Hey all, so my ac compressor was not engaging and after looking at fuses and figuring out that it isn't it I thought it was the relay. a new relay wasn't working so I checked the grounds and hots. both positives are on but the only one negative lights up with a test light. it happens to be the one that you can jump a positive to it to activate the ac compressor. does anyone know what to check next? I can't find anything. thank you

 

If the pressure in the system is too low it will never engage because the low pressure switch will be open. Have you checked your pressure and or the switch?

 

True story, that cycling pressure switch is on the accumulator / receiver dryer and opens at about 24 lbs. closes at about 42
Get a set of gauges on it
Momentarily jump the terminals in the cycling switch connector together with a paperclip (I use a ground down one as to not damage the terminals)
That will tell you if all the primary circuits work (fuse control head and WOT relay all okay most likely)

 

I recently chased this issue on a recent 2006 F150 5.4 I obtained, there's a relay located driver side under the hood in a little black box, above the front tire. My ac was intermittent with compressor kicking on sometimes, sometimes not, after a delay, just random.
Was not the relay or compressor, it was the head unit, I have the electronic unit, was able to test by using other head unit from other truck 07 that has same control head. Goodluck! Like other poster said, compressor won't kick on if below pressure threshold, have you hooked up gauges? or what I like to do is evacuate and re-charge the system to make sure it's within spec.