2002 Civic HX coupe with 5spd and 189k miles, bone stock and never had any problems, I am the second owner and owned it since 125k miles. The car was sitting for a little while and the brand new battery died so I replaced it. After that, the AC doesn't blow cold. The AC button lights up when pressed, I jumped the AC evaporator temp sensor in the drivers lower foot well and no change. I checked/swapped around the AC relays under the hood in the passenger side fuse panel, no change. I checked the AC fuses and they look good, I took the car to a local mechanic and he was able to jump some wires and get the compressor to come on and blow cold air. I let the car run and get to temp until BOTH fans came on by themselves so I know both are working correctly.I replaced the original PCM ending in L04 the first time (had the immobilizer settings transferred over) with a re manufactured PCM ending in L05 and still no change. I took the car to a local Honda dealer and they were able to jump things and get the system to blow cold too. Then they were about to take the compressor out to replace it and the AC system worked all by itself then stopped working so they didn't pull anything apart. They said they found a short in between the re manufactured L05 PCM and the original multiplex and wanted to replace the PCM again, I told them no and brought the car home. I replaced the multiplex with a new OEM Honda part, still no AC change. Then I bought another brand NEW OEM Honda PCM ending in 417 from Honda (immobilizer settings transferred over), I just installed it and still no change. I am completely baffled here and so is the Honda dealer. They had the car for 4 days or so and said there is a short after the 1st replacement L05 PCM but before the original multiplex. So to sum it up, I've put two new PCM's (went from the original L04 PCM to a L05 and a 417) in the car and a new multiplex and still no AC coming on. Any suggestions?

 

Use a multimeter to check for an open or short in the Yel wire that runs between the MCU and the ECM.

@tech8 Would you be able to post the detailed 2002 Civic circuit diagram that controls the compressor clutch (ECM--MCU--pressure switch--heater control panel)?

 

 

Based on the circuit diagram kindly posted by tech8, unplug ECM connector E24 and dash fuse box connector E10.

1) Use a multimeter to test for continuity between the Yel wire pins in unplugged connectors E24 and E10. There should be continuity.

2) Use a multimeter to test for a short to ground (continuity to body ground) at the Yel wire pin in either connector E24 or E10. You should not find continuity to body ground at either connector pin.

Post your results.