Control system for a four-wheel drive vehicle

Claim 1

In a control system for a switchable two-wheel drive four-wheel drive vehicle powered by an engine, the vehicle having a hydraulically operated automatic transmission for transmitting power of the engine to main driving wheels of the vehicle, auxiliary driving wheels, a hydraulically operated transfer clutch for transmitting the power of the engine to the auxiliary driving wheels for the four-wheel drive, a hydraulic circuit for controlling the transmission, said hydraulic circuit having a line pressure, and a hydraulic control circuit means for operating the transfer clutch, the improvement wherein: said hydraulic control circuit means including a pressure regulator valve means for regulating the line pressure applied from the hydraulic circuit to the hydraulic control circuit means to a constant value, accumulator means for accumulating oil from the line pressure of the hydraulic circuit for being supplied to the transfer clutch, control valve means having a spool for controlling said oil supplied to the transfer clutch providing clutch engaging pressure, said pressure regulator valve means for applying said constant value line pressure to one end of the spool of said control valve means, and solenoid operated valve means for controlling the spool of the control valve means by controlling the pressure of said constant value line pressure against the spool to control the clutch engaging pressure of the oil applied to the transfer clutch; first means for detecting engine operating conditions and for producing a torque signal dependent on the engine operating conditions; an electric circuit means for controlling said solenoid operated valve means, said electric circuit means including second means responsive to the torque signal for producing a clutch engage control signal, the magnitude of said control signal being dependent on engine torque; and the clutch engage control signal being applied to the solenoid operated valve means to control the spool of the control valve means and beng such that the clutch engaging pressure decreases with a decrease of the engine torque.