This is my first post after lurking here for a few weeks, and first I would like to say thanks. This forum is great, I have learned alot!

Now for my problem. I have a 1994 Honda Civic DX with A/C issues. First of all, it has the correct amount of freon (R134a) according to my buddies manifold gauge. So far I have replaced the Condensor Fan Relay and the Compressor Clutch Relay, and checked all the fuses with test light. When I turn on the A/C, the A/C light comes on the dash, the engine idles up a bit, the condensor fan turns on, but the compressor does nothing. I pulled and jumped the A/C Pressure switch, and still nothing. I used a test light to see if there was power coming to the compressor (red wire), and there is no power coming from the Compressor Clutch Relay. The relay is getting power, but the relay is not getting the signal from the blk/red wire from the ECM to turn on the compressor. According to Honda Service book, it comes from ECM pin A15, and will cause the relay to engage the compressor clutch, thus cooling the car. I jumped the compressor wire at the relay, and the compressor did engage, and cooled the car.

So my question is, what makes pin A15 of the ECM send power down the blk-red wire to the relay, causing the relay to engage and turn on the compressor? Has anybody dealt with this, or dug into their A/C system far enough know what I am talking about?

HELP!
Thanks in advance,
John "Trouper" Troup
West TN

 

That wire is an output from the ECU. The ECU accepts the command on the other wire, idles up the engine, and if it is above the minimum idle speed (i.e. not about to stall), approximately 1/2 second later the ECU drives A15 to ground which should pull in the compressor relay and engage the compressor.

Since the engine idles up, the ECU is receiving the command OK. The pressure switch and all are OK. The problem is that the ECU is not driving the output. It is probably a defective ECU. Measure right at the ECU pin it should be 12V and go to ground when the compressor is supposed to come on. Also make sure the idle speed is above about 600 rpm.

If it really is a problem in the ECU, you can work around it by simply connecting the compressor relay to the ECU input wire instead. This will make the compressor engage instantly, without the 1/2 second delay, so the engine will probably stumble every time it comes on. Also the features of the ECU to drop out the compressor under certain circumstances will not work. The compressor will not drop out to save from stalling due to abnormally low idle, or to get more power to the wheels (and possibly save the compressor from over-revving damage as the engine approaches redline) at wide open throttle.

 

The ECU provides ground for the clutch relay through the BLK/RED [text corrected] wire. Use a multimeter to check for an open (no continuity) in the BLK/RED wire between the clutch relay and ECU connector terminal A15 [text corrected]. If continuity is fine in this wire, then the ECU may be bad.

 

Thank you for the quick responses! Which wire sends that command to the ECU causing this wire to ground? Could it be a faulty input to the ECU? Also, which wire is the ECU input wire I need to bypass this? I am guessing a replacement ECU is hard to find and expensive?

 

mk is correct. The BLU/RED wire is fine, so focus on testing for a possible open in the BLK/RED wire. If this wire is fine, then the ECU is likely bad.

mk was saying that you should try grounding the BLK/RED wire.

 

Another question, by your diagram, I see that there is a thermo switch inline. Can that be a cause? Or since it's idling up, is it okay? Where would that be located on my car to check it if I need to?

So you are saying if I don't have continuity at the ecu from A15, I can just ground that wire by permanently jumping it to the blue/red wire, or by grounding it? I am not sure if I can get another ecu.

Sorry for all the questions, I am new to Honda's and this is a great learning experience! Ya'll are great!

 

The fact that the condenser fan runs proves that the thermo and pressure switches are fine.

As mk said, the proper idling up indicates that the BLU/RED wire and the corresponding ECU function is fine.

mk properly corrected my original post by saying that the BLU/RED wire does not provide ground for the BLK/RED wire via the ECU. Therefore, do not jump the BLK/RED wire to the BLU/RED wire. Instead, find a convenient ground for the BLK/RED wire to the body frame.

 

Thank You!

 

Do not ground the wire, the compressor will run all the time. With no protection from the pressure switch, when the condenser fan stops the pressure will rise out of control until the relief valve or the discharge hose blows out.

What you want to do is connect the black/red wire to the blue/red wire, bypassing the ECU. The blue/red wire is on pin B5.

Used ECU's should be available and not terribly expensive.

 

Okay, got a problem. I went out to temporarily jump the wires to get the a/c running (like mk described above), and when I started the car, turned on the a/c switch, the condensor fan did not turn on. WTF? It worked yesterday, but not today? I checked the fuses, probed the condensor relay & it was receiving no signal from the white wire (which comes from the comp clutch relay.) If I jumped the white wire in the comp clutch plug, the condensor fan would engage, wtf? I swapped relays & no change. I also tried jumping the white wire (engaging the fan) then jumping the red compressor wire, and everything worked. I am totally confused. Would the outside ambient temperature cause the condensor fan not come on, or what else? It is 15 degrees cooler today and raining. I never got around to checking the blk/red wire at pin a15, but will get to that once this is solved.