herein disclosed is a hydraulic servomechanism control system in an automatic transmission , for modulating the oil pressure of any hydraulic servomechanism to engage or release frictional engagement devices . the control system comprises : two pressure modulating mechanisms for modulating the oil pressure of said hydraulic servomechanisms ; and a shutoff mechanism for shutting off , when one of said pressure modulating mechanisms is caused to communicate with said hydraulic servomechanism , the communication between the other pressure modulating mechanism and said hydraulic servomechanism .