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Unread 08-11-2013, 07:39 PM   #1
Volf
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1997 XJ Cherokee 
 
Join Date: Sep 2011
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Cruise Control and HVAC issues. Vacuum woes.

I've been having what I assume is vacuum issues ever since I bought my Jeep last year.

The cruise control works intermittently. Sometimes it will work fine, other times it will work for a while then shut itself off while going down the road, and other times it won't work at all.

The HVAC controls are a bit screwy too. Most settings work okay, but defrost seems to get stuck halfway. The little vents that shoot air on the side windows start throwing air, but the main center vent for the windshield just whistles and will not throw air.

I have checked my vacuum lines and they all seem to be okay. Checked the check valve in the manifold, also seems okay. I convinced myself that it was the vacuum ball, so I made my own out of PVC and a brass fitting. Now the HVAC controls don't work while going down the road, if you want to change vent settings you have to move the knob then let off the accelerator until idle is reached and then the vents will switch over.

Ideas?

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Unread 08-11-2013, 07:51 PM   #2
dave564
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My 2000 had a similar problem with the cruise. Intermittently it wouldn't hold speed or accelerate on command. Turned out that the vacuum line that went from the cruise servo around behind the battery to the bulb had a hole worn in it where it went past the battery.

Suggest you put the bulb back on, then start checking your vacuum backwards from there. (IE at the bulb, then at the cruise, then farther back up the line till you get to the manifold) When you find vacuum, well you have found the leak. Fix accordingly.

Good Luck!!
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Unread 08-12-2013, 06:39 AM   #3
CCKen
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Here's the methhod I used to check the integrity of my HVAC vacuum system (using a Harbor Freight brake bleeder vacuum pump).

With the Heat/AC panel mode selector in the off (O) position, hook up the pump to the rubber coupling at the intake manifold, pump the vacuum down to around 17 in. Hg. Let it sit for awhile and see if it drops off.

You can check the operation of the vacuum operated doors in the HVAC Unit by selecting a position and listen for the door operating. Re-pump the system down again and check another door.

I pulled my engine out just so I could take this pic fotr you.

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Unread 08-12-2013, 06:45 PM   #4
Volf
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Quote:
Originally Posted by dave564 View Post
My 2000 had a similar problem with the cruise. Intermittently it wouldn't hold speed or accelerate on command. Turned out that the vacuum line that went from the cruise servo around behind the battery to the bulb had a hole worn in it where it went past the battery.

Suggest you put the bulb back on, then start checking your vacuum backwards from there. (IE at the bulb, then at the cruise, then farther back up the line till you get to the manifold) When you find vacuum, well you have found the leak. Fix accordingly.

Good Luck!!
I have vacuum anywhere I pull a line out of a connection. I can feel it pulling the skin on my finger tip.

Quote:
Originally Posted by CCKen View Post
Here's the methhod I used to check the integrity of my HVAC vacuum system (using a Harbor Freight brake bleeder vacuum pump).

With the Heat/AC panel mode selector in the off (O) position, hook up the pump to the rubber coupling at the intake manifold, pump the vacuum down to around 17 in. Hg. Let it sit for awhile and see if it drops off.

You can check the operation of the vacuum operated doors in the HVAC Unit by selecting a position and listen for the door operating. Re-pump the system down again and check another door.

I pulled my engine out just so I could take this pic fotr you.

I appreciate you pulling the motor for me.
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Unread 08-13-2013, 12:22 PM   #5
dave564
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Quote:
Originally Posted by Volf View Post
I have vacuum anywhere I pull a line out of a connection. I can feel it pulling the skin on my finger tip.2:
Well all I got now is to check to see if the vacuum bulb will hold a vacuum.
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Unread 08-14-2013, 08:18 AM   #6
cbenez1
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You may have vacuum but you may not have enough. Test it as per CCKen's recommendation.
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Unread 08-20-2013, 02:51 PM   #7
Volf
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Update


Went to Autozone and rented the vacuum/brake bleeder tool. Tested as described above.


Results:

First I just pulled the the main vacuum line off the manifold. I figured that'd be a good place to start. Pumped it down to about 17-20 and let it sit for 30 mintues. Needle didn't budge. Held vacuum just fine. So of course I'm all sorts of confused because I'm thinking the vacuum system is fine. I started the Jeep and attached the vacuum tester to the fitting on the manifold to see what idle vacuum was. It was about 17-20.

So since I was confused and needed time to think I decided to test the factory vacuum ball. Holds vacuum fine. I then pulled the tiny vacuum line that goes into the firewall for the HVAC controls (I assume anyway) and attached the tool and pumped it down to 17-20 and it holds vacuum just fine.

Now at this point I figure I'd play with the controls and try to move the blend doors around since the tool is hooked directly to the HVAC system and nothing else. Same results, cannot get the defrost door to open, it just whistles. Rest of the doors work fine. Sometimes a bit slow to change between settings, but they all work as intended.

I set the hvac controls to defrost and then took a piece of wire and ran it down the defrost vent to manually open the defrost blend door. That does work, the door even stays open for about 5-10 seconds before loudly slamming closed and whistling again.


I'm actually more confused than when I started. WTF is going on?
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Unread 08-20-2013, 03:01 PM   #8
dave564
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Quote:
Originally Posted by Volf View Post
Update


Went to Autozone and rented the vacuum/brake bleeder tool. Tested as described above.


Results:

First I just pulled the the main vacuum line off the manifold. I figured that'd be a good place to start. Pumped it down to about 17-20 and let it sit for 30 mintues. Needle didn't budge. Held vacuum just fine. So of course I'm all sorts of confused because I'm thinking the vacuum system is fine. I started the Jeep and attached the vacuum tester to the fitting on the manifold to see what idle vacuum was. It was about 17-20.

So since I was confused and needed time to think I decided to test the factory vacuum ball. Holds vacuum fine. I then pulled the tiny vacuum line that goes into the firewall for the HVAC controls (I assume anyway) and attached the tool and pumped it down to 17-20 and it holds vacuum just fine.

Now at this point I figure I'd play with the controls and try to move the blend doors around since the tool is hooked directly to the HVAC system and nothing else. Same results, cannot get the defrost door to open, it just whistles. Rest of the doors work fine. Sometimes a bit slow to change between settings, but they all work as intended.

I set the hvac controls to defrost and then took a piece of wire and ran it down the defrost vent to manually open the defrost blend door. That does work, the door even stays open for about 5-10 seconds before loudly slamming closed and whistling again.


I'm actually more confused than when I started. WTF is going on?
Okay I have never had to work on that on my jeep (yet). An educated guess tells me that at least the hvac issue will be something common to ALL the blend doors. I bet it is the selector switch. It may not be leaking vacuum, but may be leaking internally between all the doors.
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Unread 08-20-2013, 07:14 PM   #9
CCKen
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With the system pumped down with the vacuum pump, you should be able to hear the vacuum actuators for the other doors operate open and close. You'll have to keep pumping the system down in between door selections. Any selection that doesn't result in an audible reaction may indicate the door is stuck, the actutor is inop, or the Heat/AC Mode Selector is defective.

The FSM actually has a procedure to do these tests of the valves/doors but they only call for 8 in.Hg to do it. I suppose at that low vacuum any door actuator that's marginal would show its arse during the test.

Pic time:

All these pics are from other Jeep forums members. None are mine.

Behind the Heat/AC control panel showing the Mode Selector vacuum tubing. The black tube is the main vacuum supply from the firewall.



View of the HVAC Unit showing the defrost door actuator.



Defrost Door access with radio removed.



Broke Defrost Door actuator.



Closeup of actuators.

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Unread 08-25-2013, 02:44 PM   #10
Volf
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Thanks for the info! I'll tear into it as soon as I get the chance.


But this doesn't explain why the cruise is intermittent. I pulled and capped the connection that goes into the firewall for the HVAC system. Effectively removing the HVAC vacuum system all together. Cruise is still intermittent even with that removed.
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Unread 08-26-2013, 07:18 AM   #11
CCKen
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Quote:
Originally Posted by Volf View Post
Thanks for the info! I'll tear into it as soon as I get the chance.


But this doesn't explain why the cruise is intermittent. I pulled and capped the connection that goes into the firewall for the HVAC system. Effectively removing the HVAC vacuum system all together. Cruise is still intermittent even with that removed.
The cruise control system isn't just vacuum, the largest portion of the system is electrical. Vacuum is supplied to the cruise control servo to operate the servo. The servo must get the electrical info it needs to operate.

You may have two unrelated problems - cruise and HVAC.

Common faults that have been found in the cruise control system are (1) Cruise Control On-Off/Set switch, (2) Clockspring, (3) Brake Lamp Switch. Not necessarily in that order.
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Unread 11-25-2013, 07:27 AM   #12
Volf
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Update:

Cruise control ended up being the brake light switch. Which I found out only after spending $150 and replacing the steering wheel control and the vacuum actuator itself.

Still haven't figured out the HVAC situation. I'm going to be replacing my heater core soon. Replacement HVAC control modules can be had on ebay for about $40. Probably going to swap it out.
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Unread 12-11-2013, 03:54 PM   #13
Volf
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Update:

Well I pulled the HVAC box to replace my heater core. Replaced the Evap core and blower motor while I was in there. The defrost door problem was that the door was simply unhinged from the arm that is actuated by the vacuum motor. Just had to pop it back in. The air pressure simply forced it closed when the blower was on. So nice to have heat again!



It would seem that my problems are solved and I am a happy camper. And I am for the most part. However I still feel like I am having a vacuum issue. When on the hwy and the cruise is set and the heater on, it seems that I slowly lose vacuum to the vent setting that I want. Example, I have set to defrost and floor and high heat. Everything works flawlessly until I get on the hwy and set the cruise. Then after a few miles the vent doors start to slowly want to change positions. If I tap the brakes and let the motor idle the vent doors return to the correct setting and everything is fine again for a few miles. I also can't change vent settings while the cruise is on. If I want to change vents I have to coast and let the motor idle.

When I had the air handling box out to swap the cores and blower I took a vacuum tester and tested every circuit. All circuits held vacuum over a period of time. I would usually pump it down to 8-9 and let it sit for about 10 minutes. The needle on the vac gauge either didn't move or moved just a small amount. I then removed the HVAC control panel from the vehicle and hooked it up to the air handling box and then applied vacuum to the black line that runs through the firewall and ran all the tests again. Again, everything seemed to work just fine.

I'm kind of at a loss as to where to begin again. Ideas?

A few things I haven't tried yet (but will do shortly are)

Driving without cruise control and seeing if the the problem re-occurs
Hooking vacuum gauge up to measure vacuum at idle and at ~2000 rpm (about where the RPM sits at cruise speed and when the problem occurs)
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Unread 12-11-2013, 06:51 PM   #14
CCKen
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Quote:
Originally Posted by Volf View Post
Update:

Well I pulled the HVAC box to replace my heater core. Replaced the Evap core and blower motor while I was in there. The defrost door problem was that the door was simply unhinged from the arm that is actuated by the vacuum motor. Just had to pop it back in. The air pressure simply forced it closed when the blower was on. So nice to have heat again!



It would seem that my problems are solved and I am a happy camper. And I am for the most part. However I still feel like I am having a vacuum issue. When on the hwy and the cruise is set and the heater on, it seems that I slowly lose vacuum to the vent setting that I want. Example, I have set to defrost and floor and high heat. Everything works flawlessly until I get on the hwy and set the cruise. Then after a few miles the vent doors start to slowly want to change positions. If I tap the brakes and let the motor idle the vent doors return to the correct setting and everything is fine again for a few miles. I also can't change vent settings while the cruise is on. If I want to change vents I have to coast and let the motor idle.

When I had the air handling box out to swap the cores and blower I took a vacuum tester and tested every circuit. All circuits held vacuum over a period of time. I would usually pump it down to 8-9 and let it sit for about 10 minutes. The needle on the vac gauge either didn't move or moved just a small amount. I then removed the HVAC control panel from the vehicle and hooked it up to the air handling box and then applied vacuum to the black line that runs through the firewall and ran all the tests again. Again, everything seemed to work just fine.

I'm kind of at a loss as to where to begin again. Ideas?

A few things I haven't tried yet (but will do shortly are)

Driving without cruise control and seeing if the the problem re-occursHooking vacuum gauge up to measure vacuum at idle and at ~2000 rpm (about where the RPM sits at cruise speed and when the problem occurs)
If there was/is an issue with the vehicle speed control servo leaking vacuum, but not enough to drop it out of cruise, it may be affecting the HVAC controls through a defective HVAC check valve at the firewall. This check valve is supposed to trap vacuum in the HVAC system when the vacuum source drops off.

If so, there are two issues: Servo leaking and the check valve leaking.

Pic of the Check valve:



Dorman check valve from O'Reilly Auto Parts. I haven't verified fitment of this check valve but it looks like it will fit the hoses.

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Unread 12-11-2013, 07:19 PM   #15
Volf
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The speed control servo is the vacuum motor that pulls on the throttle cable correct? If so that is brand new. Used an OEM part. As for the check valve, I only have one at the manifold. I do not have a check valve at the HVAC tee. Only the tee itself. This is promising information.

Thanks for the ideas. I'll do that and anything else that anybody can think of in the next few days and report back.



I appreciate the on-going support CCKen
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