Hi reactiv12. Do I assume you are trying to add refrigerant to improve cooling? Going just by pressures is not a good way to determine state of charge because if more liquid is added, there will just be less vapor in the system, but the "at rest" pressure will be the same. When it's running, the compressor will cycle off when the high side gets to the switch setting, or, if you have a variable displacement compressor, it will just pump less volume. Either way, the pressures can read normal with some undercharge and a bunch of overcharge. What's more important is to start from empty, pump in the exact amount listed on the sticker under the hood, then look at the pressures to judge system performance.
Once you have a full system, adding another quarter pound of refrigerant will have very little affect on pressures, but the liquid will turn to a vapor too late. It is supposed to vaporize in the evaporator in the dash. That's where you want it to become cold. If there is too much liquid in the system, it will fill the evaporator and vaporize and become cold in the line going to the compressor. Liquid in the compressor will lock it up.
Since the liquid isn't vaporizing and getting cold enough in the evaporator, the sensing bulb will open the expansion valve to let more liquid in thinking there isn't enough in there. The only way to know you have the right amount of refrigerant is to recover what is in there now, pump the system into a vacuum, then pump in the measured amount called for.
I apologize if you are referring to something else. I never paid too much attention to system pressures. If the air temperature from the ducts was around 40 - 45 degrees, the system was working real well.
Monday, March 22nd, 2010 AT 9:34 PM