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Old 07-13-2010, 03:10 PM   #1
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Question Air Conditioner blows out defrost vents on freeway

I looked through the threads here and could not find my problem, so here it is:

My A/C blows ice cold air and works great around town!! But, when I get out on the highway or anywhere where I am going a constant speed, the air will switch from blowing out the dash vents to the defroster vents. If I let up on the gas pedal for a few seconds it will return to the dash vents again. Within 5 - 10 minutes it will start blowing out of the defrost again. No problems with any of the fan speeds and when it switches to the defrost vents it's still ice cold.

This is really annoying on long trips.

Any ideas

Thanks,
Chuck

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Old 07-13-2010, 03:16 PM   #2
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have you checked for a vacum leak

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Old 07-13-2010, 03:19 PM   #3
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I looked around, but when it is sitting idling it blows out the dash vents like it should.

Any suggestions on where to start looking for the vacuum leak?

Chuck
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Old 07-13-2010, 03:54 PM   #4
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I am in the process of searching for a similar vacuum leak. There is a small hose that comes off the manifold and runs towards the firewall and then tees off, towards the battery tray and through the firewall. I have checked all those connections and now I am fixing to look behind my dash to see where that hose goes. My a/c blows cold but only through the defrost vent or at my feet. Good luck!!
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Old 07-13-2010, 04:21 PM   #5
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That's because you have lots of vacuum at idle and less at cruising speed. Find the leak and you will be golden.
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Old 07-15-2010, 10:03 AM   #6
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Quote:
Originally Posted by DevilDogDoc View Post
That's because you have lots of vacuum at idle and less at cruising speed. Find the leak and you will be golden.
Sounds to me like that is gonna be easier said than done. Kinda like finding a bad ground.

I am guessing this is a very minor leak since it works so well around town and at idle.

Chuck
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Old 10-03-2010, 02:09 AM   #7
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I had the same issue, and it drove me crazy for YEARS! I even disconnected the panel vent actuator so it could stay open all the time. Having the AC blow up on defrost, and not right at you is horrible in the Florida summer!

Anyway, it is absolutely a vacuum leak. I tried replacing the check valve and the vac line that connects to the manifold, thinking that was the culprit (letting vac escape back to the engine under load). Then I took out the battery expecting to find a problem with the vac reservoir, since that FSM says that it the part that allows the vac system to work when under load, and found no problem. After some testing with a vacuum pump, and gauge, I found a really fast leak in the "half open" side of the defrost actuator. The one with the yellow line connected to it. I pulled the line off the actuator, capped it, and now everything is perfect except the defrost is always half open, or full open when actually on defrost. I will grab a new actuator next week, and be good as new.

Do yourself a favor, get a vacuum pump and just test each section of the system (I borrowed one from the local Auto Zone). It took an hour to find the problem once i had the pump. Without the pump, I would have tried replacing the reservoir, more lines, and probably the wrong actuator, since it was the PANEL that had a hard time staying open.
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Old 10-03-2010, 02:47 AM   #8
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Funny, I had the same problem on a 1998 Cherokee
It lasted for more than a year, till I found the leak, mine was due to a problem in a small rubber tube on the right side under the hood connecting some device in the front of the car to the AC system.

I thought they had these things fixed on new models!
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Old 10-03-2010, 09:32 AM   #9
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Vacuum leak for sure when you lose vacuum it defaults to defrost
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Old 03-28-2011, 08:34 PM   #10
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Vents Not Switching from Defrost to AC??? Try this....

The vent doors on Wranglers are pretty notorious for sticking... One common problem is a vaccuum tube that runs out of the firewall and goes down to a box mounted underneath the battery tray. It's an easy fix with electrical tape, but it took me over a year of tinkering to find it....

The hose running from the engine wall... (has a yellow cap on it)


End of the line...


The magic box, under the battery tray...


Hope this helps someone, someday....
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Old 03-29-2011, 12:03 PM   #11
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My leak was under the battery tray. I took it out and seen the hose almost broke all the way through.
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Old 03-29-2011, 12:22 PM   #12
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i replaced my vacuum canister with one from a volkswagon....no problems since.

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