What controls the motor drive heat exchanger bypass valve?

The motor drive heat exchanger bypass valve is primarily controlled by temperature control mechanisms within the system. Its purpose is to regulate the temperature of the fluid passing through the heat exchanger, ensuring that the engine and other components are maintained within optimal operating temperatures. When the temperature reaches a certain threshold, the system will adjust the bypass valve to either allow more fluid through the heat exchanger for cooling or to bypass it entirely to maintain efficiency or response times. This temperature-based control is vital for preventing overheating and ensuring safe and effective operation of the engine during various flight conditions.

The other options relate to different aspects of the overall system operation. Bleed air inflow, for instance, is used in various aerodynamic and environmental control systems but does not directly control the bypass valve. Ram air flow contributes to cooling as well but is not responsible for direct control of the valve's operation. Lastly, the evaporator blower is part of the cooling process, but it does not influence the bypass valve’s positioning directly. Thus, the temperature control mechanism is the correct answer as it specifically manages the bypass operations based on the cooling needs as determined by current temperatures.