Heat/AC only blows through defrost
 

I have a 96 Blazer and the heat and A/C only blow through the windshield defrost vent no matter what position the heat/air switch is in. Someone told me it was a vacuum line problem but I don't know. I didn't know if there is a door somewhere that is not switching properly. Just need an idea as to where to start looking.
Thanks.

Most likely it is a vacuum problem. I just fixed a similar problem with my oldest daughter's Jeep. Now I will be doing some ac work on my youngest daughter's blazer. You should be able to follow the vacuum line from the engine, through the firewall, and to the mode selctor switch (ac, defrost, heat switch). It will be easiest to check the vacuum line in the engine compartment first and if its ok. Go to the mode selector switch next and see if you are getting any vacuum there. If you do get a vacuum to the switch then you may have a bad switch. If there is no vacuum at the switch then you will have to trace the vacuum line to the firewall. HTH

Thanks for the info. I didn't realize that the vacuum line had anything to do with the switch. I had heard that but not being much of a mechanic I didn't know. Thanks again.

i had the same problem with my 95 blazer. It turned out to be the vac line at the back of the upper manifold was unplugged. It was a easy fix for once.

Had this same problem on my '96, just fixed it today.

The vacuum line was actually rubbing against the manifold, and had scorched a hole thru the tubing, not to mention all the vacuum lines are shot anyway.. Now they're all replaced and it works tons better :D

Yay for ventilation again

More Info:

If it's a "door" problem, the air generally doesn't blow through the defrost on all settings. When I had a problem with my air door, it was only on the "floor" setting. There are 3 "zones" for air output: upper (defrost), central (main vents), and lower (floor). If all "zones" are affected, it's generally a vaccuum which affects all of them. Defrost is the failsafe - it opens so the air goes somewhere, even if the system doesn't work.