Patent ID: 8065065

Claim:
A lock-up control system configured to control a lock-up clutch in a torque converter which is disposed between input and output elements of the torque converter, the lock-up control system being configured to bring the torque converter into a lock-up state by controlling a lock-up differential pressure between a lock-up clutch apply pressure and a lock-up clutch release pressure, the lock-up control system comprising: a lock-up differential pressure generator configured to generate the lock-up differential pressure in response to a lock-up differential pressure command value; and a controller configured to control the lock-up differential pressure command value according to change in an actual slip rotation speed as a differential rotation speed between the input and output elements of the torque converter, wherein, when increasing the lock-up differential pressure command value with time to establish the lock-up state of the torque converter, the controller is programmed to: switch a gradient of the lock-up differential pressure command value from a first gradient to a predetermined gradient that is larger than the first gradient, when the actual slip rotation speed is decreased to a predetermined slip rotation speed value or to less than the predetermined slip rotation speed value; and switch the gradient of the lock-up differential pressure command value from the predetermined gradient to a second gradient that is smaller than the predetermined gradient, when the actual slip rotation speed exceeds the predetermined slip rotation speed value again after being decreased to the predetermined slip rotation speed value or to less than the predetermined slip rotation speed value.