Modern torque converters use electro hydraulic control with the lock-up clutch. The hydraulic circuit which locks/unlocks the torque converter clutch is managed using hydraulic valves. The valves are actuated instantly or indirectly by solenoids. A torque converter simply cannot obtain 100% coupling efficiency. The basic a few element torque converter https://landenzyxtq.thekatyblog.com/32726834/the-basic-principles-of-torque-converter-clutch