The A/C is shut off by the ECM for the following reasons:

• High engine temperature (not sure if this is the same sensor as the temp gauge or not)
• If the ECM loses control/communication with the Fan Control Module (the module that runs the engine cooling fan, not the cabin fan)
• An indication that the system (Freon system that is) is of either too high or too low pressure
• A throttle position of 'full open' or near full open (indicating that to the ECM that you are demanding maximum power, maybe for passing)
• Probably other reasons that I don't know about
• Probably other reasons that I do know about, but cant remember right now

The Climate controller module in the console also plays a part here. The controller won't engage the compressor unless the cabin fan is on at least a "1" setting, that is, if you don't have the fan running the A/C is locked out. NOW, I am unsure (since they have never released an actual diagram or document on the Climate Controller) if the module is watching just the fan switch position OR if it is watching the output of the module being sent to the "fan resistor". If there is a fractured solder connection in the climate controller module's fan circuit (and that has happened and it means the cabin fan does not work), then I'd suspect the A/C would be locked out. If the cabin fan is running while the A/C is not working, I'd dismiss this as a cause.