Chrysler Repair: 2001 Town & Country: A/C compressor wont activate, powertrain control module, air relay

Question
i have 2001 chrysler T&C LTD 3.8 and the a/c is not working.  The compressor is not engaging but the heater works.  i took out the relay and took a piece of wire to bypass/jumper in place of relay and activate the compressor.  The system works, compressor engages and a/c blows cold air, relay seems good as I also swapped another relay in box.  Not sure of next step, could i need a new control unit inside of car?  Thanks.

Answer
Hi John,
For the compressor to be activated it of course has to get the signal from the control panel that the a/c was requested (the a/c switch being closed), that message (in digital form) is sent to the body computer, and then a digital message goes from the body computer to the powertrain control module which will activate the compressor if fuse 23 (10 amp) in the power module is good, the ignition switch section that uses that current is good, that current is then sent to the actuation coil of the compressor relay, and finally that the powertrain controller also gets the signal from the a/c system refrigerant pressure sensor that there is the correct pressure in the system and thus grounds the other end of the relay actuation coil. The most common cause of the compressor not activating is the refrigerant pressure level not being in the correct range (too low or too high). You could measure the signal voltage at either the pressure sensor (between pins 1 and 3) or between pins 42 and 43 at the pcm. The voltage has to be be between 0.451 and 4.519 Volts for the signal to be accepted as an appropriate pressure level. So check that first. If that test is passed (with the compressor not 'jumped' into action) then you would want to do some troble-shooting with a meter to check out the other possible causes that I listed. I can describe where to go to do those tests.
Roland
PS Please 'rate' my answer, and where you see the question about 'volunteer of he month' consider a 'yes' response if this reply proves to be helpful. Thanks