gmc sonoma AC unit not working?

[JBC]

i cant get the compressor clutch to disengage and actually run the compressor. I suspect either a vacuum leak or perhaps an electrical short. Any ideas? Basically what are typical things to look for and how can I do the Dx?
actually I do mean "disengage", because i want the clutch to NOT prevent the pulley from spinning the compressor stem. and an electrical short, being a damaged sement of wiring or component, can potentially cause the same no-op condition. Im simply looking to see where the primary causes may be located.

 

[Joseph]

Compressor controls include a low pressure switch that blocks voltage from engaging the coil and magnetically pulling the clutch plate in to drive the compressor. So if it is low on freon you won't damage the compressor. I found this to be the number one reason for no clutch engagement. Low on R134. After a visual inspection fuses and relays are tested before hooking manifold gages up to both sides for diagnostics.
You can get a 12 volt test light and test the system from the fuses and relays. Use a wiring schematic, that is the best way. A vacuum gage or manifold set are expensive and require training. The test light will come in handy for all kinds of repairs in the future. You'll get it.