HVAC Air Flow Switching to Defrost When Accelerating on Dodge Ram Van 1500

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: 128,938 (50,000–272,000)
9 model years affected: 1995, 1996, 1997, 1998, more1999, 2000, 2001, 2002, 2003
115 people reported this problem
Viewing 1 - 20 of 48 comments
A/C blows normal until you accelerate then it moves to the defrost vents then the feet vents. when I lift off the gas it imediatly returns to normal.
my problem is that it stays on defrost wont change to heat or vents. does antbody have a solution
We travel on the highway as we increase speed the ac/heater switches over to defrost ....we release the gas pedal it switches back to normal any solutions.
heater/ac only works on defrost
I have a 1996 dodge ram 1500 with 3.9. when I accelerate the ac/heater switches from vent to defrost, I dont know how to fix or check for problems
A/C blows normal until you accelerate then it moves to the defrost vents then the feet vents. when I lift off the gas it immediately returns to the floor.
ac and heat only comes out on defrost.
a/c switches to defrost on acceleration or moderate incline
I have the same problem on my 97 3.9L B1500 I installed two check valves this did the trick
AC/heater awitches to defrost when acceralating
Air switches from vent to defrost while acceleration or under a load and returns to vents when I let off the throttle
Hey.....I feel your pain! I went nuts trying to find the vacuum leak, and followed the lines to a little rubber stopper thing at the center of the firewall. It has holes in it and I noticed one was hanging loose......I had to trim some off the ends of the tubing, and then stuck it back in firmly, and whaddya know? AC was blowing out the vents again!!! No more defrosted ac. Hope this helps. Tennessee Jed.
When I accelerate the ac switches from dash to defrost.
when accelerating the AC/Heat moves from the front vents to defroster vents and when I let off the flow returns to the front
Ac changes to defrost when stepping on the gas
Heat or ac redirects to defrost when accelerating.
When the van is accelerating the AC will turn redirect to somewhere else, maybe the defrost vents, and when I let off the gas the AC comes back
AC works good at idle but switches to hot air during acceleration. Higher fan speeds makes problem worse.
Switches to defrost on acceleration and noq finally wont switch back
Have same problem with A/C Heater system will switch to defrost mode when engine is accelerated. I replaced two check valves in the HVAC system,did not solve problem. I live in a area where I drive has a lot of hills, so the system switches from normal to defrost mode quite often. This involves a 1999 Dodge Ram 1500 Truck with a 5.9 Liter Engine with automatic transmission.Need help to solve this problem. thank you.