300E AC defrost vents stuck open with AC on
 

I have a 91 300E with 170K that was recently working fine with AC vents blowing correctly. I decided to turn on the Defrost mode to see if it worked (and it did...got very hot,very fast!) but when I switched it back to normal AC dashboard only operation, it was still blowing strong out of the defrost vents and front force was noticeably lower than before. It is nice and cold but is still like that today. What should I check and any advice on how to fix...scared of the answer:) Thanks

The most likely explanation for that is that there is a vacuum leak in the ventilation sytem, or the defrost actuator elements (AKA pods) have torn diaphragms. Either way, it has the same result: the defrost pods do not close when A/C is selected, thus permitting cold air to blow onto the windshield and cutting down the airflow out the center and side vents. By default, the defrost pods are open. When A/C is selected, the pods are supposed to close (they're never supposed to blow A/C air onto the windshield). But since the flaps in the ventilation system are all vacuum controlled, if there's a large enough vac leak, there won't be enough vac to pull the defrost flaps closed, and they stay open. Ditto with the torn diphragms, they won't close because they no longer can hold a vacuum. The remedy for this is to replace the pods (there are 2 for defrost, and about 6 total). The pods are about $50 ea, but the expense here is the labor to remove the dashboard to get at most of those little critters. If you're adventurous and up for a weekend DIY project, there are lots of threads on removing the dash and replacing the pods.

Wow thanks! That's what I was afraid of. I guess I'll live with it until I feel like tearing into it. Appreciate the response very much!