Hello,
I’m new to this forum and I’m looking for some help please.

I have a 1989 300SEL that only seems to blow air out of the center vents, either hot or cold, when the outside air temperature is above about 60 degrees F. When the outside air temp is below 60F, the air is directed to the defroster, side vents, and some floor. The system seems to heat and cool properly otherwise and it works perfectly when the outside temp is above 60F. Any ideas? I looked in my climate control manual and I see there is a outside air temperature sensor under the cabin air inlet. Could that be the problem?
Thanks for your help.

 

Does this happen under automatic operation, or does it happen when you manually direct the airflow to be center vents?

If just under auto, then it may work that way by design. Most people don't like direct airflow when it's cool (definition of draft: cool airflow when you don't want it, definition of breeze: cool airflow when you do) ; that's why ceiling fans are reversable. They blow down when it's hot, but push up when it's cold, directing the air down the walls and across the floor before it reaches you; both directions exhange the air near the ceiling with that near the floor, but one does it without puttting a direct breeze on you. Outside air of 60F probably means the car is heating the air a bit, and thinks you'd be more comfortable having that air sneak up on you via floor/defrost vents rather than blown directly at you from the centers.

If you happen to live in a high altitude location, or one that's commonly arid, the climate control system may try to over-correct: heating a bit too much in fall and spring. Moist air takes a good bit more effort to change it's temperature, and high altitude locations are usually very dry, so the same effort to warm the air up a touch goes much further. They might be able to include a humidity sensor into the climate control system, but I don't know of any manufacturer who's done so yet. And I can testify to plenty of auto makes who's automatic cc systems that need a lot of manual assitance seasonally here in Denver where I live.

 

Thanks for the reply Danbrady. I understand what you’re saying, it makes sense, but I still think it’s not functioning properly. This is my fourth 126 chassis Mercedes and they all would blow cold air out of the centers vents if I set the temperature control to max cool in the econ mode. When the outside temp is below 60F this thing won’t even send air conditioned air out of the center vents. It seems that when its bad it sends the majority of the air to the defroster, with little going to the floor. I live outside of Minneapolis, so altitude should not be a problem. Additionally, below 60F outside if you put the heat on it wants to send that air to the defrost (windshield) as well and not to the floor. Again, when the temperature is above 60F, all seems to work fine.
Thanks for your thoughts.