The control of the oil force inside the lock-up clutch is completed by two valves: the relay valve as well as the signal valve. In such a arrangement the signal valve controls the pressure on just one side on the relay valve, which controls the tension in the lock-up clutch. https://torque-converter-clutch19741.bleepblogs.com/34708756/torque-converter-clutch-no-further-a-mystery