1992 Pete 379 AC

Evaporator is freezing up.

 

Fan motor might be running full blast but is the fan turning?? Squirrel cage may be spinning on the shaft.

 

Is it a slow stop to the air flow or fast? Does it come back the same as it goes away?

 

The doors closing. Probably needs a new air piston actuator. Though it usually comes out somewhere, defroster vents or floor vents. Try those settings. Not familiar with Petes. Most Trucks have about 4 different air piston actuators that control different blend doors. Tiny 1/4” airlines from each running to dash control. Schematic from Dealer helps. Take the bottom panels off, start looking and following the air lines. Each ones usually a different color, white black yellow blue red etc. My Freightliner has 4 of them. 1 is on the top, has 2 air lines running to it. The others are accessible from the bottom. Usually if ones bad, it’s cracked or o rings leaking. You’ll hear it leaking air in the dash, when adjusting the controller. Could be the controller on dash. Again usually hear it leaking air.

 

This is a symptom of the evaporator being frozen up, try setting to heat or vent (blower running, ac off), id wager that in about 10 minutes you have full air flow back. That would mean your freeze control switch isnt working. It will do this worse and faster in more humid conditions.

Costs you no money to troubleshoot that.

 

I know most of the Pete's I've worked on have electric actuators for the blends doors, not air cylinders like a freightliner. 92 could be different but would be easy to hear if you move the dial with truck not running