I have a 2005 ford excursion limited, 4X4 diesel. When I turn the A/C on it will only blow through the defrost vents (windshield and front windows). I push the other buttons to change the flow and get no response. I hear there is a vacuum actuater under the dash, any ideas
You have a vacuum leak (95% likely) or a bad air blender (5% likely) Most common spots for vacuum leaks #1 somewhere in the 4WD lines. #2 whatever lines are exposed to the most heat or vibration. That may sound like generic advice but that's how it is, vacuum leaks are tied for the most irratating problem to figure out, oh yeah, electrical gremlins are the other tough one.
This just happened on my Expy, if the Excursion is at all similar, there should be two lines going into the firewall right behind the battery. On my Expy, one of them was loose and actually open to air creating the exact condition you describe.
This has happened to me a couple of times. It was always a leak in the 4wd lines or hubs. To test the hubs just plug the vacuum lines going to the hubs (a golf tee works nicely) and see if your a/c vents start working.