HVAC (mainly the A/C) intermittently runs through the defroster

jeep2fun

jeep2fun

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Denver, CO
I have a 97 TJ 4.0L sport with Slider HVAC controls and the AC on it runs fine for a while like 30-45min and then switches to the defroster setting. Even though it blows out cool air through the defroster for some time at fan settings, sometimes it comes back to normal operation through Dash vents but then it would again flip back and forth between the dash and defroster. Sometime if I manually turn HVAC off and the fan off and then fiddle with the controls it comes back normal for a few minutes or so but mostly just blows out of the defroster. I have even noticed that the fan will start slowing down from either of the vents and then will come back up but mostly doesn't sty at the dash vents for long. It does not stay consistent which turns pretty annoying with wife and kids.
If this is due to a vacuum line leak where should I look for it? Should I replace the vaccum line and diy instructions/videos will help too. Please advise.
 
My 2005 is doing the same thing right now when i"m running 70mph on the highway. just got back from a small trip

I havent found the problem yet so not sure if small leak since mine does it under stress load on the highway at 2500+ rpm
 
Yes, the exact same behavior here 70 mph on highway. Good point on the rpms, but no idea how to approach the issue. Will wait for the Gurus to chime in.
 
From what I remember, going to the defroster mode signals a vacuum leak. Start trouble shooting with the vacuum line under the hood, starting at the manifold and following the line to the firewall, then go to the HVAC control and blend door.
 
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The are two common locations for vacuum leaks; at the rectangular vacuum storage canister and T at the firewall. The storage canister is at the bottom of the passenger fender near the firewall beneath the battery; there is a rubber fitting that attaches the the canister that is prone to leakage. There is a T where the vacuum line goes into the firewall thru a rubber insulator; the lines are prone to cracks...
 
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Look at the vacuum lines from the manifold that run across the firewall, under the battery tray & into the HVAC box. very common spot to find a cracked hose. When you're on the highway chugging the throttle your manifold vacuum is lower than it is at idle or around town. I had a similar problem last year & ended up replacing all the vacuum lines with bulk hose from Napa. Cost me about $9. Plus my time, whatever that's worth.

1626833792897.png
 
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Good point. I have a ordered replacement molar vacuum line. Let me replace it and hope (fingers crossed) that it fixes my issue.
 
Both of mine that had that issue, cracked where the line goes into the firewall. I had to twist the line to actually see the cracks. The rubber connector near the manifold is another problem area. My XJ cracked there.
vacuum line firewall (2017_11_20 00_38_12 UTC).jpg

Vacuum line break (2017_11_20 00_38_12 UTC).jpg
 
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CharlesHS said:
You can purchase some tight fitting rubber vacuum hose (3/32"), cut out the affected section and slide a small section in to replace area with the crack.
Click to expand...
I replaced the whole vacuum line with Chrysler part # 55037567AC link below, but to no avail. When the air stops from the dash fans and comes through the defrost and side vents it makes a sharp whistling noise. As I said before, If I am lucky enough then if I stop the fan and start again it starts rightly away but most of the times it doesn’t.

https://www.amazon.com/dp/B011V8WJ1M/?tag=wranglerorg-20
 
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