1999 Mercedes Benz ML320 Not working

I have forwarded your question to our M/B tech. He will be with you shortly, thank you for your patience

First I would like to clarify the issue with you. I am reading this as the compressor clutch does not engage when you send a request via the AC pushbutton unit in the vehicle? The pushbutton control unit in this model is common to replace for just such an issue, especially if the compressor clutch is engaging at start up (when it performs system checks). A pushbutton unit is something that will need to be replaced at the dealer. Also a leak in the AC system will also cause this condition. On the passanger side of the motor you will find two AC lines (high side and low side). There will be black service caps on these two lines. Remove one of the caps and with a small screw driver lightly press on the valve under the cap. Be careful to have your head out of the way as a high pressure spray of freon will come out and will be visible. If no freon comes out of the valve you have an empty ac system and more than likely a leak. Finally the AC pressure sensor may also cause this as when it fails it sends the wrong pressure values of the AC system to the AC control unit. This will cause a inoperable condition of the AC system and will keep the compressor from running. The pressure sensor is located on the the receiver dryer and does require discharge of the AC system to replace (receiver dryer will also have to be replaced). Of the three posibilities listed above: failed AC pushbutton unit, AC system leak, or AC pressure sensor all will require either discharge of the system or in the case of the pushbutton, coding to the vehicle. Unfortunately none of these will be something you can do at home. Finally in regards to the temp sensor, this would not cause this issue as the temp sensor on this system is mounted to the evaporator and monitors evaporator temp to prevent freezing of the evap. This would cause a cut out condition not a inoperable condition.