On the climate control system I am not that knowledgeable in the technical aspects of operation. What I have told you is straight out of the owner's manual of what the system should do, and that is how mine works.

In heat modes, no air or very little should come from the center vent ever. With the center button pressed, regardless of heating or cooling, air will go to the side vents and some air is leaked to defrost. The AC is always running (heat or cooling) with the center button pressed as dried air defrosts better and removes condensation that fogs up the windshield from inside.

Does you recirculating button work right? If that is stuck on your windows will fog up pretty quickly.

I guess I am fortunate in never having any problems with the system on our '95 ever. It seems to work quite well, especially in terms of the accuracy of the interior temp. It works considerably better than the digital dual zone systems in our Range Rover / Land Rover and VW.

Good luck

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro