Civic-EG.com Forum 3.3.1

- Wed Aug 08, 2012 10:30 am #313051 Hello

I have a 92 Civic Si sedan with D16A9 engine. I have a problem with the AC system. When I turn the AC on, the fan and the blower start to turn and the idle rpm increases, but the compressor does not engage. The relay for the compressor is tested and it works properly. The compressor clutch is tested and working. The problem seems to be electrical. There is a BLK/RED wire which runs from the ECU and sends the signal to the compressor relay. When I was testing the wiring, I grounded this wire for a second and released it and the compressor turned on, but after some minutes, when the car got cold and the thermostat switched the AC system off and the fan stopped and the idle rpm backed to normal, the compressor kept turning and did not stop!

The car is working good. The check light is not turned on. The refrigerant pressure is good. When I turn on the compressor manually, the car gets so cold. I have cleaned the wiring sockets near the compressor and relays and all of the sockets of the AC wiring system under the hood.

What is the problem? Anyone can help?

[suspendedHatch]

- Thu Aug 09, 2012 2:39 pm #313132 So the AC works fine, the engine runs fine, but you're concerned about how the compressor operates? I know the compressor will pulse on and off as needed on the later generation Hondas. I didn't think it behaved this way on the earlier models but I don't know that for certain.

- Sat Aug 11, 2012 4:44 am #313265 I didn't know that the A/C system and the compressor is controlled by ECU until I read the manual. I did the diagnostics step by step. Everything was OK. The final step was to change the ECU and retest, but I think that the ECU is absolutely OK and the problem is from another thing. When I was looking to the ECU connectors, I saw another BLK/RED wire rather that the one which runs to the compressor relay and I don't know what does this wire actually do. Maybe this wire is related to the problem. I guess that the ECU reads some data from the whole car system and only turns the compressor on under certain conditions. Now, I need to know that what does the ECU actually do with the A/C system and the compressor? Anybody can help please?

- Sat Aug 11, 2012 7:04 am #313276 I'd first trace that BLK/RED wire through all the harnesses and check for continuity throughout and figure out where its going.

I know on mine, I put a brand new compressor on, and once I got everything working properly, the clutch never disengaged when I had it turned on, put its only been 3 days and Ive been going to and from work testing it out, havent actually watched the clutch myself to see if its engaging/disengaging at any time, but we all know that when a clutch engages on a 1.5L, you can really tell in the tone of the RPMS, and the reduction in power you experience when accelerating.

Ive been spending a LOT of time working on my 92 DX sedan AC and Im pretty sure I remember that BLK/RED going all the way from the ECU out to the engine bay harness with lots of connectors linking it, I'd just test continuity from beginning to end first and if its showing continuity, do your best to look for shorts/abrasion on the shielding, specifically on the passenger side in the engine bay where it passes near the tranny.

Second, I'd take the three connectors off the ECU and look for green/yellow/black corrosion on the ECU itself and on the harnesses.

I forget what the stuff is called but its electrical cleaner in a red bottle at Oreillys, I have the bottle in my car, next time I grab it Ill come on here and post it, but just ask your guys at the auto parts store, its a red bottle with white on the front where the name of it is on. I sprayed it directly onto the ECU pins holding the ECU with the pins facing the ground to avoid it dripping down into the ECU, which wouldnt necessarily hurt it at all, would just take much longer to dry. Anytime I use this stuff Ill apply it twice with 15 min dry times in between. Im sure it doesnt take that long to dry cause that stuff its super cold really fast and evaporates really fast for me cause Im in a 90-100 degree temps right now, so if you're in a cold climate, might take longer to evap.

The honda specialists that Ive been having help me do stuff that I dont have the tools for told me that its very common for 92 ECUs to go bad, and that it is really common in the last 2 years for them to go bad. From what I understand, there is a way to test if the computer is bad by putting power directly to the compressor clutch and seeing if it clicks on, apparently if that happens, you know the ECU is doing its job, but dont quote me on that, there might be one piece to that troubleshooting that Im forgetting at the moment.

Id blame the ECU last.

Another thing to do is get to your evaporator, and on the top is the AC thermo switch, disconnect it and jump it with a 5A jumper wire and see it turns on then.

On mine, part of my issue was the actual heater control unit (head unit) which was shorting out unless a hammer was taken to it, I ended up buying a brand new one from the dealership, after wasting money on 2 different junkyard ones.

Keep us updated, best of luck.
Jon

- Sat Aug 11, 2012 10:41 am #313278 Thank you very much

I've already checked the thermostat and the pressure switch and they are OK. If the thermostat or the pressure switch go bad, the A/C system will not activate anyway. Also, I've already checked the BLK/RED wire from the ECU to the engine bay where it goes to the compressor clutch relay. It's OK. I've cleaned the ECU connectors and under hood sockets with the spray.

When I turn on the A/C system, the idle RPM increases and the condenser fan turns on. This means that the ECU knows that you have turned the A/C system on and the compressor should be running, so it increases the idle RPM.

There is a BLU/RED wire which runs to the ECU. It has 12V when the ignition switch is on. When you turn on the A/C system, this voltage decreases. So, the ECU knows that the A/C system is turned on and it should send the appropriate signal to the compressor clutch relay and increase the idle RPM. When the thermostat or the pressure switch break the system, the BLU/RED has 12V again and the ECU knows that the A/C system is turned off, however, it's turned on, but the pressure of the refrigerant is not normal or the evaporator is cold enough and there is no need to run the compressor.

My compressor clicks on when I ground the BLK/RED wire for just one moment. Then, it continues to work and never stops. This means that after grounding the BLK/RED wire and releasing it, the ECU send the signal to the compressor, which keeps the clutch engaged. Now the problem is that why the ECU does not stop sending this signal when the thermostat deactivates the A/C system?

I actually don't know that why should the compressor clutch be controlled by the ECU? The A/C system has a thermostat and a pressure switch which independently controls the operation of A/C system and the ECU is not involved in this process. So, why it should control the compressor clutch?

I'm sure that there is no open in the BLK/RED and the BLU/RED wires from the ECU to the compressor clutch and condenser fan relays. I there is a short in the BLK/RED, why the clutch engages when I ground it and why it remains engaged when I disconnect it from the ground? So, I think there is no short in this wire. I've checked it before, but I'll do it again.

I need a confident way to test the ECU and know if it's good or bad. As I said before, it seems to be good and I hope so.

Waiting for your help!

- Sat Aug 11, 2012 8:55 pm #313293 Im starting to think its the ECU too, the only way I know to test the ECU is to take someone elses ECU and hook it up in yours and see what happens.

- Sun Aug 12, 2012 7:23 am #313313 It's so hard to find another ECU and hook it up and test. It's very hard to find Si models in our country. It would be better to find another confident way to test the ECU. Also, I want to know that what does the ECU do with the A/C system?

- Mon Aug 13, 2012 7:23 pm #313409 Just a thought...Have you checked the low pressure switch? If it is defective, it would not allow the compressor clutch to engage. There to prevent burning up the compressor in case of a low charge condition. You said the charge was good, but the switch could be bad. Jsut a thought

- Tue Aug 14, 2012 6:33 am #313447 Yes, I've checked it. If the pressure switch go bad, the A/C system will never get activated, the fan will never start and the idle RPM will never increase. So, my pressure switch is OK.
Thanks