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).