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: 154,025 mi (85,000 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, 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.
Visitor, 2000 Jeep Grand Cherokee, 200,000 mi
Same not fixed yet. wonder if the loss of enough vacuum pressure is caused by insufficient vacuum due to a leak elsewhere? I know my brake booster needs replaced due to a leak. this has been a problem since i purchased the jeep and I don't know if with a system that has no vacuum leak if this happens. kinda hard to imagine it does that would be pretty crappy engineering. I assume i fix the brake booster or vacuum leak, i fix the problem.
Visitor, 1999 Jeep Grand Cherokee, 189,000 mi
the heater/ac system switches from front or floor to defrost when accelerating.