Automatic transmission control system

A control system for controlling an automatic transmission equipped with a lockup clutch for locking up and unlocking a torque converter installed to said automatic transmission by directly coupling and uncoupling input and output shafts of said torque converter, respectively, said automatic transmission automatically causing a downshift in response at least to an increase in engine load, said control system comprising:

• lockup pressure control means for controlling a lockup pressure with which said lockup clutch locks said torque converter;
• monitoring means for monitoring engine operating conditions and providing a downshift command signal when predetermined engine operating conditions including an engine load for downshifts are detected;
• and
• lockup release control means for detecting said downshift command signal and for causing, in response to an occurrence of said downshift command signal while said lockup clutch locks said torque converter, said lockup pressure control means decreasing said lockup pressure at a first specified rate in events where an increase in engine load greater than said predetermined value is detected while detecting said downshift command and at a second specified rate smaller than said first specified rate in events where an increase in engine load smaller than said predetermined value is detected while detecting said downshift command.

A control system for controlling engagement and disengagement of a hydraulically controlled lock-up clutch to lock and unlock a torque converter of an automatic transmission which causes automatically a down-shift in response at least to an increase in engine load, said control system comprising:

• a solenoid valve for controlling hydraulic pressure supplied to said lock-up clutch to lock said lock-up clutch;
• a shift sensor for detecting a down-shift of said automatic transmission;
• a load sensor for detecting engine load;
• a control unit for actuating said solenoid valve to control said hydraulic pressure so as to disengage said lock-up clutch when said shift sensor detects a down-shift of said automatic transmission, said control unit further controlling said solenoid valve to vary said hydraulic pressure in such a way that disengagement of said lock-up clutch progresses slower when said down-shift is caused due to a change in driving condition other than an increase in engine load detected by said load sensor greater than a predetermined value than when said down-shift is caused due to a change in engine load detected by said load sensor greater than said predetermined value.