In Jauary of this year I had a timing cover gasket and water pump installed in my 2001 Mercury Sable. Today I turned on my ac and the air started blowing out of the defroster vents. I went back to the mechanic and said that when he put the car back together he must have gotten something switched around, disconnected or maybe he forget to reconnect something. He was offended.
He said cars break down. Even though he didn't say it, I got the feeling that maybe he was indirectly saying I was trying to get another problem fixed by using the work he did as the reason the additional trouble happened.
He stated if warm air was blowing out of the vents when I used the heater, it should still blow out air when I turned on the ac. I apologized to him. I didn't mean to be accusatory in my concerns. All I can say is I never had air blowing from the defroster during the 5 years I have this car until this work was done.
I have two Questions. 1. What could the problem be? And 2. Is it possible that he could have disconnected or forget to connect a hose when he reassembled the car parts? Thank you for your time and patience.
Check if air blows out any other vents in any other modes. If it continues blowing out of the defroster, suspect a broken vacuum hose between the engine and firewall where it goes to the heater controls. Most systems that use vacuum motors to move the mode doors around are spring-loaded to go to defrost mode when there is a failure. That insures that even though your feet might freeze in winter, you will have a clear windshield. There is only one vacuum hose and check valve so there is nothing to mix up under the hood.
Besides that vacuum supply, it is also possible to have a defective vacuum motor under the dash. The clue to finding that is that some functions will change but some things will work improperly. It's when nothing changes at all when you switch modes that the vacuum supply hose should be suspect.