Present day torque converters use electro hydraulic control for that lock-up clutch. The hydraulic circuit which locks/unlocks the torque converter clutch is managed employing hydraulic valves. The valves are actuated immediately or indirectly by solenoids. With all the a few-passage torque converter, two passages are used for transmission oil (ATF) https://edwinukwku.creacionblog.com/33800073/getting-my-torque-converter-clutch-to-work