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When the system pressure drops below a pre-set level, the pressure switch contacts close. This energizes a four-way solenoid valve. When the solenoid valve is energized, oil pressure is switched to the port leading to the top of the unloader case, and the port leading to the bottom of the unloader case goes to drain.
With pressure at the top of the unloader piston, it moves downward and the unloader rod moves away from the limit switch, closing the contacts. This starts the pump motor. When the pump starts, the piston is still above the unloader drain port, thus the pump starts in an unloaded position (pumping to drain).
As the unloader piston continues its downward movement, it covers the drain port, causing the pump to be loaded (pumping to the pressure accumulator). When the tank pressure reaches a pre-set point, the pressure switch contacts open, and the solenoid valve de-energizes. This switches the port leading to the top of the unloader case to drain and the port leading to the bottom of the case to pressure.
Pressure to the bottom of the unloader case moves the piston upward. As the piston moves, the unloader drain port is uncovered. The pump is now unloaded and again pumping to sump. As the piston continues its upward travel, the rod trips the limit switch, opening the contacts. This stops the pump motor. The system is now ready for the next pumping cycle.
Replacement parts for this unloader system are available from American Governor. Part numbers may be found in the exploded diagram in the Woodward pump system manual.
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