I have a 91 XJ Laredo, and the winter here in VA is starting to get nippy with 16 degree lows and wind chills that just suck. Some heat would be nice.

So, here's my issue. When I turn the heater onto Defrost, air blows like it should right at the windshield vents.

Ok, the windshield is good. Let's heat my hands up now and feel comfy. Here's where we hit the issue.

If I pull the lever to Heat, nothing happens. The heat stays on the windshield. If I pull it over to Vent, it will pump lukewarm air from the vents, but randomly it will switch back to the Windshield vents, and then back to the cabin vents over and over.

I was told there is most likely a vacuum leak somewhere, and not a blend door issue because the difference between heat and vent is noticable and I am getting hot air when I'm on the Defroster.

Does anyone have a schematic or something for the vacuum lines to check? My Haynes manual has nothing, and neither does the WWW. Thanks.

 

well i have a 95 cherokee and i had my front bumper off and when i put it back on i forgot to hook up the vacume line going to the round vacume chamber in the bumper and i had problems with my heat switch so id say check that line that goes into your front bumper

 

I believe the vacuum line that controls your heater is a plastic cylinder about the same diameter as a silver dollar located by the firewall passenger of the block. There will be a single line running to it that at least on mine is yellow, check that out.

 

http://oljeep.com/gw/vac/90/1990_40L.jpg This was the only diagram I could find. I'll look in fsm and see if theres a better one.

under the hood, near the a/c box theres a vacuum line. Just went out and checked mine. dam it's cold! make sure it's not pinched or cracked.

 

You talkin about the Heater Control Valve? The little black thing with the heater core hoses runnin to it, with a small vacuum line hook to a cylinder? I think that controls coolant flow into the heater core. I need to find the vacuum line that controls where the air flow goes.

 

Ah you're right, long day my bad. I thought the sliders on the dash for the vents were controlled by linkage though?

 

I would think so as well, but I've heard mixed ideas on that. I hear from one guy is a linkage, and then from another that its vacuum controlled. And this POS worthless Haynes manual doesnt do anything to help lol.

 

I'm 99% sure it's linkage. I just ripped apart my dash the other day, partly to install a switch and partly out of boredom. So basically I did way more than what was actually required, but when I got to the climate control unit deal I didn't notice any vacuum lines but I didn't look that closely either. If you really feel ambitious take yours apart, it's not overly hard to get to the climante control deal. And yes, Haynes is good for about a quarter of what you need to know. I have a hard time believing they actually completely tore down and rebuilt the vehicle, as per the cover

 

It's a broken vacum line to the Heater Control Valve or the Heater Control valve itself.

 

ok i had the same issue a few months back... check the vacuum lines by the battery. my hood rod rubbed through mine.

 

Would it be a good idea to check them while the XJ is on? I was lookin where you told me to and I did see a line running beside the battery, but it was below the hood rod.

 

ya you can have it running and listen for a hissing noise. my hood rod holder is broke so it went farther down and rubbed through