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.

450 psi seems incredibly high for the system. The system is either overcharged or there is blockage.

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95 E320 Cabriolet, 169K

The pressure switch shuts off the compressor ast about 450psi (which it did). The expansion valve and receiver/dryer are the two most reasonable restriction sites (and I replaced them both). The bottom line, though, is that the heat is still on when there should be cold air. The pressures now don't indicate a remaining restriction (if that was ever the cause). How to proceed?

Hello Robert There are a couple of things with early ML's .Your aware of the fuse #44 30 amp that blows for the aux fans due to cycling. If your in a hot climate your head pressure will go to the blow off stage,cause loss of charge. "Updated aux fan harness" and 40 amp fuse. The easy way to know if fuse is blown the rear washer wont operate its on same circuit.
Also depending on last seven of the vin theres a doc about the a/c control unit on start up not controlling the air mixing flap do to low voltage causing no a/c cooling or heating.
Did you say your experiencing a vibration from blower? If so Its probably full of leaves 98 ML had a foam kit to block openings in cowl to correct debris getting into blower. good luck Tim

*Fuse 44 was burned. (F16 was missing, but F41 and F43 were ok). Forgot to put that in the orig post. So the pressure can swing that high when the aux fans aren't working! (not like R12) And this is just Indiana summer.
*This owner lives on a wooded lot. Can hardly see the house for all the trees. And the ML sits for long stretches (underhood rodent damage). So I think you're spot on again about the tree debris in the vents.
*Any suggestions on finding that specific update doc in WIS? I posted another thread in this forum about my difficulty navigating WIS (the 'Help' in WIS doesn't offer much substantive help). WIS was designed to dovetail with DAS in SDS, not just as a stand-alone information source. Unfortunately I don't have SDS (big bucks!).
*The bad news is that this customer went away mad, so I may never get the opportunity to check out this new information. Thanks just the same.

Hello Maybe you answered your own question you said you have rodent chit under hood.Did they get into car and nest in upper dash area where the air mix flap is to control temp and chewed some Wires.
Did you say you installed that updated Aux Fan harness or you just changed fuse?Its got a heavy supply wire so you can run a 40 amp fuse.
The one doc for lack of a/c or heat is Af83.40p3400a,If you have contriol unit part #s 163 820 25 89 or 28 89 or 42 89 or 43 89 it may not be controlling air mixing flap correctly.there is vin #s involved but you said cars gone! Tim