Ok, here i am with a 2001 honda civic 1.7 non vtech. The a/c was working one day and then decided not to work the next. I have checked all the thoughtful obvious things as listed in the title of this post. I had been told there is a ground wire that the ecu controls the ac with. the a/c fan does not kick on when the a/c switch is on but the ac fan does when the car gets worm to cool down the car as normal. The compressor does not kick on unless bridged at the relay. The relay is good, i have checked it.Can anyone confirm that it could be the ecu ground control thing, and if so where is the ground coming off of the ecu so i can check it? any help with this would be great. i need to know asap if possible and thank anyone who can provide reliable facts to get this fixed..thanks and god bless, Anthony.

 

When you press the button, the control panel should pull the blue wire at the pressure switch to ground. If the pressure is OK, this signal passes through to the other wire and goes to the Multiplex Control Unit, built into the underdash fuse box. The MCU sends a signal to the ECU which will cause the ECU to bump up idle speed slightly then the ECU will pull in the relays to start the fans and compressor.