When you start the truck, turn on the AC it works great while at idle. Hit the highway, in about 20-30 minutes the air not as cool. After a brief gas stop, the Ac worked again, and then failed. Switch to vent ... a hour or more later ... after hitting 80+ mph the AC started again (in vent position) and stayed on for the remainder of the drive. Possibilities? Hoping it may just be the ac clutch relay ....

 

A/C clutch relay replaced ... I'll let you know if it resolves the issue.

 

Replacing the a/c clutch relay resolved the issue of the A/C turning off after 20 minutes of highway driving. However, I still have cold air after switching to vent position (unless I dial in warm air). The dash A/C control does control the relay when you first start out. Dash switch ruled out as part of the issue issue. Later today, I'll listen for the compressor / relay turning on / off after an extended drive.