I have a 1990 Jeep Cherokee Limited that has a vacuum problem. With the AC running when I travel down the highway and start up a hill the vents in the AC stop blowing air. When I get to the top of the hill an the load on the engine eases the air will start blowing out of the vents. Has anyone experienced this and if so what did you find to resolve the problem. I have looked at the vacuum hoses and can find not leaks.

 

There is a vacuum ball behind the front bumper on the right side, check the hoses and the ball for cracks.

 

How do you remove the vacuum ball for inspection?

 

I would take the bumper end cap off of the passenger side so you can get to it easier. There are 2 screws holding the ball onto the bumper from the inside. Unscrew those and take the vacuum hose off of the ball. I would check the ball and follow the vacuum hose and see if you can see any cracks in it. My XJ is doing the same thing, I just haven't got around to checking it out.

 

I have a 2000 XJ which is suffering from the same problem. On a long hill, the air moves to the windshield and then comes back after the road levels. I already assumed that it is a vacuum leak, but I don't have that "vacuum ball" under the right front bumper. Where else could i look?

 

I had this in my 96 XJ. I didn't think anything of it. I just played with the gas pedal and realized that the more you press the gas pedal, the less the air comes out. Which means uphill, you're using more gas (so air doesnt blow out as much), and going downhill, you're usually off the gas (air blows out hard).

I don't even notice it anymore. My vacuum hose is good.

 

Your theory is flawed.
Going up hill makes the engine work harder, you step on the gas more opening the throttle, the more the throttle plate is open the less restriction there is in the intake meaning there is less vacuum (or negitive pressure). The HVAC mode doors use vacuum motors or servos to operate the doors, if the vacuum signal is lost or weak the HVAC system goes into a fail safe mode of directing the incoming air to the defrost mode.