FUNCTION OF ECU • Control of Shift Timing The ECU has programmed into its memory the optimum shift pattern for each shift lever position (D, 2, L range) and driving mode (Normal or Power). Based on the appropriate shift pattern, the ECU turns No. 1 and No. 2 solenoid valves on or off in accordance with the vehicle speed signal from the vehicle speed sensor and the throttle opening signal from the throttle position sensor. In this manner, the ECU operates each shift valve, opening or closing the fluid passages to the clutches and brakes to permit up-shift or down-shift of the transmission. HINT: The electronic control system provides shift timing and lock-up control only while the vehicle is traveling forward. In REVERSE, PARK, and NEUTRAL, the transmission is mechanically, not electronically controlled. • Control of Overdrive Driving in overdrive is possible if the O/D main switch is on and the shift lever is in the D range. However, when the vehicle is being driven using the cruise control system (CCS), if the actual vehicle speed drops to about 4 km/h (2 mph) below the set speed while the vehicle is running in overdrive, the CCS computer sends a signal to the ECT ECU to release the overdrive and prevent the transmission from shifting back into overdrive until the actual vehicle speed reaches the speed set in the CCS memory. On this model, if the coolant temperature falls below 60°C (140°F), the Engine ECU sends a signal to the ECT ECU, preventing the transmission from up-shifting into overdrive. • Control of Lock-Up System The ECT ECU has programmed in its memory a lock-up clutch operation pattern for each driving mode (Normal or Power). Based on this lock-up pattern, the ECU turns lock-up solenoid valve on or off in accordance with the vehicle speed signals received from the vehicle speed sensor and the throttle opening signals from the throttle position sensor. Depending on whether lock-up solenoid valve is on or off, the lock-up relay valve performs changeover of the fluid passages for the converter pressure acting on the torque converter to engage or disengage the lock-up clutch. (Mandatory Cancellation of Lock-up System) If any of the following conditions exist, the ECU turns off lock-up solenoid valve to disengage the lock-up clutch. 1) The brake light switch comes on (during braking). 2) The IDL points of the throttle position sensor close (throttle valve fully closed). 3) The vehicle speed drops 4 km/h (2 mph) or more below the set speed while the cruise control system is operating. 4) The coolant temperature falls below 60°C (140°F) and vehicle speed is under 60 km/h (37 mph), or 35°C (95°F) and vehicle speed is under 40 km/h (25 mph). The purpose of 1) and 2) above is to prevent the engine from stalling if the rear wheels lock-up. The purpose of 3) is to cause the torque converter to operate to obtain torque multiplication. The purpose of 4) is both to improve general driveability, and to speed up transmission warm-up. Also, while the lock-up system is in operation, the ECU will temporarily turn it off during up-shift or downshift in order to decrease shifting shock.