Hi i have a quick question about my 98 jimmy. I am not getting heat out of the defrost setting to blow onto the windshield. If i put it on the cold setting though it comes out of that area. I am curious if anyone has had this problem before or knows of a fix. Any help would be much appreciated.

 

Check if there is a leak in the vacuum lines that supply vacuum to the HVAC system and the front axle engagement. If you have the 4-button Auto-4wd transfer case, then take a look at the thread I have in my .sig file below. It is worth checking as much of the HVAC system is controlled by vacuum actuators. These hoses rot over time and need to be replaced. They either stretch and do not seal well at the connectors or small cracks open up in them reducing the amount of vacuum they hold.

 

Do you have a diagram of a 3 button setup or is it all similar to the 4 button one? I thought mine was all electronic didnt know there was vacuum at all. Whats the possibility it could be the control panel? Thank you for the reply and anymore help or information would be great.

 

The vacuum hoses inside the passenger compartment are pretty tough. One of the ones from inside comes through the firewall and connects to the engine compartment vacuum lines on the engine side of the firewall. When you find it, you can see how it is different.

This point is also a good place to to a test of the vacuum system inside the passenger compartment by using a hand vacuum pump and drawing a vacuum at that point on the interior vacuum lines. If it holds (pretty well) then that is likely not your problem.

For the transfer case, there are apparently lines that go down to it. If a seal in the transfer case goes bad, then transmission fluid can be drawn into the HVAC system! None-the-less, it si worth looking into replacing as many of the engine compartment vacuum lines as possible. I also added zip-ties to where the hoses are pushed on to the nipples of each of the connectors to further ensure no leaks.

Hopefully your problem is with the easy hoses, like the one that supplies vacuum from the left-side valve cover. There is also the vacuum reservoir hose that you can check.

That's the best I can do for now.

Good Luck