I've been trying (kinda) to trace the issue with the ac clutch. whenever the key is in the on position with engine running and not running, Not on Defrost, and the ac switch/button off. the relay is always on engaging the clutch. tried with defrost off and turning the button to the on position and then off does nothing. but when I turn on the ac from inside , the engine RPMs go up which is normal. Currently have had the ac relay unplugged for a while to prevent damage to the compressor. the clutch connector is free of corrosion, got a new relay, and still does it. if I plug in the relay I can feel and hear it click and the clutch turn on. Seems like possibly the control side of it is the issue. a wiring diagram would be helpful, would want to see if I have a control voltage coming out of the PCM (not , if so wouldn't that point to a bad ecm/pcm? p.s when the ac relay is removed after some time it throws a dtc for P0645 "A/C Clutch Relay Control Circuit".

 

Check here, I suspect they will have a wiring diagram for ya.

 

Usually the issue is caused by a faulty high/low pressure switch. Now I've never had an AC issue with this truck, so I'm not sure if it's the same setup as most cars, or where the switches are location. Also, you might just be low on refrigerant.

Edit: I take it back, I did have to change the AC clutch back in the day because the bearing wore out. I did that without disconnecting the lines though, so I didn't have to mess with too much.

 

When I pull fuse 11 (interior fuse panel) with the ignition in the on position either the engine running or not. The relay doesn't click in, as soon as I put the fuse it clicks in. When I pull the a/c fuse in the pdc the relay still clicks but the clutch doesn't work.
Grabbed my multimeter and on pin 86 to battery positive i get .55 omhz. Pin 86 to battery negative i get 12.5v. on pin 85 there is no Short to battery positive or battery negative (pcm control side). Probed pin 11 on ecm connector 3 to pin 85 on the relay/fusebox i get resistance. So it seems the ac relay control wire is fine but why is it controlling it. I unplugged connector 3 and put the relay in with the key in on position and it doesn't click in but when i plug it in, the relay pulls in. So this points to a bad ecm maybe or something commanding the pcm to turn on the ac???