The 190 has a vacuum operated heater valve. It is loacted under the cowl. Good chance the vacuum is not getting to it. If you look closely with a flashlight the arm for the valve can be seen through the air duct at the base of the windshield (I did it somehow).

There also is a mixing door that is vacuum controlled and should be shut in A/C. It will keep the compressor of in the snowflake position if the door doesn't close. Use the top button for max A/C, the one that looks like a defrost icon. This confuses most people (including myself, for years). The point to the icon is that in a defrost condition one wants hot dehumidified air. They have a separate def button that lines this all up so I couldn't tell you why they used that icon there, but it is the MAX A/C position.

Without proper vacuum the recirc door will stay open which kills A/C on the 190 especially.

The low side line should be cold not ambient, no matter what all those doors are doing. Pressures tell the story here.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician