So I am about to replace my leaky heater core. While the dash is apart I want to fix my vents. On the controls the fan speed works properly but it always blows out of defrost vents. In cant seem to ever get air to flow through vents on any setting. What should I look at to fix THIS? I checked the backs of all the connections and no corrosion orany of those issues at the panel where the ***** are. I have a 99 Xj 4x4 4.0.

 

The heat control works by vacuum and if there is any break in the vacuum lines it blows out of the defrost as it's default setting. You will need to trace all the vacuum lines from the ball up behind the front bumper all the way to the heat switches and see where the break is. To give you an idea...The same thing happened to me. It turned out one time when I pulled my battery out and put it back in I accidentally nicked the vacuum line running along the fender behind the battery and that's all it took to have mine just blowing out the defrost only

 

I think the 99 has electric motors that change the flaps in the heater box. You will have to check the voltages on them.

 

Open this link and scroll down to post #51.

https://www.cherokeeforum.com/f2/gre...e-6558/index4/

There's a series of photos and schematics that show the HVAC/Cruise Control vacuum system in your engine bay. Visually inspect each area for cracked/chafed/broken lines and rotted rubber couplings.

 

i had the same issue on my '99, and the problem was that the vacuum line had come off the ball behind the front bumper as 'Programbo' had mentioned. It took about 5 seconded to put it back and the vents directed air the as they should. The ball is black and is located behind the front - right bumper, I'd look there first.
Good luck.

 

Check your heater control switch