AC malfunction



1998 Jeep Grand Cherokee (Laredo, 6 cyl).
The AC, when set on the vent oulets that blow at chest level, will shift to defrost when
accelerating or going up hill. It returns to the proper setting when you leave off the gas feed.
One can hear the blend doors changing. I am mystified, because I thought that vacuum operated
blend doors went out decades ago. Any ideas what this could be?


Here’s the vacuum diagram for the HVAC system.

And here’s the vacuum reservoir behind the front bumper.

These are the areas to check.



CLASSIC vacuum problem . . . could be the check valve. It’s supposed to prevent exactly what’s happening to you

BTW . . . The Chevy Astro, for example, used vacuum-operated hvac doors. And it was built until 2005, which wasn’t all that long ago. Mercedes-Benz used it until 2003 in some cars, and that’s a luxury car maker . . . !


I typically found the vacuum hose connected to the left side of the engine/manifold to be split or swollen from oil on those Grand Cherokee’s.

BTW those 1993-1998 ZJ’s were manufactured decades ago and the vacuum operates the mode doors.



this website said a post has to be at least 20 characters . . . outrageous



I saw your post, with a severe leak you can get enough vacuum to operate the HVAC system, perhaps 5" of vacuum. When the throttle is opened there isn’t enough vacuum.


Thanks to everyone. I could not imagine that they could still be using vacuum for this.


5" of vacuum is enough to operate some hvac actuators . . . ?!


So…there is no perceptible rough idle. Does this mean it is likely the check valve rather than a vacuum leak?


If you pulled the hose off the manifold it will still idle smooth, that is not an indicator. Did you test for vacuum to the firewall connection?


It used to be an indicator. I suppose modern (1998) cars may be able to compensate for a vacuum leak?


The leak would have to be near a cylinder to cause a disturbance on this engine, the throttle body is a vacuum leak, each situation is different.

Open the hood, inspect the connection on the manifold, walk over to the passenger side, disconnect the vacuum line from the firewall and connect a vacuum gauge. Is there sufficient vacuum? Does it hold after shutting off the engine?


OK, thanks. I am going to take this to a mechanic. I just wanted to be informed.


OK. It turned out to be a vacuum leak where a hose went into the firewall. Fixing that fixed the blower door problem completely.
Round 2: This car is putting water out into the passenger compartment–sometimes up front, sometimes in the back seat. I am aware that this is usually caused by the drain line being plugged up. The mechanic my mother uses seems to be kind of clueless about this side of things. Am I right about the probable case, and is blowing out the line with compressed air or some other procedure a good approach for this?


Please forgive me.

Would anybody be able to tell me how I start a new topic? I have stumbled around this site lost for 15 minutes and for the life of me cannot see where I go to start a new topic.

Thank you.


eller221 solved their problem.

This was Mellott’s thread not eller221’s-sorry for not making that clear.


After the hijacking…
But, mine still exists.


Hey- I did what I had to do