Heat does not blow out of defrost window vents.

I have a 96 Cherokee and all functions on the HVAC work except when on defrost, heat will not blow out. The cold air blows great but when I slide it to warm air the fans continue to blow but no air is coming out. I have seen that a common problem is that air only comes out of the defroster but nothing else but I don't have that problem because everything else works fine. What could be the problem?

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This is for a 95 but likely to be the same as your 96. The last time I had something similar (really odd function) the vacuum connector came partially unplugged from the HVAC selector assembly, my bad as I had recently replaced my radio/stereo. All other times it was system low vacuum caused by the reservoir or the tubing. Looking at the drawing the defrost door almost has to have some kind of spring loading (or vacuum) for open (just a guess). Possible (another guess) something came disconnected or broke and the high airflow is blowing the door closed?

That kinda makes sense but wouldn't it make it not work when the cold air also because it does work with the cold air on.

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I don't get it either the schematic shows the defrost door open with NV (no vacuum). Why would it V (vacuum) when you switch to defrost?

Just a WAG (wild arsed guess) but my defrost always comes on a full fan, it doesn't have steps 1-4. Maybe just quirk of my particular 96 XJ or maybe a change after 96? Possible full fan bows the defrost door closed and there is something wrong with the linkage. Possible soething wrong with the vacuum switch, a kinked line of stoppage and the vacuum isn't releasing from the defrost door actuator? The only other thing I can see that might influence operation any is the hot water valve.

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