First and second hysteresis zones for shift and lock-up control system in an automatic transmission

A control system for an automatic transmission mounted in a vehicle and having a fluid transmission unit and a lockup clutch arranged in the fluid transmission unit, the control system comprising:

• running state detection means for detecting the running state of a vehicle and for outputting running state signals;
• an electronic control unit for making a shift decision and outputting a shift signal, responsive to the running state signals, and for deciding ON/OFF for the lock-up clutch and outputting a lockup ON/OFF signal responsive to the running state signals;
• a hydraulic control unit for selectively actuating a plurality of solenoid valves, responsive to the shift signal and to the lockup ON/OFF signal, to selectively engage/release a plurality of frictional engagement elements and to thereby establish a selected gear stage and to engage/release the lockup clutch;
• wherein said electronic control unit comprises:
• a lockup diagram having a first hysteresis region defined by a lockup OFF line and a first lockup ON line, and a second hysteresis region defined by a second lockup ON line, formed in said first hysteresis region, and said first lockup ON line;
• lockup decision means for deciding OFF for the lockup clutch when the detected running state is below said lockup OFF line, for deciding ON for the lockup clutch when the detected running state is over said first lockup ON line, for deciding ON for the lockup clutch when a shift is effected on the basis of said shift decision and when the detected running state is in said second hysteresis region, said lockup decision means making a lockup decision of ON or OFF by comparing said running state signals against said lockup diagram and basing said lockup decision on said comparison;
• and
• lockup signal output means for outputting a signal to a predetermined one of said plurality of solenoid valves to engage/release said lockup clutch responsive to said lockup decision reached by said lockup decision means.