Replacement solenoid coils are needed in hydraulic systems when the solenoid coil used to control a hydraulic solenoid valve wears out or stops functioning. A solenoid is an electromagnet that consists of a tightly wrapped metal coil around a solid metal core. They produce a controlled, uniform magnetic field, and the metal coil can be inserted or removed which creates a method of control for various devices, including a hydraulic valve. How do solenoid coils work in a hydraulic system? In a hydraulic solenoid valve, solenoids are used to control the flow of fluid. Most commonly, they are used as a low-power switch that opens and closes a pilot valve, which in turns controls the main valve by applying pressure to a connected piston or diaphragm. Types of replacement solenoid coils. There is little difference between different types of replacement solenoid coils, but they can have various power supply voltage which may affect which one you choose. They are also available from a range of different manufacturers.