I have an 88 300E and no matter what setting I have the climate control on, air always come through the upper vents at the windshield.
Does anyone know if this is a vacuum problem or an electrical problem? Or is it something else?

Any input is greatly appreciated.
Thank you in advance

 

Vacuum my 87 300D does it also. behind the heater controls is a vacuum piston that opens for air in other area's check if it has vacuum or is bad

 

Where did you say the valve is located- I have an 84 300SD- any help will be appreciated. Windshield fogs on outside when AC is fully charged.