Newbie here so let me know if I'm doing anything incorrectly. I originally posted this in the 97-02 F150 section so reposting here after I realized there was an HVAC section.
I replaced the AC compressor, Accumulator, orifice tube and the high side hose assy with muffler on my son's best friend's truck (1999 F-150, 4x4, 5.4L eng) this week-end. His prior compressor was making a horrible noise and was needing to be recharged every 3-4 months or so until the last time we charged it last summer when it only lasted 3 weeks. I think he had some other problem than losing freon because his charges were lasting several months previously but I didn't troubleshoot it then so I have no way of knowing for sure. Anyway, I evacuated the system and charged it but after the 2nd 12 oz can of freon was going in compressor still wasn't engaging. Checked the plug at compressor and it came loose so I plugged it back up and the compressor clutch started trying to engage several times but only stayed engaged from less than a second to maybe 3-4 seconds tops. It would make several attempts then stop like it was timing out (maybe the PCM smarts possibly). I put 12V directly to compressor and it engaged continuously, the pressures were roughly 45 low side and 170 on the high side which to me seemed pretty close to normal for only 22-24 oz freon (capacity is roughly 34 oz) and outside temps in upper 90s. I tried jumping the low pressure sw on the accumulator and the high pressure sw near the high pressure connector but clutch still would not engage. Also jumped the AC relay power contacts that sends 12 V to compressor and that did engage the compressor (tried it with switch and ac on but truck not cranked and heard the clutch engage). Also swapped the relay with one of the other like relays that were working and compressor would not engage. I've looked high and low on the internet for a schematic but couldn't find one so I ordered a service manual on CD from ebay but I won't get it until later this week. I could still have a wiring issue from AC dash switch to the LPS/HPS or from the PCM to AC relay wiring and also think it could be the PCM? Originally thought it might be an issue with the new (not rebuilt) compressor or clutch but since it worked when I put 12V and ground directly to it and the pressures "looked" normal, I pretty much took that to mean the compressor and the mechanical stuff is working properly and that my problem is purely electrical. Just FYI, the pressures equalize at about 110-120 PSI but it was really hot here this week-end so I thought that was normal too.

Project just provided a schematic that will be very helpful. I didn't check fuse F5 yet but I am sure it is good because I did have 12V at the relay where the 12V comes from the fuse. The compressor has been trying to start so it seemed to be getting power at least intermittently. I have read about the AC diode and realize that's for feedback off the electromagnetic clutch and agree with Project that it wouldn't be the problem.
I plan to check the wiring on the pressure switches tonight to make sure 12V is getting thru there for the PCM to energize the relay and the schematic also shows a connector between the HPS and the AC dash switch so I'll try to locate that and check it as well.

Has anyone ever heard of the PCM causing problems with compressor running continuously?
Any suggestions before I have to go into the dash and does anyone know where the PCM is located?