wareagle32789

Complete newbie, just bought a 98 Cherokee yesterday. Previous owner stated that he had it 3 years and the A/C never worked. On the way home from picking it up, I was playing around with the A/C controls, and found that in defrost mode, ice cold air would blow out of the side vents.

I can't get air to blow through the dash vents, no matter which setting I turn the **** to. It seems like the compressor is kicking on, and since it was 95 degress outside, I was able to keep the interior cool using the defroster.

Any ideas on what could be going on here/where to start sorting it out? I would rather try to tackle this myself, instead of just running off to a repair shop and bleeding cash immediately.

Thanks!

CCKen

Your HVAC unit vent doors are vacuum controlled. A loss of vacuum to the HVAC Mode Selector will default to Defrost.


More than likely you have a broken vacuum line in your engine bay that you should try and find. Below are pics of the vacuum lines you should examine.


Schematic of the vacuum lines:



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Start at the intake manifold and move across the firewall:



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You may or may not have a check valve where the small tube enters the firewall. This small tube is the vacuum feed to the HVAC Mode Selector.
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This small tube has been found broken. The pics below show a broken tube and the fix by using a rubber vacuum rated hose to couple the two broken ends.



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If the small tube is okay, move along to the cruise control servo.


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If okay, the tube that goes from the servo drops down to between the battery and the right inner fender. The tube has been found broken here (no pic). From here the tube goes to a Vacuum Reservoir behind the front bumper, right end cap. The connection at the reservoir has been found disconnected or broken. You can access a view of this connection by laying on your back and pulling down on the right plastic wheel well liner. Using a flashlight look up at the reservoir connection.



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Do the inspections and get back with your findings. There could be other issues.

cwmoss

The PO of the 97 I just bought said that the ac was only blowing through the def. When I went to pick it up, my buddy, who has been a mechanic for over 50 years, said the that a vacuum leak was the first thing to look for, to address this. I am happy to have come across this post giving me a process to follow, to help me find the leak.

wareagle32789

Been busy, will do the investigation & report back.

Thanks!

cwmoss

How much vacuum should your mightyvac hold on the line that goes thru the firewall to the control switch? Mine holds about 5 inches, I can pull more, but it leaks down to about 5 and holds. The vac res. holds and the cruise holds. The lines that run between these hold.

CCKen

When testing my system I pumped it down to 20 in/hg and it held. I think I read in the FSM the minimum for operation of the system is 8 in/hg.


I don't know why yours would leak down to 5 then hold. Where are you tapping in to pump the system down? At the small tube going through the firewall, or at the intake manifold?

cwmoss

I got that reading at the small tube going thru the firewall. I did a quick pull from different places yesterday. I am gonna hook everything back up an start my pulls from the intake. Yesterday I made a pull from the vac. res. and it leaked down pretty fast, but I realized that I can't make my pulls from there.

CCKen

When checking the vacuum at the intake select defrost (5) on the mode selector. This will trap the vacuum at the mode selector (see vacuum chart below). If it leaks down the leak is in the black tube that goes through the firewall or the fault is in the mode selector. If it holds, select some other mode, like OFF and see what happens. Continue making other selections and watch the results.




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cwmoss

Thank you for the chart. I just remembered that I had downloaded the Factory service manual that you recommended. I have found the section on how to test the system.

wareagle32789

OK, so I traced the line as recommended by CCKen, and the line appears to be intact. Traced it all the way to the canister under the bumper. I don't have cruise control, so it's pretty simple. The only thing I noticed is that the 90 degree fitting where the vacuum line connects to the intake manifold seems a little dry rotted. Could the vacuum leak be there? If not, what's the next thing to check?

CCKen

Did you do a leak check with a hand vacuum pump from the intake manifold, as discussed?

cwmoss

Yes, you will need a vacuum pump, a Mighty vac is a nice inexpensive one to get. There are a lot of different things you cab use it for.
CCKen, I found my leak. I took the entire vac line from the manifold to the canister off and then took that apart to check each component. It was the line from the intake to the first tee that had a pinhole in it. I had to check it in the sink that I filled with water, found the pinhole, cut the line there and then joined the line back together with a piece of rubber vac. hose. Thanks for the tips!!!

CCKen

Good work and thanks for the follow up.

travisrr

I'm having a similar issues with my 99 cherokee. Sometime the air will work great for an hour long trip, blowing out the vents like it's supposed to. I used it as a wheeling rig. It's about 25 minutes to the trails. Usually what happens is the air works great until I get out on the trails. Then once I get out there, it starts to blow out the defrost. I visually looked for leakes and cannot find any. I also sprayed carb cleaner all down the lines while it was running in an attempt to locate the leak. I need try the Mighty Vac I guess. Ive replaced a dry elbow and both check valves. Whats weird is its an intermittent issue. Like I said, the last time we went wheeling, It worked great on the way, after we were out there for awhile, it started to blow through the defrost, then after about 2 hours, it started working great again after hitting a bump.... Another symptom it has is most of the time when I get in to start it up, you can here the ac doors "suck" into the position they should be in. I read this could be caused by a bad check valve not holding vacuum, which is why I changed them. Any ideas other than rechecking for leaks??

CCKen

travisrr,


Check for leaks using the vacuum pump. Look for a fault at the vacuum reservoir.