Intermittent air vent selection not working properly

Tiny
CARLSP
  • MEMBER
  • 1998 RAM 1500
  • 5.2L
  • V8
  • 4WD
  • AUTOMATIC
  • 250,000 MILES
When I accelerate the air vents will switch from whatever setting I choose to defrost. Once I am up to speed and the RPMs drop back down the air vents will switch back to the setting I set on the selector switch. So if set the selector switch to say, the floor, and then accelerate, it will switch to defrost by its self. When I let of the gas it will switch back to the floor again. Same thing happens if I am stopped and put it in neutral. When I increase the RPMs it will switch to defrost and when I let off the gas it switches back to what is on the selector switch. I am not sure where to start looking for this problem and would really appreciate some guidance on this. Thank you for your time on this.
Do you
have the same problem?
Yes
No
Wednesday, September 15th, 2021 AT 3:52 AM

4 Replies

Tiny
CARADIODOC
  • MECHANIC
  • 33,689 POSTS
Actually, this is a real common problem with an easy fix. Your mode doors are operated by vacuum motors. You're losing engine vacuum when it's under load or when accelerating. There can also be a leak in the system that prevents the controls from operating correctly. If that were to happen, these systems are designed to default to the "defrost" position. Your feet might freeze, but your windshield will be clear. It's designed that way for safety.

At the mileage you listed it's normal for the engine to not develop as much vacuum as it did before. The first thing to do is switch to "defrost" with the engine idling. Stop the engine, then try to select a different mode. You should hear some squeaking or rubbing as the doors start to move. If you do, there's little chance there's a leak in the vacuum hose. If you don't hear anything, we might still have to check that hose for a leak later, but lets go with the fix first and see if that solves the problem.

My red arrow in this drawing is pointing to the check valve in the vacuum hose that runs into the passenger compartment and runs the heater controls. It's on the firewall under the wipers. That valve is about the size of a stack of three nickels. The dealer's parts department has a replacement check valve that is roughly the diameter of a 50 cent piece, and about an inch and a half long. This one includes a small vacuum storage canister. It can be used to solve this problem on other brands of vehicles.

All you have to do is describe the symptoms to the people in the parts department, like you did here, and they'll know just which part you need. I don't remember if there's an arrow on it to tell you which way to attach the two hoses. If the heater controls don't work at all with the new valve, just switch it around, then reattach the hoses. There's a good chance you can find this valve in a salvage yard too.
Was this
answer
helpful?
Yes
No
+1
Wednesday, September 15th, 2021 AT 6:05 PM
Tiny
CARLSP
  • MEMBER
  • 32 POSTS
Thank you for the information. The check valve was bad and I found a spot in the tubing that had a pin hole in it. System works fine now. Thanks again for all the help. You guys are awesome!
Was this
answer
helpful?
Yes
No
Saturday, September 18th, 2021 AT 7:32 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Just to add, I attached a picture for you. When these valves fail, you lose the vacuum on acceleration and the doors change.

I would go to the dealer and replace both check valves. Once you do that, it should be good. There is no vacuum leak in the system at all.

Roy
Was this
answer
helpful?
Yes
No
Tuesday, September 21st, 2021 AT 11:56 AM
Tiny
CARADIODOC
  • MECHANIC
  • 33,689 POSTS
Happy to hear you solved it. Please come back to see us again.
Was this
answer
helpful?
Yes
No
+1
Tuesday, September 21st, 2021 AT 4:30 PM

Please login or register to post a reply.

Sponsored links