08 AC clutch not engaging - HELP.
 

2008; 2.2; auto. I start the car and turn on the AC. AC light comes on but clutch does not engage. The clutch relay (#15) appears to be okay. (I swapped it for the fuel pump relay and the car started with no problem.) When engaged, the voltage at the clutch is about 0.2 volts - not the 12 volts you would expect. I shorted the pressure sensor (Orange and Gray) wires to check the pressure sensor and the clutch still didn't engage. I hooked up the low pressure gauge on the system and the resting pressure was around 24 lbs... so there doesn't appear to be a leak in the system. Prior to this problem, the AC worked fine with no cycling (which usually indicates some loss of Freon). After 15 seconds after shutting off the engine with the pressure sensor shorted, the cooling fan kicked on. The engine was still cold. When I pulled the jumper wire, the fan shut off. The connection between the compressor clutch and the cooling fan runs through the ECM. I tried rebooting the ECM (no power for two days) and that did not fix the problem. All the fuses check out okay. I'm running out of ideas - what should be checked next? Am I going to have to replace the ECM?
Any help would be greatly appreciated... Jimmy

Resting pressure of 24 ? The static pressure (with the compressor not engaged) should be around 100-120.

There is a diode for the AC fan in the under hood fuse box.

Thanks for the quick reply...
The chart I have lists 25-35 for the low side (at 65 F) and 135-155 for high. Of course, I didn't have any previous readings for this vehicle when it was running because it failed quickly. The AC has been working flawlessly without any issues since new. The belt tensioner pulley started making a grinding noise last year, but that was an easy fix and did not require any changes to the sealed system or the clutch.
I could run a separate 12V power cord directly to the clutch to force it to engage which would enable me to get the running pressures.

I haven't checked it yet, but I will tomorrow morning. Do these diodes ever go bad?

Yes, they sometimes get overloaded.

As I think about it, a faulty diode could be the issue that is preventing the 12v signal from getting to the clutch. The relay has power and seems to be working properly. My next question is why would the cooling fan run after the engine is shut off while the engine was still cold? The engine had only been running for about 45 seconds; just long enough to see if the clutch would engage.

I pulled out the diode and tested it with a continuity tester... It seems to be working properly as there was continuity in one direction but not the other. There was some a resistance load in the positive direction. Is it possible that the diode has failed where it still establishes the flow, but with an increase in resistance. The resistance reading I got was 0.59. Could this be the reason for the low voltage (0.2V) measured at the clutch? I think the next step will be to apply 12 volts directly to the clutch (with the everything shut off) to hear whether or not the clutch is engaging.
Any other ideas?

Also, There are no codes present and no trouble lights on. The car starts and drives with no problems.

Could also be the clutch assembly simply died, out is 18 years old!

crazy question, but do you have a check engine light on? and check temp on dash, if it is 3 dashes, that would be your thermostat. If the thermostat messes up, it will disable the A/C from working. Been there.