Abstract: A lock-up clutch controller for the fluid coupling of an automotive automatic transmission, capable of making a lock-up clutch provided in the fluid coupling for directly coupling the input shaft and the output shaft of the fluid coupling engage or disengage smoothly responsive to an electric duty signal applied to a lock-up operation control solenoid valve. The lock-up clutch controller includes, as essential components, a lock-up clutch relay valve (a spool valve), a lock-up clutch control valve (a spool valve) and the lock-up operation control solenoid valve which operates in accordance with an electrical duty ratio signal to control the lock-up relay valve and the lock-up operation control valve so that the working fluid is supplied to the fluid coupling or the fluid coupling is drained appropriately for the smooth engagement and disengagement of the lock-up clutch.

Abstract: A stepless automatic transmission for a motor vehicle includes a V-belt type stepless transmission mechanism having input and output pulleys mounted respectively on input and output shafts, each pulley being provided with a hydraulic servo for varying the effective diameter thereof, and a V-belt linking the input and output pulleys to transmit power therebetween, in combination with a forward/reverse change-over mechanism and a coupler such as a fluid coupling, centrifugal clutch or friction clutch, and a hydraulic control system.