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.
HVAC Air Flow Switching to Defrost When Acceleratingon Jeep Grand Cherokee
Average mileage : 150,675 (67,000–240,000)
39 people reported this problem
18 people shared problem details
2002 Jeep Grand Cherokee127,000Visitor
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.
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???
My a/c system blows from defrost only, I don't need to accelerate, it happens every time I turn it on.
2003 Jeep Grand Cherokee89,000Visitor
AC only on Defrost doesn't come out of the vents,I checked for vacuum leak but found nothing.
1999 Jeep Grand Cherokee112,000Visitor
was doing this when accelerating now stuck on defrost
1999 Jeep Grand Cherokee171,000Visitor
accelerate goes to defrost let off throttle goes to vent
1996 Jeep Grand Cherokee238,000Visitor
Loss of vacuum causing AC to switch from top vents to defrost on heavy acceleration or medium RPM at highway speeds
1998 Jeep Grand Cherokee166,000Visitor
HVAC Air Flow Switching to Defrost When Accelerating, now air only out on defrost vent, also water on floor passenger side.
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.
Vacuum hoses are terrible in this car. Remove passenger side headlight and replace the ones you can reach back behind it. Almost certainly the problem is one of them.
2001 Jeep Grand Cherokee152,000Visitor
Same issue as everyone else. Vacuum and pcm modules should be checked.
2003 Jeep Grand CherokeeVisitor
When accelerating the AC will blow out through the defrost.
1993 Jeep Grand Cherokee145,400Visitor
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).
1994 Jeep Grand Cherokee190,000Visitor
I think it is a vacuum issue. I will see what solution there is.
2000 Jeep Grand Cherokee200,000Visitor
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.
1999 Jeep Grand Cherokee189,000Visitor
the heater/ac system switches from front or floor to defrost when accelerating.
1998 Jeep Grand Cherokee85,000Visitor
ac/heat defaults to defrost when accelerator is pushed. Stops immediately when brake is used. Still not fixed
HVAC switches to defrost when accelerating above 1800 RPMs. Not fixed yet.
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