That doesnt sound right.
I posted some info in this thread :
http://67-72chevytrucks.com/vboard/s...d.php?t=776599
As far as I know that info is correct, but I have not tested it myself yet. I can confirm that within HPtuners there is a table for AC system pressures and fan set points. I am hoping that the switch talked about in the above post relays that info to the ECM and it runs the fans according to the pressure relayed by that switch.