THe low pressure gauge isn't really much of an accurate measure of system charge.

It gives you a general idea of WHAT is going on, but it mostly gives you an idea of the evaporator temperature, which is a function of ambient temperature, cabin temperature, and refrigerant temperature/volume/pressure.

I don't want to make this sound more complex than it really is, but it's good to understand exactly what's going on and why you might be getting readings like that.

First, the clutch obviously cycles off when the pressure gets to about 20. Then the heat exchange from the cabin causes the evaporator (low side of system) to climb because more heat = more pressure. When the system climbs to the high thirties, the comp cycles on to suction out the warm refrigerant and push in some new, cooler refrigerant for the evaporator to exchange for cabin heat. It sucks out refrigerant until the low pressure side of the system gets to 20, and again the process runs it's course.

That's one part of the equation. The other part is, what the ambient temperature is when this happens. If the ambient temp is sixty five degrees (for the sake of example) and the cabin temperature is 80, and it's cloudy (not much radiant heat from the outside) then the compressor will certainly cycle on and off quite a bit, showing lower pressure readings than it would if the weather was warmer.

Now take a day where the temp is 85, it's bright and sunny outside, and you're running the inside fan at full blast to exchange the most cool air from the evaporator. That will spike pressures in the evaporator and on the entire low pressure side of the system. It'll spike them enough so that the compressor never even turns off because the low pressure side is ALWAYS exchanging a massive amount of heat from the cabin.

But on a cool day (back to the beginning) the temperature between inside and outside is not much different, and therefore the load on the refrigerant (heat exchange) is low, and as such the compressor doesn't need to cycle on and off much to get decent, efficient heat exchange.

Obviously this all ties together. The moral of this story is that the low pressure side has too many variables to judge proper charge accurately. A "proper" low side charge in cool weather can be enough to explode the system (there are protective measures against this, but the idea is the same) in hot weather. Or the system will just shut itself down until it gets back to realistic pressures.

R134a is a very heat sensitive refrigerant. Where R12 has a steady heat-to-pressure rise, R134a gets very high pressure under lower amounts of heat. That's why it's very crucial to have the RIGHT amount of 134, and if anything, slightly less than slightly more.

So what can you do to find out more? One thing is to buy a high pressure gauge, or gauge manifold with high and low pressure gauges. That will help you immensely when it comes to figuring out what the compressor's output is. You should see solid high side pressures that are more in line with a 134a system, while the low side can vary quite a bit, over different periods of time, depending on what the ambient & cabin temps are.

If you don't want to spend the money for a manifold set, consider buying a vent temperature gauge that shows you, in degrees, what the cold air temperature is at the vents. You should see temperatures in the low 40s when the car is moving along with good airflow over the evaporator, with the fan on lower settings. Some systems, in top condition, see temps in the thirties. My cars have usually had 38 degree vent temps even in very hot weather, and upper 40s at idle. That's very respectable. If you get to a charge level that allows you low 40s or even high 30s at speed, and mid to high forties at idle, then you've reached an accurate charge (within a few ounces). But the key to remember is, you have to tune the system to the WARMEST weather, not to somewhat cool weather. Otherwise there is a chance that it will be totally overcharged.

The best way is to vacuum out the system and measure in the proper amount, but in your case it sounds as if that might not be the wisest solution, nor the longest lasting. It's worth a little experimentation, but try to avoid overcharging as it will cause LESS cooling than a slightly undercharged system, especially in higher temperatures.
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Chris Herbst
1992 945, 71k
Volvo shop tech