The lock-up clutch slip control system

To prevent an alteration of the target slip amount of a lock-up clutch slip control due to changes over time of a automatic transmission, and moreover to improve fuel consumption without reducing drivability, a lock-up clutch is mounted inside a torque converter. Inside the lock-up clutch, operational oil is supplied while switching between a release state and an engaged state of the clutch according to the switching function of a clutch control solenoid. Furthermore, the arbitrary slip amount (slip rate) is regulated using these two components. As a result of these controls, there is mapping of the target slip amount in the operating ranges having as parameters the throttle aperture change rate and turbine rotational speed of a throttle valve included on the intake pipe of the internal combustion engine and the turbine rotational speed of the torque converter. If the throttle aperture change rate goes above a predetermined value, the target slip amounts of the operating ranges are updated based on the target slip amount of the operating range at that time.