I have recently noticed that Air from my heater or ac only blows from my defrost vents regardless of the setting.I can still control weather or not its hot or cold. (91 Cherokee 4.0)
When I was replacing spark plugs a few months ago I broke a vacuum valve that was in a 3 split with some coolant hoses. I replaced the broken valve with some cheap looking one from shucks for like 8 bucks. I am 99.9% sure I put it in correctly so I do not think this is the problem. I have done a lot of pulling apart my dash and putting it back together, and my car has been down many a wash board road at 60 miles an hour shaking the crap out my rig. I have not investigated this problem at all but am going to tackle it tomorrow afternoon so I figured I would see if anyone has any ideas of what this might be before I start.
Thanks

 

reading through posts seems that this is more then likely a vacuum issue. guess I will check that first tomorrow

 

Hey Ihaving this same issue and i was just wondering which vacuum line was giving you the issue. Any help would be greatly appreciated thanks

 

Sounds like vacuum. Check from canister all the way back. Mine wasnt working correctly after canister relocation from front bumper install. Would only defrost. fix leak, get back all function.

 

Alright thanks. Ill try spraying some carb cleaner around the canister and see if it shows itself

 

I have the same issue, i didnt bother with it but now its cold as heck out and it would be nice to have some heat

 

i have the same issue, i don't know much about what is what on my jeep , so anyone who can walk me through some troubleshooting would be appreciated.