96 Xj, Strange AC problem

My 96 xj with 260000 miles 4.0 has a very odd problem :screwy: when i turn on the AC it will blow cold air out the vents BUT when i am going down the hwy it will suddenly stop blowing out the vents and the air will come from the windshield vent as if i had put it in defrost mode. but when i let off the gas pedal it will come back out the side vents:confused: it seems to be getting worse as time goes by. any ideas ? it doesn't matter if its on max or regular AC it will still do it and it seems only to happen when I'm on the hwy and not when I'm driving around town :banghead:

You have a vacuum leak. It's either a small hole somewhere in the vacuum line or a bad check valve.

At high throttle settings, acceleration, climbing hills, etc, the engine sees low vacuum. Normally, the reservoir holds enough vacuum to get through these relatively short periods. As soon as you back off the throttle, you increase engine vacuum. If you have a small leak or bad CV, the vacuum bleeds off faster and the vents default to defrost.

You don't see it around town because the low vacuum periods are typically much shorter than on the freeway. Starting from a red light on a side street takes less time than accelerating to freeway speeds or climbing up a long hill.

[QUOTE=Saudade;6912664]You have a vacuum leak. It's either a small hole somewhere in the vacuum line or a bad check valve.

whats the best way to check for a leak? :confused: also can you tell me where to look for the check valve?

I check for vacuum leaks by going over the vacuum lines -tubing and hoses - and closely examining each joint. With the mileage on your jeep, you can expect some really deteriorated rubber hoses, cracked plastic tubing, or joints that have begun to pull apart.

I'm not sure about Cherokees, but in Wranglers there is a junction point that goes bad; I'd check the schematic to see if there's a similar junction in the Cherokee.