Just in case someone else installs a new AC Compressor and when they start it up, the clutch doesn't engage, I learned today that no pressure in the system due to no freon, can cause the clutch to not engage. Strangely in all of the reading and research I've done before replacing mine, nowhere did it mention this. Not even in the repair manual. I was 5 minutes from uninstalling the new compressor when I found this out. Obviously you need the clutch engaged to be able to charge the system, so its a catch 22, fortunately you can apply 12 volts to a relay terminal to get the clutch to engage. Then charge\fill it. Then the system should behave normally assuming everything else is correct. On my 2006 Envoy SLT, I had to apply voltage to the relay terminal that has the image of the swinging switch on the schematic on the side of the relay itself. Just pull out the relay and jump a wire from the positive battery terminal to the relay terminal mentioned. I applied voltage to the other terminals as well and it didn't hurt anything. There is also info on how to manually engage the AC clutch online if you search for it. Hope this helps someone.
