The AC/heater (HVAC) system may unexpectedly switch to the defrost mode when accelerating. This system is operated by engine vacuum and should be inspected for any vacuum leaks if this problem develops. There is also a revised vacuum check valve available to address this concern.Google+
Car Problem Reports
Jeep Grand Cherokee HVAC Air Flow Switching to Defrost When Accelerating
Jeep Grand Cherokee Problem
Average Mileage: 152,284 mi (67,000 mi - 238,000 mi)
Under acceleration when the engine pulls vacuum, the small check valve on the manifold vacuum line does not hold allowing the AC/heater directional system to default to the defrost system (no vacuum position). Check valves are netorious for failing or leaking. I was able to "stall" the problem by "cleaning" the check valce with wd40 but it is only a short term fix and will only work if the check valve is dirty and not broken. A check valve this size should cost less than $10.00 but buyer beware... it is probably a "special" component and may cost more???
Visitor, 1999 Jeep Grand Cherokee, 112,000 mi
was doing this when accelerating now stuck on defrost
Visitor, 1996 Jeep Grand Cherokee, 238,000 mi
Loss of vacuum causing AC to switch from top vents to defrost on heavy acceleration or medium RPM at highway speeds
Visitor, 2002 Jeep Grand Cherokee, 127,000 mi
AC switches to Defrost when accelerating, especially if climbing a hill. This doesn't happen with every acceleration, but when it does happen if I ease up on the gas, it switches back.
Visitor, 2003 Jeep Grand Cherokee, 89,000 mi
AC only on Defrost doesn't come out of the vents,I checked for vacuum leak but found nothing.
Visitor, 2001 Jeep Grand Cherokee, 152,000 mi
Same issue as everyone else. Vacuum and pcm modules should be checked.
First the AC would stop blowing out the dash vents when I accelerated redirecting the air to the defrost or floor vents. Dealership told me it was a redirection of power when I accelerated. Didn't sound right but I went with it. A few years later cruise control doesn't work, heater and AC both do not blow out the dash vents only defrost and sometimes floor vents. From my online investigation I know there is a vacuum leak. I had one fixed under the hood which helped a little but I am afraid the other might be in the dash. On top of that if the blower runs too long I start to smell a burning electrical smell. So I think the connector/resistor gets too hot and starts melting.
Visitor, 2003 Jeep Grand Cherokee
When accelerating the AC will blow out through the defrost.
Visitor, 1993 Jeep Grand Cherokee, 145,400 mi
whenever the engine is under extra load such as uphill on the freeway or passing, the air stop coming from the console and switches to defrost. Dealership changed a vacuum motor (no help).
Visitor, 1994 Jeep Grand Cherokee, 190,000 mi
I think it is a vacuum issue. I will see what solution there is.