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Do you know how much freon you have in the system?
I understand. The computer isn't going to ground the wire if there system pressure is to high or to low. This would be if there is less then half a pound in the system, then it would be to low and the compressor won't come on. The pressure switch will stop it. That being said, if there were .3 in the system and you force the compressor on, it will blow cold.
I would try jumping the pressure switch. Jump the green/orange stripe and brown/yellow stripped wires at the connector and see if the compressor kicks on. If it does, you either need a pressure switch or you need the freon level adjusted.
I believe this is mounted either in the high pressure line or the receiver dryer.
Try the jumper and let me know.
The ecm won't ground the relay if the pressure switch is no good. So, since you don't have the ecm grounding, we have to make sure the other sensors or switches aren't the ones telling it not to ground. Does that make sense? This is why I wanted to check the pressure switch first.
Now, since we know that switch is working as it should, you have to check continuity on the brown wire, from the ecm to the relay. Make sure it's not broken, if the wire is good from one end to the other, then it's likely the ecm is the culprit as you suspect.
Also, if the control head isn't telling the a/c to come on you won't get any compressor action either.
Sounds good Lloyd, hopefully we can get this worked out.