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Floridajeepcherokee
08-06-2013, 12:05 AM
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. 1998 Jeep Grand Cherokee 4.0L

XJ Wheeler
08-06-2013, 01:15 AM
I don't know for sure if this is the problem because i don't know if the Grand Cherokees vent doors are vacuum operated. Cherokees are, and i have a similar problem. And it sounds like a vacuum problem. Most likely there's a leak in one of the lines somewhere. Good thing is they're cheap, bad thing is they can be hard to trace.

Sent via messenger pigeon - i talk, he types.

4.3LXJ
08-06-2013, 05:24 PM
That is right, it is a vacuum problem. Check for leaks, especially at the vacuum reservoir located behind the front bumper.

prcjeep
08-06-2013, 05:38 PM
x3 on the vacuum pressure problem. My 96 used to do this everyday. make sure the little ball looking thing attached to the rear of the front bumper isnt cracked and is connected. They way i always searched for vacuum leaks was to listen for the sucking sounds or whistling sounds, turn my ear towards it and move m y hand over where it think is coming from. If it gets quite then that may be the leak or crack in a line.

Have you or has anyone else done any work under the hood recently?

Floridajeepcherokee
08-08-2013, 09:21 AM
That is right, it is a vacuum problem. Check for leaks, especially at the vacuum reservoir located behind the front bumper.

Thanks, I was under impression it was under the battery tray.

The problem was like this when I bought it, and obly work Ive done under hood was tune and it was preexisting.

nickxj94
08-08-2013, 12:43 PM
Also check at the heater hoses there is a grey vacuum hose that plugs into there sometimes when changing Plugs or oil filter they get disconnected.

prcjeep
08-08-2013, 01:25 PM
Thanks, I was under impression it was under the battery tray.

The problem was like this when I bought it, and obly work Ive done under hood was tune and it was preexisting.

Check all of the areas that you have worked or had your hands around...

Floridajeepcherokee
08-08-2013, 07:24 PM
It was doing this before I bought vehicle, and I believed it to be a vacuum leak because my other vehicle did the same except it only blew through defrost until I fixed the vacuum leak. I just don't know where all the vacuum hoses are on a jeep. I read some are below the battery box

prcjeep
08-09-2013, 10:29 AM
It was doing this before I bought vehicle, and I believed it to be a vacuum leak because my other vehicle did the same except it only blew through defrost until I fixed the vacuum leak. I just don't know where all the vacuum hoses are on a jeep. I read some are below the battery box

Man i wish i could remember what fixed mine... I had the same issue

Floridajeepcherokee
08-11-2013, 03:56 PM
The hose going underneath battery box wasn't going to anything, I tried plugging it and it still does the same thing.

4.3LXJ
08-11-2013, 06:00 PM
you still have a leak to find, unless you nave no reservoir. If not you need one

Floridajeepcherokee
08-11-2013, 06:03 PM
I don't see a reservoir. it should be under bumper on passenger side right??

4.3LXJ
08-11-2013, 06:13 PM
behind it

Floridajeepcherokee
08-11-2013, 08:06 PM
I saw the 2 horns but don't see a reservoir.

XJ Wheeler
08-11-2013, 08:33 PM
I searched around a little and someone said the ZJ has its vacuum reservoir mounted below the battery and is square.

Sent via messenger pigeon - i talk, he types.

Floridajeepcherokee
08-11-2013, 08:37 PM
wow, I didn't see it.

oderdene
08-11-2013, 10:28 PM
under battery tray,

http://www.jeepforum.com/forum/attachments/f13/436321d1342741231-1998-limited-5-9-questions-new-owner-image-1654502640.jpg

Floridajeepcherokee
08-11-2013, 10:34 PM
thanks, I don't have that, and tore my dash half way apart checking interior vacuum lines.

Mudderoy
08-11-2013, 10:45 PM
I remember I was driving 80 mph on a freeway shortly after installing the 4.5" lift and 32" tires. I was really having a hard time keeping it at 80 mph because of the 3.55 gears.

My AC would stop blowing through the main vents and blow through the defrost. I didn't have a vacuum gauge at the time but I think this was due to low engine vacuum since I was having to have the throttle so far open.

drakan1908
08-12-2013, 06:22 AM
According to my manual the vaccum comes straight from the motor. There is a check valve between the motor vaccum source and the heater control that sometimes goes bad.

Sent from my XT907 using Tapatalk 2

Floridajeepcherokee
08-12-2013, 12:32 PM
Arrggghhhh!!!

Floridajeepcherokee
08-12-2013, 01:42 PM
According to my manual the vaccum comes straight from the motor. There is a check valve between the motor vaccum source and the heater control that sometimes goes bad.

Sent from my XT907 using Tapatalk 2

Then why is it defaulting to defrost event only at higher speeds?

drakan1908
08-12-2013, 03:00 PM
If it is loosing vaccum it might default to defrost. At higher speeds it might not have enough vaccum. The manual shows testing the check valve with a hand pump other than that nothing. Mine has a vaccum line chart under the hood. if you have that you would need to start checking the lines for leaks and signs of wear/dry rot. Like mine most all the lines are 15 yrs. old.


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gary63
08-12-2013, 04:07 PM
I did read if you ever found a vacuum resuvore if not it may have been removed and pluged in that case the motor will make some vacuum but anaf to keep all the vacuum acca. running right. You may have to install one to get ti working right.

drakan1908
08-13-2013, 09:04 AM
Got off my butt and went and looked at mine last night. The first picture is where vaccum is picked up from the intake. The line runs around the firewall to the passenger side where the line tees with the line that goes to the interior (2nd picture). The line then goes around coolant bottle along the passenger fender and tees with the cruise control, then down under the battery to the resevoir. Does your cruise work correctly?



http://i219.photobucket.com/albums/cc250/drakan1908/IMG_20130812_175746_294_zps88d54977.jpg (http://s219.photobucket.com/user/drakan1908/media/IMG_20130812_175746_294_zps88d54977.jpg.html)


http://i219.photobucket.com/albums/cc250/drakan1908/IMG_20130812_175823_211_zps90d9826a.jpg (http://s219.photobucket.com/user/drakan1908/media/IMG_20130812_175823_211_zps90d9826a.jpg.html)


This is a picture from under the front looking at the bottom of the battery tray and resevoir. Sorry about quality I had to stick the camera up and guess.....should see the ones I deleted. lol

http://i219.photobucket.com/albums/cc250/drakan1908/IMG_20130812_175550_438_zps4e648b53.jpg (http://s219.photobucket.com/user/drakan1908/media/IMG_20130812_175550_438_zps4e648b53.jpg.html)

http://i219.photobucket.com/albums/cc250/drakan1908/IMG_20130812_175621_118_zps5eb71ac4.jpg (http://s219.photobucket.com/user/drakan1908/media/IMG_20130812_175621_118_zps5eb71ac4.jpg.html)

Floridajeepcherokee
08-13-2013, 07:00 PM
Thanks, I bought a ball reservoir and tried that. It's still doing it. Yes The cruise works.

drakan1908
08-13-2013, 07:36 PM
You probably need to check the check valve. It must be in the dash somewhere :banghead:

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4.3LXJ
08-13-2013, 09:43 PM
The check valve should be visible in the engine compartment

drakan1908
08-14-2013, 07:11 AM
The check valve should be visible in the engine compartment

It's not unless it is real small and under the rubber boot. The 2nd picture above showing the tee to the main vaccum line is all there is, from there it is straight through the firewall.

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bulrid8
08-14-2013, 07:40 AM
The check valve is in your first picture. Did you run the entire length of the big vacuum line and look for breaks? That hard line cracks easily and dry rots over the years.

You vacuum reservoir may be cracked too. You need a test pump and start testing lines to find the problem.

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drakan1908
08-14-2013, 05:27 PM
The check valve is in your first picture. Did you run the entire length of the big vacuum line and look for breaks? That hard line cracks easily and dry rots over the years.

You vacuum reservoir may be cracked too. You need a test pump and start testing lines to find the problem.

Sent from my ADR6400L using Tapatalk 2

Haha you are right I didn't pay much attention to the first pic:thumbup:

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Floridajeepcherokee
08-14-2013, 07:15 PM
I'm about to give up, put a ball type reservoir and it still does it.Dodge won't verbally help m e, They just want my money.

4.3LXJ
08-14-2013, 08:24 PM
What kind of higher speeds are we talking about and how far down is the pedal mashed to get there

Floridajeepcherokee
08-15-2013, 01:39 AM
around 2,000-2.5 thousand rpms and /or 55 mph

Mudderoy
08-15-2013, 09:32 AM
Have you put a vacuum gauge on the engine yet? It would also be good to know what it reads when the A/C air flow changes.

4.3LXJ
08-15-2013, 10:58 AM
Have you put a vacuum gauge on the engine yet? It would also be good to know what it reads when the A/C air flow changes.

That is kind of where I am going. If you don't have any vacuum at all, even a reservoir won't save you from this. But I still suspect there is a leak somewhere that is not real serious, but enough to affect the system at partial throttle. Also we don't know if the check valve is working yet.

Floridajeepcherokee
08-16-2013, 12:09 PM
no guage, called Dodge Jeep and asked for price on check valve and he talked like there were numerous valves.

4.3LXJ
08-16-2013, 12:15 PM
Check valves are cheap at auto parts stores. You should have one in the main line and one on the CAD line if you have one

drakan1908
08-17-2013, 07:12 AM
Swing by a junkyard and pick up a check valve or two.

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