I've been trying to figure this ac out for a long time. If I jump across pins 3 and 5 where the relay plugs in, the clutch engages and it starts blowing cold air, with a known good relay in it, the clutch doesn't engage, and it just blows barely cool air. I have 12V every where I'm supposed to, the pressure switches both have continuity, about the only thing I can think of left to check is the control on the dash. It doesn't seem to be available new anywhere though. Am I missing anything?
I'm almost to the point of putting a toggle switch in and hard wiring it, but with my luck I'd burn up the compressor.

 

The A/C compressor clutch is actually controlled by the PCM and which in turn takes the hi and lo pressure switches and the climate control switch as input. The switches are in series so when all are closed the PCM input is pulled to ground and the compressor turns on. That makes it easy to troubleshoot, just measure resistance to ground at various points. Start with both sides of the high pressure switch while the climate control is set to an air-conditioning setting (engine does not have to be running, you can even have the key out). If that checks out do the same at the low pressure switch. If that's ok then check the relay driver side with a voltmeter. Pin 86 should have 12V, if not check the inside fuse panel (fuse number depends on year of the truck). Pin 85 gets pulled to ground by the PCM to enable the clutch so you can measure voltage between battery positive and that pin. 12V means PCM does request A/C clutch. For this test the engine needs to run.

You can not just use a switch because it bypasses the high pressure switch and that's not a good thing .