Two common problems seen here. Loss of vacuum under the hood, or the potentiometer for the blend air flap is faulty or out of calibration. In any case a tech should take care of this.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM