A passive wireless sensor and control transmitter subsystem is provided for operating irrigation or other hydraulic system. The system includes a master module unit hardwired to and in communication with an existing hydraulic system controller, and a field module unit that is hardwired to and in communication with field electromechanical control devices such as valves, solenoids and servo motors, and field sensors indicating, for example, atmospheric conditions. The master module unit and field module unit communicate with digital wireless communication and can act as a simple wireless bridge providing pass through data connectivity. The master module and field module units are capable of digitizing input signals from the devices to which they are hardwired and transmitting them to the opposite module; they are further capable of recreating the digitized signals received back into the analog form to reproduce the original input signal and communicating it to the device to which it is hardwired. Full duplex communication enables remote sensors to be hardwired to the field modules unit rather than directly to existing control apparatus, as well as being able to transfer valve, pump and relay status back to existing control apparatus. One embodiment enables the subsystem to receive sensor inputs and process them accordingly even though the hydraulic control system may not be capable of utilizing them. Supplemental functionality may be added to an existing hydraulic control system by including the WSCX and its system features of sensor functionality and control.