Let's start with jumping the pressostat at the accummulator and see what that does. If the compressor kicks in you have one of two problems, you are low/out of refrigerant or you have a bad pressostat, I have seen both. If it fails to kick in at that point then you have one of two problems there, either you are not getting enough current to the pressostat in the first place (bad solder joint on the PCB) or you have a break in the wire somewhere between the pressostat and the compressor. If it does not engage I would jumper a wire from the fat red B+ wire on the back of the alternator to the compressor lead right there at the compressor. It should fire up, if it does go see if it will blow cold. If it does blow cold this indicates that you have enough refrigerant in the system to close the pressostat and make things work and that your problem is somewhere in the control side. If it does not engage then you have an open in the compressor clutch or the ground cable for it is broken.

I have seen all of the options I have laid out here happen. Bad clutches, broken ground wire, bad pressostats, faulty PCB's, no refrigerant. In fact I had one of the fellow board members in my shop the other day and while stuffing in his pre-pump I gave him a quick down and dirty diagnosis on his a/c, bad solder joint on the PCB. Well he went home and fixed that but still no a/c. He came back to the shop and paid for diagnosis and I went ahead and diagnosed the rest of his problem, bad pressostat and it is a bit low on refrigerant. So as you can see these problems can exist on multiple levels, follow the guide lines I laid out above and see if you can't narrow it down.

Mark