Abstract: A lock-up clutch control system for an automatic power transmission includes a torque converter mode, a full lock-up mode and a clip lock-up mode for controlling operation of a lock-up clutch which is disposed between a pump impeller and a turbine runner of a torque converter. The slip lock-up mode is a transitional mode between the torque converter mode and the full lock-up mode and connects the pump impeller and the turbine runner such that a relative rotation therebetween is allowed. In the system, one of the above-noted three modes is selected based on preselected vehicular operating condition indicative parameters, such as, an engine load and a vehicle speed. When a rapid depression of an accelerator pedal by a driver is detected after the slip lock-up mode is selected, the torque converter mode is executed in place of the slip lock-up mode until the rapid depression of the accelerator pedal is not detected.

Abstract: According to the lockup device of a fluid transmission under the present invention, at the decelerating running when the change rate of engine load exceeds a set value, a lockup control pattern is changed so that the area where the lockup device is controlled to perfect friction or to the specified friction force. By this change, at the decelerating running large engine brake force can be worked and a good feeling of deceleration can be given to a driver. In the case where fuel supply to the engine is suspended at the decelerating running of the engine, transmission of large engine brake force as mentioned above results in comparatively longer time of maintaining high engine speed and longer time of fuel supply cutting off, with consequent saving of fuel consumption. Moreover, by changing the target slip quantity to the increasing side at the accelerating running, torque increasing action at the accelerating running is displayed effectively and accelerability can be improved.