If the 50 is with the compressor ON, then you're overcharged. Your compressor has no chance in that situation to cycle off. The specs for your compressor is 25-33 psi, Low Side pressure (ie, compressor ON).

The compressor cycles off in normal operation to prevent freezing up: "compressor is connected in series with the low-pressure switch, high-p switch, and safety switch . . . Low-pressure switch (pressostat) turns a/c compressor on and off to maintain pressure within limits" (from Volvo N.A./Mitchell manual).

As I recall (though not 100%), the low-p switch cycles the compressor off around 20-22 psi??? If you're overcharged (very common among DIY'ers I think), that might be your problem. Is your a/c cold as Hades, like 38F inside ducts? (Temp's not a reliable measure of overcharge but just wondering.)

Hang on now, whether your compressor cycles or not is dependent upon a few different factors most of which are pressure related. The blanket statement of "I know it should be cycling every 30 seconds between 45 and 20 or so." does not really hold water. On a fully charged system you will find that it really only cycles on and off when you have the fan speed set on low and it does this to prevent icing of the accumulator. On full fan speed the compressor will pretty much stay engaged. It reads a steady 50 on the guage on the low side huh? Well for one it is not going to disengage until it gets too high on the high or too low on the low side. Since you cannot read the high without a Volvo VST and it is not disengaging I would say the odds are that it is not seeing high enough pressures on the high side to disengage it and it sure as heck is not seeing low enough on the low. You'd need somewhere under 30 PSI on the low to take the compressor off line. What should they read when shut off? That depends upon the amount off refrigerant in the system but what happens is the low and high sides equalize so you have the same number on either side.
Why are you puzzling over the a/c, does it not blow cold? You might want to try disconnecting the low side switch, it is in the low side pipe at the firewall, and then try disconnecting the high side switch, it is by the acummulator in the right front corner of the car. If either one of these individually cause the compressor to cut out then I would say that your compressor is working fine and that it is being commanded on by the control unit as it should be.


Mark

stuck clutch?

check if there is voltage at the one wire connector near the compressor pulley when it is always running. If some times the voltage drops to zero, your clutch is mechanically stuck to the pulley...

if there is always voltage, suspect the pressostat

try to disconnect the a/c relay under the glovebox. Does it make the compressor to disengage? if yes, it is probably a sensor... or the relay which is stuck closed.