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Discussion Starter #1
Hey guys I've read through the AC diag sticky and read all the posts in search results and couldnt find an answer. Ive seen the diagrams shared but nothing shows the BCM. What is the role of the BCM when it comes to the AC components and the high speed fan? When the car is on and idling, I turn the AC dial on then hit the AC button. The blower runs fine and blows appropriately in all the dial settings. The AC button lights up, but the compressor clutch does not engage nor does the high speed fan. Both fans work as they should when the car gets up to temperature.

I am getting power at the AC relay but not at the 2 pin plug that goes into the compressor clutch. Also I tried the test with the connection at the relay jumpered and compressor did not turn on and no power at either pin, I added a pic of the pins where Im not getting any readings
502645
. I should be getting power at the green cable correct? If I plug it back in can I put 12v to the green and a ground to one of the rd/black wires to direct test the compressor?

Also I checked the BCM and put a cover over the wiring loom like the BCM thread suggests. Out of all the wires the RD/BK wire was kinked , not chewed through. The metal inside the wire was not exposed. I looked at the BCM diagram and the RD/BK wire is supposed to be an "AC Request" line. What exactly is this doing? Where is this line allowing power to go to? Does this line also request the high speed fan to come on? If I have power at the AC relay does that mean the connection at the BCM AC request line is good? Or can I still have power at the relay with the BCM AC request line being bad? The relay I am using to test all this is brand new. Also the Eng Sens 15a fuse is good.

Any help is appreciated. I've been without AC for about a year, which was fine since its a cuising car/weekend toy. But now Im passing the car onto my nephew I would like to pinpoint where the fault is in the system before I pass it on.
 

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Discussion Starter #2 (Edited)
Update, I sliced open the green cable and gave it 12 volts and the compressor turned on and the high speed fan came on as well. Cold ac came out the vents, so I take it my lines are good and freon is still good.. Seems to be an electrical issue. Could it be the BCM? Can anyone chime in? What exactly is the purpose of the BCM in the system? Is it absolutely necessary? Can bypass the BCM and I setup my own wiring with seperate fuse and relay?
 

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This is from the service manual

A/C Request Signal and A/C Clutch Control
The (BCM) requests the ECM to turn the A/C compressor ON or OFF via the powertrain interface module (PIM) and the serial data bus normal mode message. The BCM monitors the voltage at the BCM A/C request circuit in order to determine the status of the momentary A/C switch. When the A/C switch is pressed, 12 volts are applied to the BCM A/C request circuit. The BCM sees this high voltage as an A/C switch input signal. On receiving the A/C switch input signal, the BCM requests the ECM to energize the A/C clutch via the PIM and the serial data bus normal mode message, if the ignition is ON and the blower motor is operating. The BCM also provides a ground to illuminate the A/C ON LED within the A/C switch. If the A/C switch is pressed again the BCM will request the ECM to turn OFF the A/C compressor. The operating status of the system will be remembered by the BCM, when the ignition is switched from ON to OFF or when the blower is switched OFF. If the blower is OFF and the A/C switch is pressed, then the next time the blower is switched ON the air conditioning will be turned ON. Turning the ignition OFF will cancel this button press function. The system will reset to OFF when the battery is disconnected. The ECM uses this signal to:

• Adjust the idle air control (IAC) valve position to compensate for the additional load placed on the engine by the air conditioning compressor.

• Energize the A/C compressor relay to operate the A/C compressor if the pressure in the A/C system is within the correct operating range.

The BCM monitors the blower motor switch setting to determine whether the blower motor has been selected or not. When the blower motor is running, BCM blower input circuit is pulled to ground. This causes the voltage at the BCM to change from battery voltage, +12 volts in the OFF position, to less than 0.3 volts, which is seen by the BCM as the blower ON signal. The BCM does not have any direct control over the operation of the air conditioner. When the A/C switch is turned ON at the HVAC controller, it passes this request received at the BCM to the ECM. The ECM then determines from other engine and A/C parameters whether the A/C clutch will be energized or not. Communication between the ECM and the BCM takes place via the Primary Serial Data bus.

The ECM monitors the A/C pressure sensor to determine A/C system pressure.
 

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Discussion Starter #6
This is from the service manual
I believe I've read that in another thread. Forgot about the ECM and the BCM connection takes place via the "Primary Serial Data bus." I was looking for wires to test at the BCM to probe them to see if I was getting voltage. And trying to see which wires went to the ECM. Also to see if I can bypass the BCM, but Ive been reading threads like this where the BCM or some custom built BCM has to be used.https://www.ls2.com/forums/showthread.php/841396-How-does-the-BCM-talk-to-the-PCM-on-2005-GTO-(E40-PCM)

I gave 12v direct to the compressor again today and checked the pressures,high and low are within spec of the chart according to humidity and temp. I have a strong feeling the glove box backing plate did do some damage to the wires. I'm gonna get in there again tomorrow and hopefully I dont have to remove the entire dash to be able to probe the wires.

Also as I was writing this I recalled when the AC went out the heads up display went wonky. I got the overheating warning, traction control warning went on and off multiple times and also abs off flashed a few times. I turned the car off and then back on and everything was fine after that. Since I really only use the car on weekends or evenings, I rarely turned on the AC. I really only use the AC during the daytime so I think I noticed the AC wasnt blowing cold weeks later. Does that sound like a BCM issue?
 

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How do I test that and wouldnt there be other consistent issues other than just the ac signal not going through?
you might want to go back and recheck the wiring, find the wires that are related to the communication lines and make sure there are no breaks, shorts, etc. the physical part of the bus is basically the part of the wiring harness that carries the signal. the BCM and ECM communicate via the PIM, as well, since they both use different protocols.
 
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