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.
Bosch Master, ASE Master, L1
33 years MB technician