I have a 2004 F-350 6.4L 4x4 Powerstroke and cannot get the A/C to blow out. I have replaced the fan motor or blower, and the speed control resistor inside the cab of the truck. It looked like the compressor was working and noticed that when I took the cap off where the freon goes, it was slightly hissing. I dont know if that is why the a/c wont come on or if it is something I am overlooking. Please help since its been over 100 degrees everyday