. As the pressure increases, the friction between the clutch plates increases, finally resulting in full lockup of the clutch plates. In this condition there is direct mechanical drive from the engine crankshaft to the transmission planetary gear train.

The standstill decoupling feature is new for the eight speed transmission. When the vehicle comes to a standstill (with the brakes applied), the converter is disconnected from the driveline, so that only a slight residual load remains. This further reduces fuel consumption. Decoupling is by actuating clutch B in the transmission, and is dependent on load and output speed.