tdwpgtp

Hello everyone, my wife's honda civic's air conditioning sometimes runs on its own. It seems to be when it is humid, but I don't believe that is the only time. The system runs while the A/C button is off, and the car is not set to defrost. Sometimes the system turns on and off rapidly, sometimes it stays on for a few minutes, and sometimes it will run for days at a time. We have replaced the relay several times, and have replaced the computer. I have also tried disconnecting the HVAC control panel plugs while it was running, to no avail. The relay is getting ground from somewhere when it shouldn't be. Our local shop has traced the wire everywhere they can, without tearing out the whole dash, and could not find a short to ground. Is there anything else that could cause this, other than the wire grounding somewhere along the way?

All input is appreciated

Slumpertcivic

Not something I have seen with OEM ac clutch, but plenty times I have seen bent clutches from aftermarket that basically turn into a solid pulley so the compressor runs “on it’s own”.

I always suspect it’s combination of cheaply made part and over tightening belt.

If you unplug the compressor, and you can still see the middle section spinning that’s the problem.

tdwpgtp

Thank you for the input. If this is the case, though, the relay would still remain off, and no ground would be present on that relay socket, right? The problem is that the relay is getting ground and turning on. The clutch seems to engage/disengage when the relay closes/opens. If I take the relay out, the clutch disengages

Slumpertcivic

Well switching the air controls to the “defrost” vents also will turn the AC on.

Possible that controller is bad, there is a factory way to override that “feature” but that’s something for you to look up.

tdwpgtp

I've tried the defrost override and also made sure it wasn't set to defrost in the first place, all with no luck. Additionally, it runs regardless of whether the heater control panel is plugged in or not, which would suggest that it doesn't involve that, correct?

tdwpgtp

I cut the A/C clutch wire coming from the PCM, which killed the compressor, so it is NOT getting ground from a short somewhere between the PCM and relay like I had thought. I guess I should have known that, since the condenser fan is running, too, and that is a different relay. Is it possible that I got a second PCM with the same issue? That seems exceptionally unlikely

Is there anything else in the system that can command the computer to give ground to the relay?

Slumpertcivic

Not sure what your thinking, the heat dial only controls coolant flow into the heater core. You can have heat turned to max and still turn on the ac.

The fan speed selector should turn off the AC system when in off position.

The vent selector can trigger the AC system, generally it’s only supposed to do that when you turn it to the two defrost options but anything can fail.

The separate AC button below the radio can also fail so that even when the “light” goes off it could still be triggering the ac on.

Your odds of having two PCM with that exact problem are highly unlikely.

tdwpgtp

I meant anything other than the HVAC controls. Nothing on the control panel will successfully shut off the A/C compressor. It runs regardless of the settings, as does the condenser fan.

I've seen mention, several times in various places, that if the fault was that of the HVAC control panel, that unplugging it would turn off the A/C system. This does not happen when it is unplugged, whether while running or prior to starting the car. The only way to turn it off still seems to be to pull the relay (or cutting the PCM-to-relay wire).

I agree that the odds are against that, but what other possible cause is there?

Slumpertcivic

So that includes unplugging the separate actual recycle-AC-rear defroster button control switch.

I don’t have have the electrical diagrams on my tablet, but it’s likely both of those controls pass into the MICU which might be the root of your problem.

tdwpgtp

Is there a way to test the multiplex unit? I'd rather not spend the time and money replacing it without verifying, if possible. Also, is it possible that either the pressure or temperature switch are somehow telling the PCM to ground the AC relay? Or do they simply pass/terminate the signal sent by the multiplex?

tdwpgtp

So I did replace the multiplex unit, since it was 15 bucks at a junk yard. So far it seems to have fixed the issue, but since it has always been intermittent, I am going to give it some time. HOWEVER, now with the new multiplex unit, the keyless entry no longer works. There shouldn't be any programming that needs to be done for the multiplex, correct? I did notice that the original fuse box had a fuse for heated seats, even though the car never had heated seats, and the new one does not have contacts for a fuse there. Is it possible that they used the heated seats fuse for the keyless entry receiver? Or is there something else I am missing?

Colin42

is it factory keyless or dealer installed keyless?

tdwpgtp

Looks factory to me, as far as I can tell. The harnesses are all wrapped together, which I doubt a dealer would have spent the time to do.

Colin42

take another fuse from a non critical circuit, stick it in and see if the keyless works

tdwpgtp

There are no contacts for a fuse in that fuse location in replacement box

Colin42

Then you bought the wrong multiplex which could also be the problem

Honda lists 4 different ones for 05, and there could be others in different years as well

tdwpgtp

Where can I find which one to buy? The part number that I saw molded into the fuse box was the same, but I didn't look for any other numbers on them.

tdwpgtp

Update: I got a new multiplex to fix the door locks, which it did, but the original A/C problem is back. I suspect it was never actually fixed, and just coincidentally didn't show up for a while before. What are the chances the multiplex unit I pulled from the junk yard had the same issue? Anyway, now I am frustrated and at a loss for what to do next. I'm going to go to the yard to pick a new hvac control panel, since that is the only part I haven't replaced, but I don't believe that is the problem, since unplugging it does not solve the problem.

tdwpgtp

HVAC panel did not fix it, problem still persists. What the hell could be causing this?

CraigW

iTrader: (1)

Try this method...…..https://www.civicforums.com/forums/1...et+A%2FC+codes

tdwpgtp

I have already tried that, unfortunately.

tdwpgtp

Update: After replacing the multiplex AGAIN and the HVAC controls, the problem remained. So, I did what any normal person with no more ideas would do: Started poking wires and harnesses with a plastic stick. Well, what do you know!? When I poke the two wires coming into/out of the thermal protector on the compressor, the A/C and condenser fan (that are running with the button turned off) turns off. While holding the stick on these wires, I had my wife push the A/C button on and off a few times, and the A/C turned on and off with each press. See, the thermal protector is designed to open the circuit if the compressor gets too hot. That means it can only PREVENT the compressor from turning on, it cannot provide power on its own. So, what does this mean for my system? How is gently poking the wires in the thermal protector shutting the A/C and condenser fan off, but still allowing me to turn them on with the button push?



According to the diagram there is no way that the thermal protector can provide power to the compressor clutch, but it seems that not only is doing that, but it is also providing power to the condenser fan, as well. I'm at a loss, yet again.