Guys (and gals)- I'm not the smartest guy in the room, but this one's got me stumped. 2000 F350 w/6.8L. AC was working fine a week ago. Took it out two days ago, and nothing but hot air in all the positions. AC clutch not engaging. Alright, let me check some stuff:
1. Have pressure. Slowly stuck a screwdriver in the low pressure port. Lots of pressure. Also attached the crappy gauge I've got off the top of one of the refrigerant refill bottles, and it pops up into the red (should be- high side and low side equalize when the truck is off, so should be higher pressure- least that's how I understand it)
2. Checked voltage on low side pressure switch connector with the AC running: ~14V, aka working as advertised.
3. Checked voltage on high side pressure switch connector with the AC running: ~14V, same as above.
4. Jumpered low side pressure switch with truck running, AC on. No clutch engagement.
5. Jumpered high side pressure switch with truck running, AC on. No clutch engagement.
6 Pulled off the AC clutch wire, and jumped it with the battery. (truck off, cause I like my fingers) Clutch engages, so the clutch works.
7. Said screw it, jumpered high and low side switches at the same time, truck running AC on. No clutch engagement.
8. Checked the breakers under the dash- 21 and 24 if I remember right. Checked the breakers under the hood for the blower. Both fine, cleaned the contacts.
9. Checked the diode under the hood- tested fine with the voltmeter, but swapped with the other one anyway. No change.
10. Checked the PCM with a OBDII diagnostic tool, checks good. (only did this because the only wiring diagram I can find shows a lead coming in from the dash panel, and one from the PCM- admittedly that diagrams for a diesel, but figure the wiring is similar)

I'm sure I'm missing something obvious, but I'm too dumb to figure it out. Thinking about getting some more refrigerant and just hooking it up to see what happens, but don't want to overpressure the system. Randomly also got the service engine soon when I was troubleshooting as well- I'm pretty sure it's not related, but figured I'd fess up in case there is some weird connection. P0171 and P0174- cleaned the MAF, and they cleared. Hive mind have any ideas?

 

This is a PCM-controlled A/C clutch. If the PCM either isn't getting the correct input signal from the pressure switches OR there are certain PCM-detected faults, it will not enable the A?C Clutch relay which is supposed to power the clutch.

Quick-checks, in order:
1. Scan PCM for fault codes which might inhibit compressor clutch. Misfires and open COP/injector codes are common ones.
2. Check fuse F06 under the hood.
3. Check to see if the A/C clutch relay (in the underhood fuse box) "clicks" as an assistant switches the climate mode switch between OFF and A/C.

Start there, post explicit results for further advice.

Here are the schematics that are specific to your truck:

 

Sorted!
I'll preface this with- I'm and idiot, do the simple things first, and RTFQ.
Threw the OBDII reader back on, and was able to verify that the PCM was sending a signal to the compressor to turn on, but the compressor wasn't running.
The verbiage in the manual was throwing me- for some reason when I saw AC CASS relay, I just tuned it out. Still don't know what CASS stands for, but AC should have been a major clue. Swapped the blower motor relay and the AC CASS relay, and sure enough, no blower motor. One trip to the parts store and a new relay, and I'm back in business.

ProjectSHO89, thanks for reading my novel, and pointing me in the right direction. Next time, I'll do the easy stuff first before jumping feet first into the deep end!