I just went out and checked. Static is ~57 psi. When I forced the compressor on in my 60 degree garage it was about 30/125 initially and would climb to 30/275 and then one fan would kick on and it would go to 30/125 and the process would repeat.

 

OK... you're on to something here. Yes, I can run that test. In fact, I just realized that there is a bit that is "AC disable (pressure)" which I believe is tied to the AC pressure sensor.

When the HVAC is on (even when the compressor won't kick on), HPT shows the AC request as "Yes". If I turn the HVAC off, it goes to "No." This is good news as it means the HVAC module is communicating with the PCM even when the AC compressor won't kick on.

When the compressor won't kick on, the "AC disable (pressure)" bit is "Disabled". If I force the compressor on, the bit changes to "OK" as the pressure builds. So, it appears it is indeed the pressure sensor circuit on the PCM preventing the compressor from kicking on. Now I just need to figure out if it is electrical, sensor, or mechanical.

HPT reads AC voltage. It appears that below 0.5v the compressor won't kick on. Mine was at 0.37v when I started messing around tonight and the compressor wouldn't kick on. After I forced the compressor on, it didn't go below 0.53v, and then I could turn the compressor on with the HVAC button. Per the FSM, over 50 psi static HVAC pressure should be enough to make the PCM happy. I have 57 psi and yet the sensor voltage is telling it no.

 

You are a much smarter man than I.
Good luck!

 

Thanks for sending the info. It turns out, that procedure is in my 98 FSM as well (in the PCM section)... it just isn't referenced from the HVAC section unless you have the DTC P0530 code (which in my case I didn't). Regardless, I did the test as it basically runs you through the circuit test for the AC pressure sensor.
Looking at Bill Dearborn's post, it appears the 2000 FSM corrected their oversight and consider the pressure sensor as part of the HVAC system.

Running through the pressure sensor circuit test confirmed my wiring is all good. That left the pressure sensor, so I swapped out my new pressure sensor for my original just to see what would happen. The voltage reported doubled from ~.4 to ~.8 which is above the 0.5v threshold needed to allow the PCM to kick the compressor in. It appears that putting the original sensor back in will solve my problem (which is great)... however the whole reason I had replaced my sensor was because of the AC compressor not kicking on. So, I've come in a bit of a circle here (learned a lot in the process though). I've checked to make sure the AC will kick on now and so far it has consistently with the old sensor (it's been less than 24 hours, but the intermittent condition would repeat quite often). I think the new sensor I got is either bad or the wrong sensor (the new one has an aluminum housing and the old one is a steel housing). Anyway... I'm holding out to see if the same problem pops up again with the original sensor. If it does I will get another new sensor...

Thanks for all the help guys. This had been driving me for quite some time. This forum is the best.