2001 f-150 ac ompressor not engaging

Truck is a 2001 f-150, 4.6L, 185,000 mi.. It seems that all of a sudden my ac compressor will not engage. When I switch it to ac, max ac, defrost, floor, or floor and vent the compressor does not even try to engage. This seemed to happen overnight, so I assume it is not refrigerant, because before, it blew as cold as it did new. There is also no oily residue around the compressor. My first guess was electrical, so i swapped the compressor relay with another that i knew worked, to no avail. I also checked the diode (swapped it with the pcm diode and the truck still ran and the compressor didn,t; I assume this means the diode is ok?) I have also checked the fuses that i thought had anything to do with the ac. The blower works fine. Any advice would be greatly appreciated! Thanks in advance!