If the PCM thinks the fill is low, it won't turn the compressor on. There may be a bad pressure sensor. There are a couple other things that I don't recall offhand.

I don't have the wire diagram nearby, so I cant answer why you are seeing 12v all the time. When I get back I'll try to take a look.

 

I suspect one pin maybe gets 12v and the other pin runs to the pressure switch and then to ground. Makes sense in my head.
Mr Case is the man and will answer that directly.
Id try jumping the pressure switch. Low charge is a likely answer, if it was just below the threshold it would probably still blow cool.

 

I suspect one pin maybe gets 12v and the other pin runs to the pressure switch and then to ground. Makes sense in my head.

From what I'm reading, this doesn't have a switch, it's a 3-pin transducer and can't be jumped. I tested the lead going to the transducer and I'm getting 5v, 5v, Ground... which doesn't seem right to me. I think it's supposed to be 5v, signal (so 0v at the lead), Ground.

Id try jumping the pressure switch. Low charge is a likely answer, if it was just below the threshold it would probably still blow cool.

I gave power to the condenser and checked the pressure and it's at about 34PSI

 

i havent tinkered with anything this new to give any more advice. id guess computer talks to the switch and commands the compressor.
**** just getting more and more complicated.
i miss the old days. im just a dinosaur i guess.
mr case will fix you up im sure.

 

A/C clutch engagement is controlled by the Powertrain Control Module (PCM). When the A/C-heater control is set to any A/C position, it sends a request signal on the CAN-B bus to the Totally Integrated Power Module (TIPM), which then transfers the request on the CAN-C Bus to the PCM, which determines if operating conditions are correct for A/C clutch engagement. When all operating conditions have been met, the PCM sends a signal on a dedicated hard-wired circuit back to the TIPM to energize the internal A/C clutch high side driver. When energized, the A/C clutch high side driver provides battery current to the A/C clutch field coil.

The pressure switch has three terminals. One is ground, one is a 5V reference, the other is voltage indicating the pressure. If I'm reading the FSM correctly, when everything is working properly you should see 5V on the sensor circuit. You would probably get a check engine fault if there was a short there. You could try disconnecting the sensor and shorting the 5V to the sensor signal wire, the clutch should activate if other conditions are right (like AC is enabled on the HVAC controls). So most likely low pressure (measuring ~0V) or perhaps a TIPM fault, or the transducer failed to the low pressure state.

The compressor inlet should show somewhere between 20 and 50 psi depending on ambient temperature, 25-35 at 80 degrees. Outlet is 200-300 at 80 degrees.

 

By the way, the clutch control circuit should be tested with a typical test light that applies some load. The brightness will vary with the clutch duty cycle desired.The PCM will adjust duty cycle by turning the 12V ON and Off with varying durations.

 

I just had the same issue with my a/c clutch on my Liberty. The clutch had lost ground and power and couldn't get it back so I ran ground back on to the block and when I reconnected the ground I still couldn't get voltage out of the power wire. I pulled the the transducer plug to test it. That all came back normal as Cold case mentioned. Once I plugged the transducer plug back in the clutch kicked on. I repeated this process of removing ground to the clutch and reconnecting it and having to pull the transducer wire connection and plug it back in for the a/c clutch to get the 12v/power to it again, this is with the ground ran to the block still. It seems like when I pulled the transducer plug and plugged it back in it reset the system and allowed the system to supply 12v/ power to the a/c clutch. Hope this may help.