Took my 98 Dodge ram 1500 4x4 5.2 automatic to the dealer because AC would quit when I accellerated. They said the check valve was stuck shut and replaced it. I picked it up from dealer and truck still had same problem. They said the new one they installed was bad and replaced it. I could see they replaced a couple of vacuum lines also. Everything was fine for 2 weeks but yesterday I switched my fan to the highest speed and the AC suddenly switched from vents to defrost vents and turning my selecter **** changed nothing in all positions. This morning the selector switch works fine. I did find a vacuum hose on the right side of engine compartment that is not connected to anything. It follows the brake lines down towards the tranny. Too hot to get under there today. Any idea of what is going on? Thanks Tom.

 

Does the cruise work? Check under the battery tray for a broken line or elbow where the line connects to the servo motor. And if the lineyou see in back running down is green, thats just the vent line. If it is white or lite gray then does the 4x4 engage properly?

 

Just went for a ride, 4 wheel drive works and so does cruise control. Beginning to think that hose that connects to nothing may be a breather/vent. It has a black rubber button thing on the end. Seems after sitting over night it loses vacuum and things work O.K. until I turn the fan to higher speed. It immediately switches to defrost vents and stays there no matter how I turn the selector switch. Tom

 

Has to be a vacuum leak somewhere then. Fan shouldn't have anything to do with it.....

 

I agree- vacuum leak. Yesterday when it happened I was driving at 70 mph with the AC running on next to max fan speed with climate swtch on dash vents. As soon as I turned the fan speed to max (next speed) the air switched to defrost vents and have been unable to get it back sp air comes out of the dash vents. It's almost as if it blew a vacuum hose off climate control switch. Still over 100 degrees today so will check under the dash when I get a chance. Thanks, Tom.

 

Since it's a 98, I happen to have the whole dash control unit if you are interested.

 

Have you found anything out about your problem? I was searching today and found that you have about the same problem I do. My a/c fades on acceleration, then will come back on once I am at speed. I hear the fan running, but the air just quits coming out of the vents. Thanks in advance.

Cale

 

Read this thread, https://dodgeforum.com/forum/2nd-gen...ild-vents.html

 

I noticed a weird thing that when I go uphill I hit 3000 RPMs and up with my 5.2 liter 318 in my Dodge Ram van 2500. I also noticed under the hood that it says that they climate control is attached to the vehicle speed control. Another problem is when I accelerate my AC and my heater will switch to the defrost vents so it blows on the window instead of in my vents pointing at me. This is potentially a problem going into open loop. I think I'm running lean when I'm going into open loop. Since the climate control is also attach the vehicle speed control I think that's why the vents are switching to the defrost vents. So you might be running lean in open loop I will check that out. I still can't figure out why it might be one of the coolant temperature sensors or something like that maybe a map sensor I don't know I'm leaning toward coolant temperature sensor right now because my coolant temperature is a little bit off from what it was when everything was working I think. My coolant temperature is a slight hair below what it was when I first got this vehicle.

 

Some of the actuators in the hvac box are spring loaded. Vac overcomes the tension on the spring and moves the actuator. Any time the vac gets low it will go back to the default position witch is defrost. You should have two check valves one on the passenger side of the intake and the other by the ac lines going into the firewall. Also you need to check the line going to the vac reservoir. Keep in mind your engine produces no vac on acceleration so the system relies on the the vac in the reservoir. Also if you have a leak anywhere in the cab(actuators or vac lines) you loose vac and it goes to defrost. Use the tsb for reference it is for the early rams that didn't have the second check valve.

http://dodgeram.info/tsb/1996/24-01-96a.htm