Hydraulic control system

A hydraulic control system of a vehicle power train including a belt-type continuously variable transmission that clamps a belt using a hydraulic pressure to transmit power and that changes belt winding diameters to vary speed ratios, and a hydraulic lock-up clutch that is provided for a fluid power transmission device arranged between a power source and the belt-type continuously variable transmission and that directly couples the power source to the belt-type continuously variable transmission, comprising:

• a line pressure control valve that regulates a line pressure, which is a source pressure of hydraulic pressures used at various portions;
• a first control valve that outputs a hydraulic pressure supplied to one of a drive pulley and a driven pulley of the belt-type continuously variable transmission;
• a first electromagnetic valve that controls the hydraulic pressure output from the first control valve;
• a second control valve that outputs a hydraulic pressure supplied to the other one of the drive pulley and the driven pulley;
• a second electromagnetic valve that controls the hydraulic pressure output from the second control valve;
• a third electromagnetic valve that controls an engagement pressure of the lock-up clutch;
• and
• a fail-safe valve that switches a hydraulic pressure supplied to the one of the drive pulley and the driven pulley between the hydraulic pressure output from the first control valve and the line pressure, wherein
• the fail-safe valve is switched to a fail position in which the line pressure is supplied to the one of the drive pulley and the driven pulley when a rapid deceleration state is likely to occur in the belt-type continuously variable transmission,
• the fail-safe valve is switched to a normal position in which the hydraulic pressure output from the first control valve is supplied to the one of the drive pulley and the driven pulley during normal times other than times when a rapid deceleration state is likely to occur in the belt-type continuously variable transmission, and
• the fail-safe valve is switched by a combination of a hydraulic pressure controlled by the second electromagnetic valve and a hydraulic pressure controlled by the third electromagnetic valve.