I believe what is said in the owners manual although I'm calling BS
on MBZ. It's humid here often and in the summer the car can get hot and you need the 20 -30 degree drop of AC air to cool it off. If it was maintained at 3 degrees it would never cool the interior.
Car manufacturers favor low air flow when the interior reaches the set point for several reasons. Less noise, less air blast, and less air through the system so the AC compressor can cycle and be more efficient. Less air through the pollen filter too so it lasts longer. I agree with all of that.
The problem is less air flow needs cooler air to maintain the temp. When they bleed it on the windshield it condenses on the outside. I have a patch on the passenger side that is over 1 foot long and 6-9 inches tall. Very annoying. It comes back in 2-5 sec after using the wipers on it.
So now I wonder if the defrost flapper is hung open more than it should be or if it is controlled by the ECM can it be reset by MBZ? I'm just wondering if my car has a problem or are all W209 CLK like this??
I am considering buying one of those carpet dash mats to put up there and tape off the defroster holes on the carpet. It would fix it but I think they degrade the interior and I would have to yank it off when I needed defrost.
What does it take to dissassemble the dash to get to the defrost flapper valve? Or is there some other bypass built in the system? Could be some weather stripping have come off internally or hung the flapper open?
Wonder if Rain-X on the outside of the windshield would help?
Please help, and thanks for comments so far.