Several possibilities, although I'm not totally "clued in" on the 190 CC.
Bad monovalve. Don't know which type, but if stuck open or leaking, you have heat all the time.
Bad fuse -- if the main CC fuse or the controller fuse is blown, system defaults to full heat out the defroster vents, and that's all you get no matter what button you push. Sounds like this isn't the problem though, as the unit works on economy.
Bad CC pushbutton control unit. Will not control heat.
No freon in AC system.
Slipping belt, so that AC compressor comes on for a while, then stops until you shut the car off.
Bad KLIMA relay, won't run AC compressor.
Best and easiest things are check teh main fuse -- it can get corroded fail. Heater hoses should not be hot with the AC on and temp selector set for max cold -- if they are, the monovalve or PBCU is bad.
And the center vents MUST be open to get AC, otherwise there is no way for the cool air to get out!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!