First and second hysteresis zones for shift and lock-up control system

Abstract

A control system for an automatic transmission including a fluid transmission unit and a lockup clutch arranged in the fluid transmission unit. A running state sensor(s) detects the running state of the vehicle and outputs signals to an electronic control unit for deciding a shift and outputting a shift signal and for deciding lockup clutch ON/OFF and outputting a lockup ON/OFF signal. A hydraulic control unit selectively actuates a plurality of solenoid valves, in response to the output signals from the electronic control unit, to selectively apply/release a plurality of frictional engagement elements, to establish a new gear stage, and the lockup clutch. The electronic control unit (ECU) has stored in memory therein, 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 the first hysteresis region and the first lockup ON line. The ECU compares the output signals of the running state sensor(s) against the lockup diagram to decide lockup clutch OFF, if the running state is in a region below the lockup OFF line, lockup clutch ON, if the running state is over the first lockup ON line, and lockup clutch ON if the running state is in the second hysteresis region. The ECU also outputs a lockup signal to a predetermined one of the solenoid valves to apply/release the lockup clutch in accordance with the results of the comparison.