ML320 air conditioning

[Robert Squires]

Hello,
This '98 ML320 air cond was blowing mostly heated air, but not exclusively. Sometimes it acted like the AC was trying to work. There were no codes in AAM. (No communication with AAC; guess no AAC for this model). I put pressure gauges on it and, with the engine fully warmed up and idling, watched the compressor cut out at about 450psi and cut back in when the hi-side had dropped back down to 300psi. Low-side pressure was ok during all of this. The compressor cut in and out several times a minute.
I replaced the expansion valve and receiver/dryer, and the pressure switch (although it was obviously working). Vacated and recharged. Now the compressor doesn't cycle (i.e pressure doesn't spike to 450psi) but the air out all the vents is always heated air (not just ambient air). It no longer even acts like it's going to cool.
I couldn't find a duo-valve (heat valve) under the hood. Is the vent temp varied exclusively by the blend air flap? While the AC was briefly working, the blower was quiet. As soon as heated air comes out of the vents, the blower (or something else) sets up a vibration/rumble in the top center of the dash. Does anyone have an idea what's going on here, or can you suggest a systematic test approach. (The information I need is likely in WIS, but I'm not sure I'll ever find it.) Long post. Thanks for taking the time.