Hey everybody, I am new to the site having run across it looking for info on google. I am working on a 2003 civic lx. The customer brought it to me and wanted the a/c compressor changed since he was told it was bad. I changed it for him and when I went to charge it the compressor clutch would not cycle. I checked the wiring and noticed there was no power to the compressor. I checked the relay and it was fine. But in my testing I found there was no ground coming from the ecm. If I jump the relay terminals I can get the compressor to kick on, but not without the jumper. I did a continuity test on the relay control circuit and I have continuity to the ecm connector. Anybody have this trouble before? Any help would be greatly appreciated!!

 

Check that the pressure switch and thermal switch on the compressor (if not the type that's wired in series with the clutch coil) are closed. Check that the wires at the pressure switch go to ground when the A/C button is pressed. If not, problem is with the control panel. Likely the evaporator temperature sensor if the light in the A/C button still works like it should. If the control wires go to ground but the fans and compressor don't start, problem with the multiplex control unit or ECU. Note that the ECU is programmed not to engage the compressor while the idle speed is abnormally low.

 

There is ground to the blue wire going to the pressure switch when the button is pushed. The compressor is a reman unit, I wonder if the thermal protection switch was changed or not.

 

Check that the pressure switch is actually closed now that you have pressure in the lines.

Is the thermal protection switch 2 small wires with it's own plug, or is it connected directly to the clutch coil? The type with its own plug is wired in series with the pressure switch to interrupt the A/C command to the MCU if the compressor is overheated.

 

When I replaced the compressor I had to wire the connector from the old one onto the new compressor. It shares the plug with the compressor. I jumped the pressure switch to eliminate it for this test and it made no difference. I am just not getting a ground to the relay from the ecm.

 

The ground from the pressure switch signals the ECU via the Multiplex Control Unit which is mounted on the fuse box under the dash. The MCU generates a digital code which goes over one wire to the ECU. When the ECU receives the command it will increase the idle speed and pull in the relays for the radiator / condenser fans and the compressor. Typically if the MCU is bad there will be failure of other systems it controls, such as the dome light and the keyless entry.

 

Everything else is working. So it sounds like the mcu is not defective.

 

I wanted to make another post to let everyone know what I found. After testing everything I was pretty sure I had a defective ecm. The owner of the car had already fallen victim to one bad diagnosis from a shop which resulted in a new compressor and drier. After talking with the owner we decided it was best if he takes it to the dealer and has them look it over since you can't return the ecm. I heard from him this morning and it ended up being a bad ecm. They will have it later today and are going to put it in for him. I just wanted to post my results to maybe help someone else with the same problem.

 

Thanks for following up. It looked like it was definitely either the MCU or ECU and I was hoping that RonJ would post some testing procedures from the service manual. The proprietary scan tools at the dealer would make short work of that and it's essential to have a good diagnosis since both those parts are expensive.