The 1998 Dodge Ram my son had experienced what I have found to be a very common problem with the air from the vents redirecting from the front vents to the defroster vents under acceleration or load. I researched and found this to be a very common problem with that era Dodge vehicle. Turns out the check valve in the vacuum line that is in the tee from the vacuum line off the manifold to the under dash diverter control cannot hold the vacuum under these conditions. Vacuum is what causes the diverter to direct air to the front or floor vents. Absence of vacuum causes the control to direct air to the defroster vents (a safety issue I am sure).
A new vacuum check valve can be obtained from the dealer, although I found it to be more expensive for me than others quoted (I paid $18 Jimmy!!!). Others I have seen quoted being able to get the part for just a few dollars. The old check valve is a very thin disk with a barb on each end to fit in the vacuum line. It looks very much like a watering nozzle for drip lines and is easily removed by pulling the vacuum lines off each barb. The new check valve is much larger, slightly larger and longer than an old 35mm film canister. In addition, the nipples on either end of the new valve are not the same size as the vacuum lines in my 1998 Dodge and are not barbed. The valve comes with one elbow reducing joint but it is not the same size as the tubing in my 1998 either so it was a waste. Perhaps it fits newer models with this as a replacement part, I don’t know.
So I had to run down to the local parts store and buy some other vacuum line connections and tubing sections to piece together a set of connections that would allow the smaller nipples on the valve to fit in with the larger ID tubing that exists on the truck. No biggie, other than it seems like a hassle Chrysler didn’t get this replacement part to fit as is with older vehicles. The extended length of the new valve and connections is no big deal as it all has plenty of room under the hood.
The valve is found under the hood, near the firewall on the passenger side. It shares an entry point through the firewall with an AC line. The picture attached shows the replacement (in place).
The original check valve
Shallowbottom 
Where did you find the new check valve. Was it for a different truck. I have the same problem with the new one I got at the local store. It is to small and will not fit in the hose.
I didn’t get a different check valve. The only one I got was from the Dodge house. The one I got had barbs that didn’t fit the existing hoses, so I just went to a local auto parts store and got enough short hose pieces and adapters to make it fit, in one package of such things.
Did this cause a terrible whistling noise when you reach a higher speed? I’m experiencing this noise when I hit 60 or above, and cannot figure out where the noise is coming from.
It didn’t result in a whistling noise for me, just the inability to direct air through the dash vents.
So did this completely fix the problem? I’ve been living with this for quite a while, but if I knew the fix was this easy I would have taken care of it a long time ago!
It worked for me Matt, but it depends on if this is really the cause in your case as there is more than one possibility. My searching did find it to be a VERY common and well known problem with Dodges.
I have a ’94 Ram. I think I’ll give it a try, it can’t really hurt anything (famous last words, right?).
I would like to add that Rams have two one way check valves, one described in the post above, and one directly off of the passenger side manifold. In my case, the check valve described above was working, and the one near the manifold was defective. How do you tell? Take each valve off and blow on each opening of the valve, air should flow through only one opening (i.e. one way check valve) not both.
I bought my replacement check valve from Autozone, it is a universal check valve by Motormite, part number 47149 for $5. I also noticed the same part at Oreily’s. Fits like OE except where the air exits the valve is smaller and needs an adapter to fit OE vacuum hose. I found a package of rubber adapters with different IDs on each end for $5. All the parts mentioned are sold out in the lobby of Autozone and Oreily’s as universal vacuum parts, not behind the counter.
Also, the manager at Autozone had no clue about any of this, but was having the same problem with a Chevy truck (AC through defrost on acceleration). So, apparently this can be a problem with many different cars.
Did you ever manage to get any part numbers? My 97 Jeep Cherokee uses the same check valve and no dealership parts department can seem to find it for me. I’d prefer to go with a Mopar replacement over a universal from a parts house.
Went to dodge dealer to get the part. Not the best people to deal with. Dodge part# 4549209
$5.36.
auto zone, part # 47149
Dodge Part # 4549209 Valve Check.
I replaced both parts.. and I still have the problem. any Ideas ?
Same issue, thanks for post. I am under the hood now.
Seems same issue with tubes, going to change it all out.
Changed the valve, fixed the prob and ended up changing the plugs while I was at it. Although the back drivers side plug was tough. You’ll need a swivel head socket.
Hey guys I had the same with air only coming out of defrost. I checked all the valves and they were fine, but all the suddon I pulled the hose going in the fire wall out and it had been melted in half. All this time and I just found the problem by accident. Not easy to fix, but got some tubing that was of slightly different size and some other off the shelf tubing parts and spliced it up. So check the valves then check the tubing…Hope this helps
I had the same deal as Aaron, when I went to pull the ‘boot’ off, the hose just snapped. I just pulled the broken piece of hose out of the boot, pulled a couple inches of good hose carefully from the firewall and threaded it back thru the boot. I checked the valve by blowing into it, it’s a very simple one-way valve so it should open/close when you blow/suck. No new valve was needed (or hoses/adapters), air works great thru the correct vents! Thanks guys, the location pic was the biggest help of all Shallowbottom!
Having this same issue gents. But a bit worse. The line running into the firewall by the heater core line is badly brittle. The truck was my brothers and it sat for 3 years doing nothing. Anyway. I Dont have any idea where that line runs. But the vaccum on the check valve is working GREAT. the line was just disconnected. Can i get a new line and run it to the control box. or does that line connect to something on the heater core and then it goes to the control box. Let me know. I AM SWEATING DAILY IN THE MISSISSIPPI HEAT TRYING TO FIX THIS!
Like Mare Said the location pic was great help. Thanks Shallowbottom.