Correct, your S90 does not have a high pressure port for the AC.

The low side pressure with the A/C compressor cycling should be 23-45 psi which is what you have. Cycling time depends on outside temperature. Cooler temps have the compressor cycling more frequently. Hotter temps keep the compressor engauged longer.

The heater control valve is located on the firewall and has a samll vacuum line going to it. Check for vacuum and check for movement when your dashboard temp is moved from hot to cold.

If your system is working fine, your accumulator should still get cold and you can feel that whether or not your heater control valve is working properly.

It's hard to tell if the orifice valve is partially blocked without taking it out. If you haven't had any problems with the system such as a seized compressor that would distribute particles throughout the system, your orifice valce is probably o.k. .... Unless all the r134a you've been adding has a leak sealer in it. That stuff us bad news. The orifice in the valve is very small and can be plugged up with sealer gunk. I find it difficult ot find just plain R134a without any other bonus stuff added to it.

It is possible to overfill these systems. The only proper way to do it is to evacuate the entire system, repair leaks, then add only the required number of ounces to the system w/oil. But this is not alwyas possible in the home garage.

What's your system's static pressure with a cold engine at 75 degrees ambient?

DEWFPO
--
1998 S90 068,300 and 1995 964 152,300