Compressors are specifically designed to operate with designated refrigerant and cannot be interchanged. The compressor is an electrical (as well as mechanical) device, so use caution when working near compressors.
The Copeland Scroll Ultra-Tech compressor has an internal bypass port, which allows the system to run at 67 percent part-load capacity and 100 percent full load. This unloading (Y1) and loading (Y2) process are accomplished by the use of an internal DC solenoid and rectifier. When the compressor is operating in low stage (Y1 only) the 24Vdc compressor solenoid coil is de-energized. When the compressor is operating in high stage (both Y1 and Y2) the 24Vdc solenoid coil is energized.
If it is suspected that the unloader is not working, it can be verified by the following method. Call for Y1 (unloaded) and let run for 15 minutes until the system is stabilized. While it's running unloaded, measure the amperage of the compressor. Than with Y1 energized also energize Y2. There should be approximately 20 to 40% change in amperage.
If the expected result is not achieved, shut the compressor off and remove the rectifier from the compressor. Verify there is 18 to 28Vac applied to the rectifier leads then check the output of the rectifier for 24Vdc. If you receive 24Vdc you may have a bad connection where the rectifier plugs into the compressor or the internal solenoid are defective. You can measure the internal compressor solenoid coil resistance. The resistance should be approximately 330 or 1640 ohms depending on the unloader supplier. If the coil resistance is infinite or grounded, the compressor must be replaced.
NOTE: Refrigerant pressures are not typically used to verify the operation of the solenoid because the liquid pressure is very similar between low and high stage operation, so it should not be used for troubleshooting the solenoid.