With the engine off, you put 12 volt + to one wire of the compressor and the other to ground the clutch magnet will energize and the clutch should CLICK onto the drive pulley. If not then give a little push on the clutch toward the pulley. It it is to far away from the magnet then it will not engage on it's own. But it should when pushed. If not then the magnet is bad and needs to be replaced. If it does engage then we can move on to figuring out what is going on. When running and the compressor is running and the pressures are the same. There can be only to problems. A bad compressor or the TXV.
Also in the manual section look for the UTI training manuals. In there is a full section just on refrigeration and how to find problems. Along with the operation of each part with breakdowns.