HVAC Air Flow Switching to Defrost When Accelerating on Jeep Grand Cherokee

Problem Description and Possible Solution

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.

Problem Data
RepairPal Verified
Average mileage: 159,264 (57,000–283,000)
12 model years affected: 1993, 1994, 1995, 1996, more1997, 1998, 1999, 2000, 2001, 2002, 2003, 2012
65 people reported this problem
Viewing 1 - 20 of 29 comments
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???
AC only on Defrost doesn't come out of the vents,I checked for vacuum leak but found nothing.
Loss of vacuum causing AC to switch from top vents to defrost on heavy acceleration or medium RPM at highway speeds
accelerate goes to defrost let off throttle goes to vent
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.
My a/c system blows from defrost only, I don't need to accelerate, it happens every time I turn it on.
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).
Switched to defrost mode and stopped blowing from dash vents
During Acceleration the Vacuum lowers and the air vents shut to the main cabin area
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.
was doing this when accelerating now stuck on defrost
My problem now is nothing works on the heat or any part of the heater fan nothing the blower motor is fine but nothing in the switch controls work they all got,power need help to fix please.
When my a/c or heater is on and I accelerate, it will start blowing out of the defrost. When I hit a constant speed it goes back to where I had it set.
with vent selector switch set to any setting but defrost, when I accelerate, the air switches to defrost vents. When I let off the gas, it blows back out of front vents like it should. not repaired. need help.
When I have the AC on and I accelerate on the freeway it switches into defrost mode then switches back after 30 seconds what goes on back and forth every time I accelerate
Under heavy throttle, vents switch to blow on defrost, release throttle and vents return to normal dash.
A/C switches between the selected vent setting and defrost. Noticed it switching back and forth when driving through the mountains.
When climbing a hill or under heavy acceleration wile AC is blowing it will switch to the defrost.