The compressor in this thread should be scared.
Follow the link in alcomechs post and read the second page.
Unloader's are used for capacity control and in the case of this semi hermetic compressor will unload the cylinders when the load on the cooling coil decreases. This topic is long but in short unloader's are added to a compressor to maintain suction pressure and saturation temperature.
You may as an example have a DX cooling coil and it is desired to maintain a 40 degree coil temperature. You will then need to figure out the suction saturation temperature in relation to refrigerant pressure. This equates to somewhere in the range of 68 PSI or 40 degree liquid saturation temperature at around 10 to 12 degrees superheat.
It is important to be with in the design range of the DX coil at all times and in the case of a system with a TXV it will hunt to maintain the desired superheat. Hunting of the TXV will depend on the load or conditions such as low evaporator air flow, or low ambient outside air temperatures, which will decrease head pressure.
When suction pressure is below the design it is desirable to begin unloading so that the suction pressure does not fall too low. When suction pressure is low the suction saturation temperature will also be low. Again you must consider the design which is normally 105 degree condensing temperature for R-22.
A system that works out of its design must be capable of handling this condition. Otherwise it is certain that problems such as oil migration or incorrect oil velocity through a system will occur and more than likely will result in a compressor failure.
You will have good results to load the compressor at somewhere in the close range of 68 PSI and begin to unload at around 62 PSI. I have seen many set to 66/59 but you are now out of the design range and saturation temperature will be as low as 33 degrees but above freezing.
There is much debate as to when to start unloading cylinders and in the case of a multi-stage unloading compressor it is important to figure out your design conditions first.
Trouble shooting is not part of the repair......understand the symptoms and you will find a solution.