fresh air duct

[BurbanPena]

when selecting max ac the blend door on the lower passenger side opens up but there seems to be no difference in cooling and if i have the fan on low and driving on the highway i can feel warm outside air being forced in from the max ac vent is there another vent that is supposed to close in the cowling to stop outside air from coming in?

 

[75gmck25]

On max A/C there are two vacuum motors that change it to recirculate mode. The vacuum line has a T in it and you should be able to access it if you remove the glove box door.

The actuator on the passenger side kick panel is easy to see, and it pulls air in from inside the truck.

There is a 2nd actuator up in the cowl and on my ‘75 you access it from outside the truck. It shuts off most of the outside air, but IIRC the manual says it does leave a small amount coming in.