A/C with aftermarket ECU
 

Hi all, trying to finish up my wiring, but have run into a little problem.

On the A/C dryer, there is a pressure switch. The wire from this goes directly to the stock "Engine Control Module". I have removed the engine control module, what do I do with this? Is it ok to leave it unplugged?

Thanks.

run it into a input of the ECU.

That is your A/C request.

Then run a DPO or output to actually engage the clutch.

This way your ecu can idle up before the A/C bogs/kills the motor.

the clutch for the A/C is still controlled by the stock relay and switch. This is just the pressure switch on the Dryer. I'm not sure how that works with the rest of the system.

Anyone know where the PCME "Powertrain Control Module (Engine)" is? that's how the relay is hooked to the pressure switch. I can't find it for the life of me. i can find the thermoswitch for the A/C on the blower, which is tied into it as well, but not the PCME itself.

I think I can just ground this wire, but then I have no protection for my compressor if it ever malfunctions and pressure builds up.

Take a look at the Body Electrical Troubleshooting Manual, page G-9 for an AC circuit diagram.

The PCME is Mazda's label for what most people would probably call the ECU, or the engine control module. It lives near the passenger-side footwell and has 4 rectangular plugs attached to it.


Pin 1E on the stock ECU connects to the refrigerant pressure switch. The refrigerant pressure switch is just a pass-through, as long as the pressure is in the 'safe' range it will allow the signal from the thermoswitch to pass through. The thermoswitch is another passthrough, as long as the temperature is in the 'safe' range it will allow the Air Conditioning request signal from the AC control panel to pass through. The Air Conditioning request signal depends on the position of the blower fan dial and the AC switch.


Pin 1L on the stock ECU connects to the AC clutch relay. The stock ECU activates the AC clutch relay depending on the voltage of pin 1E and some other factors like RPM and throttle position.

More info on pages Z-34, Z-68, and Z-70 of the Mazda wiring diagram.

Thanks, I've been spending a lot of time in the wiring manual. My stock ECU no longer exists, and i'm relocating my relay box.

I'm still not entirely clear on what to do.

There are 4 wires going to the A/C relay. I have 3 of them figured out.

These two wires are how the Relay actually triggers power on and off to the A/C clutch.
1. Power into the relay via the Joint Box A/C fuse
2. Power out of the relay to the A/C clutch


These two make the relay itself activate:
3. Power to the relay via the Joint Box Cigar fuse
4. Ground for the relay via the PCME

The PCME is no longer there, but the relay still needs to have a ground. I could just ground this wire, and everything would work, but then I'll have no protection from the pressure switch or thermoswitch. I know some A/C systems don't have this stuff (older ones), but i feel like I should have it.

Does anyone know how the pressure switch works? I know it's a pass through, but if pressure gets too high does it interrup itself, or does it rely on the PCME seeing that resistance is off, and cutting the signal internally (breaking the ground to the A/C relay resulting in cutting power to the A/C compressor clutch).

From the info I've found, the pressure switch and thermoswitches are both simple on/off continuity switches, when the refrigerant pressure or temperature is out of range they will interrupt the signal from the AC control panel on the dashboard. It should be possible to measure the signal wire that originally went to pin 1E on the stock ECU with a multimeter, check for voltage with the blower fan dial at positions 1-4 when the AC switch is on and off.

I don't remember the details, but the voltage or resistance of the blower fan dial may be different in each position. I don't have a Power-FC, but I've seen threads that make me think it was once a challenge to make the PFC control AC correctly, but I think people have gotten it figured out... I'm sure you could find more info in the PFC subsection of the forums.


The reason the ECU is in control of the AC clutch relay is because it puts a LOT of load on the engine and needs to be compensated for by adjusting the idle solenoid and possibly fuel and ignition settings. If you're trying to take the ECU out of the equation, it's very likely your car will stall when you switch the AC on. If your aftermarket ECU is capable of controlling an AC clutch, it's a very good idea to use that capability.

The ECU activates the AC relay by pulling the connection you label as 4 to ground.

You could get creative and connect one side of the the stock AC dash switch to ground and then wire its output through the thermoswitch and pressure switch to relay connection #4.

You could then activate the AC by pressing the switch and have the protections afforded by the thermoswitch and pressure switch.

However, the load issues mentioned above are not addressed by this solution.

Do not bypass the pressure switch. It is there to protect the compressor.

All you need to do is join the input and output A/C wires where your factory A/C was. The ECU receives a ground signal as a "request" for A/C and it sends a ground signal out to the relay.

As others have said, the best way to do this is to use inputs and outputs on your aftermarket ECU so that it knows when the A/C is about to put a load on the engine.

And aftermarket can't control the A/C on/off button though. I'll look into the idle issues later on, but first I need to make sure that when I press the A/C switch, something actually happens.

After a little more reading, this is what I've come up with, I think I can still use the thermo switch and the pressure switch without the PCME.

Can anyone make sense of this? I'm basically bypassing the PCME because its not there, hoping that the A/C switch will still pull to ground through the thermoswitch and pressure switch.
Attachment 718673

My question in pic 1 asks about connector X-07 and it's ability to connect to the A/C on/off switch via the heater control unit. More detail on why X-07 comes into play is here:
Attachment 718674

What you're doing will work assuming you don't run into some of the known grounding issues in the FD A/C electrical system. As you've noticed, a signal starts from the climate control, goes through the pressure switch and all that, and then goes to the ECU. Then the ECU decides whether to kick on the compressor or not based on however you decide to set it up. When you bypass the ECU, the clutch will come on, and your idle quality will take a hit.

I have done this bypass on a 2nd gen car, which has basically the same style wiring for the A/C.

I have no idea what your wiring looks like at this point. I don't know how much of your dash harness you've got left. If you were to take the original plugs the go into the stock ECU, cut the two A/C wires from the plug, and stick those two A/C wires together the A/C should work.