Ron D. wrote: "in a 1972 Chevelle, the AC and Defrost was linked although it may have been possible that
at full hot, the AC got turned off. I can't remember, but i suspect not. "


I always made the simplistic assumption that the only
difference between "AC" and "Defrost" modes was the
direction the compressored air was ducted: either toward
your face and feet(AC), or out the top side of the dash
toward the windshield(Def).

A lot of owners manuals suggest a temperature setting
between "10 and 2 o'clock" when using Defrost, vs
extreme/full cold or hot, respectively, for AC or Heater.