If the AC system is plugged anywhere but the suction inlet of the pump, it will shut off due to high pressure, since all the freon will be stuffed into the high side. I've blown a hose from just this on the Volvo -- orifice tube completely plugged, and boy, was that a devil to get out!
Sounds to me like an open expansion valve! 85 psi on the suction side and 100 on the high side indicates no restriction in the AC at the evaporator, or a seriously defective pump or wide open bypass, if there is on (not that I know of, anyway!). More like the needle on the expansion valve is broken off, actually! Must be one hell of a hole in there for there to be no pressure drop -- the volvo orifice tube is about 2mm diameter and about 1 1/2 inch long -- gives 300 psi high side, 35 psi low side at idle!
The expansion valve is up by the brake booster, down low. You can trace the lines up there and see it. It should consist of a block with two lines attached (big and little), a block with a little diaphram thingy on it, and another block (the evaporator, actually).
The air shouldn't be warmer than outside if the temp control is set all the way cold (shuts off re-heat). The temp control valve can fail wide open, causing excessive reheat, and keep the expansion valve wide open. I would suspect this if the air is actually hot. Be warned, failure of this valve can trash the pushbutton control unit! You can check this exactly as you have described -- if the air coming out of the vents is significantly warmer than the outside air, it is being heated, ergo the valve is open.
Check the temperature of the AC lines -- if the suction side (large hose) isn't cold or cool, something is seriously wrong with the compressor or the valve is missing or broken in the expansion valve.
It isn't too difficult to change the expansion valve EXCEPT that the screws are small and easily stripped, and there is no way to stick a driver in them and give them a whack to start them -- firewall is in the way. Have a mechanic do this! Expansion valve is cheap ($40 or so!).
I'm still mystified at how there could be such a small pressure difference if the compressor is actually turning!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!