GMC Troubleshooting Questions? Ask a Mechanic for Answers
So you can see that the compressor doesn't engage when the AC is turned on right?
That is most likely due to low freon- the system has a pressure switch that won't allow the compressor to engage without enough freon present in the system since the freon acts as a lubricant.
Can you see any signs of freon leaks on any of the lines? A leak would look like oily/dusty spots on the AC lines.