Replaced the ECM replacement under warranty and I was able to get rid of the O2 sensor OBD codes, but the A/C still not turning on. The only way is by bypassing the A/C clutch relay. I hope it has nothing to do with the compressor since I had it replaced twice, because the first one had the clutch defective. I need help troubleshooting this problem. The mechanic who change the compressor said he believed the problem was with the A/C control inside. How can I check if that is really the problem. I have a multi meter to test, but not too handy with it r reading diagrams. Please help!!!!!!

The pressure switch outside of the car? it has 3 wires, one supplyes 5volts signal from PCM, one is ground, 0 volt, the other wire (Lightblue/brown tracer) should sit between 0.5-4.5volts. Test all of these wires and see if any of them are in spec.

The A/C control unit and The Evap temp sensor inside the vehicle will require Enhanced scanner to diagnose. If you want to buy one, they cost at least $500+
You need to look at the scanner and see if any data is doesnt seem right. Such as does the computer recognize the signal when u press the button, and if Ambient temp sensor is reading Freezing temp outside, etc...

Thanks...I check the voltages with the car on and with the a/c clutch relay bypassed or just the car on?

Key on engine off, or key on engine running, doesnt matter. But you have to back probe the connector with the sensor connected.

correct me i'm wrong but isn't there a pressure switch in the coolant circuit? the switch will not allow the compressor to engage unless the system charge is adequate .

Question...To replace the high pressure switch, Does the system freon have to be recover and the system needs to be vacuum? I am not an expert in this matter, but it look like its just unscrew the old switch and install a new one. Why cant I just replace the high pressure switch without the recovery and vacuum?

Because the switch needs to be in the refrigerant circuit to sense it's pressure, therefore there is a passage into the A/C system for that switch and when you remove it all your refrigerant (and lots of oil) will go with it.

Don't just throw parts at the problem, check the function of the temperature switch inside as suggested above, and check the function of the high and low pressure switches.

I removed the high pressure switch and did not see any refrigerant come out or oil. I watch this great video on youtube regarding the a/c switches and how to test them. Here is the link http://www.youtube.com/watch?v=tMi3VDkPdCo and let me know if the person explains it well. He talks about testing the high pressure switch for continuity to see if its a bad switch. I took the switch out of the system and with a multimeter tested for continuity and it did not have any. Let me know what you think and thanks for the reply. Where would that temperature switch be located and how does it look like and how do I tested.

I think we are talking about different things? The pressure switch does not need to be removed from the car to be tested, you only need to unplug the wires to it. Unplugging the wires does not involve venting the refrigerant in any way. Removing the switch itself would release refrigerant as far as I know.

As for the temperature switch you'd need somebody with a wiring diagram. It will be located on or touching the evaporator and turns off the A/C compressor when the evaporator reaches a cold enough temperature to prevent icing. Usually the wires will run to the BCM or similar and can be backprobed there, or usually there is also a connector near the air con box inside the car where it can be tested.