enzoferrari

Hello
I have a 92 Civic Si sedan with D16A9 engine and P29 ECU. I think that the P29 is the same as P28 but without VTEC.

I have a problem with the A/C system. When I turn the A/C on, the condenser fan and the blower start to turn and the idle RPM increases, but the compressor does not engage.

The car is working good. The check light is not turned on. The refrigerant pressure is good. The A/C indicator light works normal. The blower works normal. When I turn on the compressor by hot wiring it, the car gets so cold. I've already checked the thermostat and the pressure switch and they are OK. The relays and the compressor clutch are tested and they work properly. The problem seems to be electrical. So, I have cleaned the wiring sockets from the compressor and relays to the ECU. I've already checked the wires. I found no shorts and no opens in the wiring. I've already done diagnostics noticed in the manual and I've reached to the final step: Substitute a good-known ECU and recheck!

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 a confident way to test the ECU. But I think that the problem is not the ECU.

I opened the ECU and I couldn't find any bad things inside it. I cleaned the socket and tested connections. Everything seems to be OK.

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 from the condenser fan relay to the ECU. This wire is indicated as ACS (Air Conditioning Switch) in wiring diagrams and ECU pinouts. When you turn on the A/C system, 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 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.

There is a BLK/RED wire which runs from the compressor relay to the ECU and is actually the ground connection of the compressor relay. The ECU should ground this wire to engage the clutch. My compressor clicks on when I ground the BLK/RED wire for just one moment. Then, it continues to work and never stops, even when the thermostat breaks the system. 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? Or, maybe I'm wrong and there is another problem. 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 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. If 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. However, I've checked it before.

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.

Anyone can help me in this? It's a long time that I'm working on it but no results. How can I test the ECU without changing it? What does the ECU do with the A/C system?

rountreesj

i think there have been problems before with the actual climate control getting hot and cracking at the a/c button. maybe check into that/ research there...

rountreesj

have you tried running a jumper wire straight to the clutch and seeing if it will engage?

Former User

Your ECU is most likely bad and needs to be replaced.

The clutch relay is wired to the ECU so that the engine idle can be raised a few seconds before the clutch engages to compensate for increased engine load.

enzoferrari

Yes, of course. The clutch engages and the compressor works fine.

I'm going to check it out.

More suggestions?

Former User

You can rule this^ out by the fact that the A/C button works to turn the condenser fan on.

Start shopping for an ECU.

redvelvetsi

Have this problem but as stated if the A/C button stays on all the time then this obviously isn't it.

enzoferrari

I reply this after about four years! The ECU was bad. Removed it and tracked the pin to the board. Found a transistor array (STA413A). Tested it. Was bad. Couldn't find the same part. Purchased a similar one (PUB4120) and replaced it. It worked.

deschlong

iTrader: (8)

I seriously love it when users come back after YEARS to post the solution to their obscure problem. Thank you. This may just help someone else out. Including me since I now have a D16A9 also.

I might have also suggested testing the car temporarily on a different ECU, just to see if the AC engages properly.

Former User