A/C issue, Tune or mechanical issue?

[1992-25th]

messing with my truck and noticed that the A/C clutch is not engaging at all. at first i figured that i had fried my HVAC controls when doing a LED retrofit.

after messing with it a bit i found that i can engage the clutch (Fan comes on as well) with the scanner when the truck is not running. as soon as i start the truck it no longer works. the HVAC control in the truck appears to be working fine, blower and blend doors all work and all lights come on (A/C, Recirc, Ect) . i have yet to start probing wires and checking voltage, just made me wonder where it will engage with the scanner as long as the truck is not running.

anyone run into this?

truck is a 2000 silverado running a 2002- 156 OS with 3 bar SD applied.

[Jason B]

The pressure switch disables the compressor if the refrigerant pressure gets low.
You really want to check the pressure before you force it to run.

[1992-25th]

I will try to get a set of gauges on it, would the switch allow me to engage the clutch while not running with the scanner but not while it's running?