Just had my 98 ML 320 a/c looked at. The first thing you should check is the aux cooling fans. Do they operate when the a/c button is depressed? If not, check fuse #44. It's a 30 amp fuse that is known to blow often. MBUSA has a recall on the aux fan harness and should replace the harness and install a 40 amp fuse at no charge. The operating pressure was about 250/high and 35low with the outside temp at 84. The vent temp at idle was 41 (fan at position 2) at which point the a/c cycled to off and then back on.
Good luck.

P.S. More info on the #44 Fuse problem can be obtained using the search function.

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