I really didn't want to start a new thread, as I know there are many already discussing HVAC issues with the Dodge Ram, but I would really like to figure this out. I checked the recirc door and it hasn't fallen down and doesn't appear loose. The AC gets plenty cold, but as i drive down the highway the airflow fades even though the fan speed doesn't change. there is still cold air coming out of the vents but barely a trickle. The cab gradually will warm up because there is not enough air to combat the heat from outside. If I turn the truck completely off and then restart it, it will be fine again, for a while. As if something resets each time, then gradually fails. This is really annoying on road trips. Could it be the mode 2 door falling down? Doesn't seem that it would be the blend doors as the air is still cold that does make it to the vents.
I have a 2008 Dodge Ram SLT quad cab 4.7L flexfuel

 

Just a thought---you might try pulling the fan motor out and letting it run for a while outside the hvac housing, see if perhaps t slowing down because the bearings are bad--or something along that line. While you have the fan out, look up inside the housing, see if anything looks out of place

 

Thanks Gone, but I did take a look up inside, even used a lighted inspection mirror, but didn't see anything out of the ordinary though you can't see past the ac coil. I am pretty certain it isn't the fan as there is no change in the sound or pitch from it. And you can still speed it up and slow it down.

 

I understand

My next best guess--not being there to troubleshoot, I think your refrigerant is slightly low, and the coil is freezing up. An evaporator sensor is supposed to monitor the evap temp to prevent this, but, if yours came loose so it wasn't in contact or very close next to the coil, it may be causing a freeze condition. And once this freezing gets started, the frost blockage grows accelerating the blockage process, which is why when you shut it off, it melts and you have air flow again. The other possibility is that something from outside is partially blocking the evap coil and it isn't getting enough total airflow, starting the frosting process again, but this would be my second approach. You can try proving this out a couple of ways. The text book way would be to pull the charge down to 0psig then weight the charge, the other would be to add a very small amount of 134a(I assume) and raise the suction pressure to avoid the frosting. Less than 1/2 of a small can would be more than enough. Too much, you wipe the compressor or blow out a line.
Hope this gets you headed in the right direction---good luck!

 

It doesn't sound like it's freezing related to me. Evaporators take a few minutes to defrost. With what he described with shutting the truck off and back on it sounds like one of the mode doors is slowly closing off airflow. Cycling the ignition could reset it's position.

 

All a very good possibility. The door status I believe has to be checked using an OBDIII scanner available at your local Dodge dealer. This is basically the simple stuff he can check before he resorts to that.
Perhaps a DTC has been set without the MIL illumination.

 

DTC set without the MIL what? Gonna have to google that. I am thinking big sloth may be right, because he is correct, I just have to turn the truck completely off and then turn it back on to temporarily fix the problem. It's just not the kinda thing you want to do driving down the interstate at 70 miles an hour. I really don't want to butcher up the plenum housing the way the heater treater shows to save a couple hours not pulling the dash back. The only real advantage i see to that is not having to evacuate the refrigerant and then getting it recharged. I do like the metal doors he has created though.

 

There are simple ways to check, but blindly adding refrigerant to the system can cause more problems than it solves.

The standard test to rule out evaporator freeze ups in airflow problems is to cycle the temperature to heat, drive with the windows down(bear with me here, its gonna get hot) and see if the condition continues. If the same problem applies, then the problem is in the mode doors. If airflow continues unobstructed, slowly cycle it back to cold to see if that once again replicates the condition.

Next is to narrow down the system, run it on defrost, floor, vent, etc one at a time to see if the condition is present in all systems. Also under normal vent operation see if its just diverting air elsewhere or if its a complete blockage. See if recirculator affects the condition.

try to eliminate as many variables as you can.

If it comes to it, its really not that hard to pull the dash. its modular, I did one in a durango yesterday.