Control system for a transfer device of a four-wheel drive vehicle with a continuously variable belt-drive transmission

Claim 1

A control system for a four-wheel drive vehicle with a continuously variable transmission for transmitting power of an automotive engine to main drive wheels of a motor vehicle, the transmission having a drive pulley including a hydraulically shiftable disc and a hydraulic cylinder for operating the disc, a driven pulley including a hydraulically shiftable disc and a hydraulic cylinder for operating the disc of the driven pulley, and a belt engaging with both of said pulleys, the control system comprising a transfer clutch engaged in a four-wheel drive condition of the vehicle for transmitting output of the transmission to auxiliary drive wheels of the vehicle, and respectively, disengaged in a two-wheel drive condition of the vehicle, and a hydraulic circuit including a pressure regulator valve having a valve body and a spool slidably mounted in the valve body for regulating line pressure of oil in the hydraulic circuit for the hydraulic cylinders of the pulleys of the transmission, an actuator with a first port and a second port, for selectively engaging the transfer clutch in response to oil pressure to be applied to the first and second ports, and a transfer valve with an inlet port for supplying oil from the inlet port to one of the first and second ports, the control system further comprising: a line pressure level changing port formed on the pressure regulator valve for changing level of the line pressure in response to oil pressure applied thereto; a conduit connecting the pressure level, changing port to the inlet port of the transfer valve; a solenoid operated valve connected to the conduit for supplying the line pressure to the pressure level changing port in order to set the line pressure to a low level; and means for temporarily restricting supplying of the line pressure to the conduit for changing the line pressure to a high level in response to supply of oil in the conduit to the actuator through the transfer valve when a driving condition of the vehicle is changed between the four-wheel drive condition and the two-wheel drive condition, so as to prevent slipping of the belt on the pulleys.