Air through defrost when only dash air intended
 

I am geting significant air through the defrost vents when A/C is on and button depressed that should sen all air through dash. This causes condensation on outside of windshield which is very annoying! Any suggestions as o the problem? Thanks, TK

MBFlyer

It isn't clear what you are asking about your 86 420 SEL .... you say there is condensation on the outside of the windshield?

Specifically, which button is depressed?

Haasman

Thanks Haasman, the condensation occurs on the outside when the button with the solid down arrow is depressed. The manual shows that no air should be coming out of the upper defrost vents when that button is depressed and a/c on. Thanks! TK

Default setting for the defroster
 

Mercedes designs the climate control failures to default to the defrost on. If you get any air throught the defrost when it should not be, it means that the servo for the defrost vent has failed and is stuck in the default mode (Defrost on all the time).

Try turning on the heat and see if it comes through the defroster as well. If the defrost will not go off under any condition it is most likely the defrost vent servo.