Automatic transmission lockup clutch control apparatus

A lockup clutch control apparatus for use in an automotive vehicle including an internal combustion engine having a throttle valve, and an automatic transmission having an output shaft, an impeller, a turbine and a lockup clutch for producing selected one of a plurality of modes of gear change, comprising:

• first sensor means sensitive to a speed of rotation of the engine for producing a first sensor signal indicative of a sensed engine speed;
• second sensor means sensitive to a speed of rotation of the transmission output shaft for producing a second sensor signal indicative of a sensed transmission output shaft speed;
• and
• a control unit coupled to the first and second sensor means for controlling the lockup clutch, the control unit including means for measuring a lapse time after the selected gear change mode starts, means for calculating a gear ratio from a relationship specifying the gear ratio as a function of the selected gear change mode and the measured lapse time, means for storing the calculated gear ratio, means for calculating a speed of rotation of the turbine based upon the sensed transmission output shaft speed and the calculated gear ratio, means for calculating a difference between the sensed engine speed and the calculated turbine speed, means for operating the lockup clutch in a slip lockup mode to bring the calculated difference to a predetermined value, means for monitoring changes in the sensed engine speed to measure a time interval required for the selected gear change mode, and means for modifying the relationship to correct the gear ratio stored for the selected gear change mode based upon the measured time interval.