The Copeland 4R, 6R, 8R, 4D, 6D and 8D compressors all use blocked suction unloading. An electric solenoid is mounted on the compressor head which when energized opens a port from the discharge of the compressor to the top side of a piston internally in the head. This piston is pushed down closing the suction port in the valve plate. When de-energized the high pressure on top of the piston is vented back to the suction or low side of the compressor and a return spring pushes the piston off the valve plate suction port.
These compressors are shipped with the unloader ports flanged. If the solenoid is not installed the compressor operates in the loaded.