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Q: when i turn on my ac and speed up it comes out the defrost and then the vent on 1999 Jeep Grand Cherokee

when i speed up in comes out the defrost when i slow it comes either out the vents or at the bottom never the same when it comes out the vents its cold please help
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Sounds like you have a vacuum leak. Some of the doors in the air distribution box are actuated by engine vacuum. So whats happening is that under a load/acceleration the engine creates less vacuum then at cruising or low speed, high vacuum, operation. If there is a leak in this system of sufficient size it will over come the weak vacuum signal during acceleration and all the doors will return to there default - defrost. Keep your ears open an listen for a hissing sound most likely under the hood somewhere but could be under the dash. The vacuum circuits are small plastic tubes, red green black - one or more, I can't remember, that go through the fire wall to the distribution box. They break very easy but you can mend them back together with some hose. Good Luck, Cheers.
agreed , seen lots of broken(brittle from age) vacuum lines mostly around pass fender and back towards firewall your ears will probably be your best friend the break might be in an area that's hidden from sight .......use the force , luke )sorry(
You might be lucky, the problem could be as simple as a bad vacuum line check valve. When the valve fails it allows the vacuum in the vacuum line to fall when accelerating or under load. Locate the valve (mine is in the vacuum hose right where it attaches to the engine) and remove it to test. It should allow airflow in one direction only.
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