This is a common problem with just about any heater/ac  system. The mode doors behind the dash are controlled by vacuum from the engine. These mode doors control where the air is delivered-defrost-dash-floor- etc.

 

What happens is a leak will develop in the vacuum source to the mode doors. When this happens the mode door defaults to full defrost and all the air will blow on the windshield.

 

Now the trick is finding the vacuum leak. There are many vacuum lines that run from the engine to the a/c control head and to the mode door actuators. A leak any where in the system will cause this problem. The most likely place for a leak to occur is in the engine compartment, these rubber hoses deteriorate and leak over a period of time.

 

It may be a little difficult for a do-it yourself-er to locate and repair this at home but give a quick visual inspection at all the rubber vacuum lines under the hood. Replace any lines that are cracked, broken or swollen.

 

Your mechanic should be able to locate and repair any leaks quickly and this is usually in-expensive to repair.

 

If you want an online repair manual that can give you illustrations on the vacuum lines for your heater I offer one from ALLDATA, this is a great repair manual that many of my readers use and have great success with it. With this repair manual you may be able to check and repair this problem yourself.

 

Thank you for asking YourAutoAdvisor your question.