Great post! Personally, I found the AC forum at fordforumsonline to be very useful in understanding how AC systems work. I was troubleshooting the AC in an f150, but most of it also applies to Volvos, except I understand there is no access to the high pressure side. The reason that pressures alone are not too useful is that the vapor pressure of the refrigerant controls the overall pressure, and this is the same whether there is a low charge of refrigerant or a full charge. This is not like a "pure" gas, where the pressure is directly linked to the amount of gas in an enclosure.jimmy57 wrote: The low side will get into upper 20's and when car gets cooler inside it will cycle off compressor at 22-24 psi and then come back on when pressure on low side goes up to 42-45psi. IF you turn fan down one speed and it cycles on for 25-30 seconds on a upper 80's day and then turn it up a fan speed and cycling gets longer on period, maybe 60-70 seconds then charge is prefect. The right charge level will also be noted by distinctly chilly low side line all the way back to compressor.
The high side will be over 200 and after car cools down inside the high pressure will be 150-180 unless the ambient temp is upper 90's low 100's and then the high side may stay close to 200 or a bit higher.
I found in charging my system, that the compressor on cycle time gradually increased from just a few seconds when I started to 60 seconds or more, over about 1/2 hr of charging. The pressures changed just like jimmy57 indicated.