Hydraulic control apparatus for controlling an engaging state of a lock-up clutch of an automatic transmission

A hydraulic control apparatus for an automatic transmission of a motor vehicle, the transmission including a lock-up clutch having a first and a second fluid chamber, the apparatus comprising:

• a lock-up relay valve which is switchable between a first position where said lock-up relay valve permits a working fluid to be supplied to said first fluid chamber of the lock-up clutch and permits the lock-up clutch to be placed in a fully released state thereof and a second position where said lock-up relay valve permits said working fluid to be supplied to said second fluid chamber of the lock-up clutch and permits the lock-up clutch to be placed in one of a fully engaged and a partially engaged state thereof;
• a lock-up control valve which is operable for regulating a hydraulic pressure of said first fluid chamber of the lock-up clutch and thereby controlling a degree of the engagement of the lock-up clutch;
• a lock-up relay valve disorder detecting device which detects a disorder of said lock-up relay valve by identifying that said lock-up relay valve is stuck to said second position thereof;
• a vehicle speed sensor which detects a running speed of the motor vehicle;
• and
• a control device which, when said lock-up relay valve disorder detecting device detects said disorder of said lock-up relay valve and simultaneously when the vehicle running speed detected by said vehicle speed sensor is higher than a reference value, operates said lock-up control valve to decrease said hydraulic pressure of said first fluid chamber of the lock-up clutch for forcibly placing the lock-up clutch in said fully engaged state thereof.