If this is a stupid question, then please forgive. I have a '91 XJ, and when I turn the heater on, it will blow nice and warm at the vents, then suddenly switch to the windshield defroster, then switch back to the vents. It repeats this over and over, with varying times with each on. Also, when I want the heater on, I have to let the engine warm up, and put it on "VENT". If I put it on "HEAT" it just stays at the windshield defroster. What's going on here??

 

Possibly a vacuum leak somewhere behind the controls. If I am not mistaken I believe the vents are controlled by vacuum and the switch controls where the vacuum goes. Could be a bad control then too.

 

on the back of the heater controls is where the vaccume lines connect. it could be a leak there, but it sound like when I took my heater controls apart and put a gear and lever back together just two teeth off. it made it so the vaccume was actually going to two positions at one time. it would switch from one to the other and back and forth like it had a mind of fits own.
HOW LONG HAVE YOU OWN THE JEEP? DID YOU OR COULD A PREVIOUS OWNER HAVE TAKEN THE CONTROLS APART?

 

I've only owned it for a little over 13 months now. As far as I know nobody took the controls apart. So what is the recommended remedy? Pull the controls out and look for leaks, and if no leaks just get a new control panel?

 

Sounds like a bad switch on your dash... pop that sucker out and make sure all the connectors are good.

 

nothing "stupid" to ask here.

 

i would suggest you check for a leak first, before you buy new controls. take the screws out that keep the heater controls secure in the dash.then pull heater control out as far as the the wires and vaccume line will let you.(remember vaccume lines are basically long plastic straws they can and will break if you pull them to hard) after you do that run your heater and put your ear down close to the controls(make sure radio is off the less noise the better) when mine had the leak it sounded kind of like a hissing-sucking sound like a hose on a mini vaccume cleaner. I know they make a vaccume tester, but that is one tool I never bought..if you do hear a leak most junk-wrecking yards sell them for around twenty bucks.

 

Leak. The HVAC system in cars defaults to defrost as a safety feature - to keep your windshield clear if/when you develop a vacuum leak.