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 TM 55-1905-219-14-8
4-28. AIR CONDITIONING SYSTEM - MAINTENANCE INSTRUCTIONS (Continued).
(3)
Pilot Operated Solenoid Valves.
Larger solenoid valves are pilot operated. This type of solenoid is actuated in the same manner as a
direct acting solenoid, but the plunger does not open the main port directly. Plunger action opens the pilot port.
Pressure trapped on top of the piston is released through the pilot port, creating a pressure unbalance across
the piston. Pressure underneath the piston is then greater than that above and the piston moves upward,
opening the main port. When the valve is de-energized, the pressures above and below the piston equalize
and the piston drops to close main port. The pressure difference across the valve, acting on the area of the
valve seat, holds the piston tightly closed.
(4)
Checking Solenoid Valve Operation.
When a solenoid valve opens, a definite click is heard. When it closes, a click of less intensity is
heard. An open solenoid can be detected by pressing the ear against the valve casing. A humming sound
should be heard. To check the operation of a solenoid valve, open or close the thermostat contacts with an
insulated screw driver and listen for the valve to click. (The operation of a king solenoid valve can be checked
by pressing the motor controller STOP button. The valve should click shut.)
4-856
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