Dodge Repair: HVAC air outlet controls: 99 ram 1500, dodge ram 1500, electric actuators

Question
the ac on my 99 dodge ram 1500 works fine but i notice if i accelerate or go up a steep hill when driving my air goes from the vents to the defrost. im thinking its a check valve issue but dont know where to look.

Answer
Hi Keith,
You have made a good guess as to the cause, but I can't be certain.
I don't have manuals earlier than '04 for the trucks so can only guess at the answer. What you describe may well be related to the air distribution being controlled by engine vacuum via a vacuum line between the intake manifold and the HVAC unit. Were that the case you would notice a vacuum line going through the firewall at mid-body on the firewall. Inside the cabin between the firewall and the HVAC vacuum-operated internal vent switching/distribution device is where I would look to find the check valve. That is how it was set up in the '80's and '90's car vehicles prior to electric actuators to move the 'mode' and other control doors. In theory the check valve would hold the position of the door when you started to accelerate/climb until the intake manifold vacuum returns to it typical level. It could be a small crack in the vacuum line or a loose fitting of the vacuum hose to the distribution device.
Please 'rate' my answer (see below).
Thanks,
Roland