Why doesn't my ac pump turn?

I have a 1999 Ford expedition Eddie Bauer version SUV. The blower is working in the cabin. The ac compressor does not turn. I replaced the relay and diode in the fuse box located out on the driver side fender. I also checked the fuses inside the vehicle. I jumped out the pressure switch. What else can I do to check to see what the problem is. I am not getting power to the AC clutch.