The control of the oil pressure while in the lock-up clutch is finished via two valves: the relay valve plus the sign valve. In this type of arrangement the signal valve controls the force on just one side from the relay valve, which controls the pressure during the lock-up clutch. https://torqueconverterclutch87417.blogitright.com/34379464/little-known-facts-about-torque-converter-clutch