This is what mine does, I'm not sure if nothers have this problem. There are only two temperature settings it seems on my car, HOT, and COLD. I can only get cool air if I keep the thermostat on MIN. If I move it a little bit, the air instantly turns hot, and does not cool down. I replaced the rubber hose in the glove box, so that probably is not it, although I probably should check it to see it the connections are snug. I put a piece of tissue paper on the little grille on the dash to test it, it didn't get pulled on the grille, but I think it's because the bit of paper is too big to be pulled by the little force of the suction of the sensor. What could be the problem? I really don't find the MIN being the only cool setting on the a/c, it's going to be pretty hot down here for the next few months anyway.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.