Control of lock-up clutch

A lock-up clutch control device which controls a lock-up clutch installed in a torque converter interposed between an engine and a transmission used with a vehicle, comprising:

• differential pressure generating means for engaging or disengaging the lock-up clutch by adjusting a differential pressure supplied to the lock-up clutch;
• detecting means for detecting a vehicle speed;
• determining means for determining whether or not the vehicle speed is equal to or larger than a first threshold value (V1) corresponding to a lock-up ON vehicle speed;
• commanding means for commanding the differential pressure generating means to engage the lock-up clutch, when the vehicle speed is equal to or larger than the first threshold value (V1);
• determining means for determining whether or not the vehicle speed has reached a second threshold value (V2) larger than the first threshold value (V1);
• setting means for setting a lock-up OFF threshold value (V3/V4) which depends on whether the vehicle speed has reached the second threshold value (V2);
• and
• commanding means for commanding the differential pressure generating means to disengage the lock-up clutch, when the vehicle speed is equal to or less than the lock-up OFF threshold value.

A lock-up clutch control method for controlling a lock-up clutch installed in a torque converter interposed between an engine and a transmission used with a vehicle, comprising:

• detecting a vehicle speed;
• determining whether or not the vehicle speed is equal to or larger than a first threshold value (V1) corresponding to a lock-up ON vehicle speed;
• commanding a differential pressure generating device to engage the lock-up clutch, when the vehicle speed is equal to or larger than the first threshold value (V1);
• determining whether or not the vehicle speed has reached a second threshold value (V2) larger than the first threshold value (V1);
• setting a lock-up OFF threshold value (V3/V4) which depends on whether the vehicle speed has reached the second threshold value (V2);
• and
• commanding the differential pressure generating device to disengage the lock-up clutch, when the vehicle speed is equal to or less than the lock-up OFF threshold value.