Mmmmm.
Well, I just went out and drove it.... this is odd...
So, from home and starting off.... temp on min, fan on low and ECO button pushed.... nothing felt from center vents... they are open... hot from side vents and screen vent... and I mean HOT!
Drove two miles..... stopped and picked up my son from work.... he drove and I fiddled.
Hit defrost.... HOT from all vents and Fan comes on max....
Hit Eco button again... suddenly, I get cool air from facia vents and hot from screen vent and then it cools... giving ambient air from all vents.... as it should I think.

Does that make any sense?

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2009 Mercedes ML320 Bluetec