Air con connection help

[MegaMicra]

Hi guys I'd just like your advice on which connections would be best to connect up my factory aircon. I have a MS3X currently and don't want to blow it up, so I thought I'd get your opinions.



So as you can see in the diagram of how Nissan have it setup I have 3 wires going back to the oem ecu.
Now I'm OK with the lower one as that's the gnd switch to bring the AC relay on.
I'm just wondering if I can use any of the others as an input as if I press the AC switch without the interior fan on they go to +12v.
As soon as I then switch the interior fan on they both drop to gnd.
Any feedback would be welcome.

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[MegaMicra]

Come on guys, anyone fancy advising.
I'd rather use the existing wiring if possible rather than running a wire back from the compressor live to Nitrous in.

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[Matt Cramer]

The "Pressure switch" wire looks like a normal A/C turn on wire. What's the deal with the separate fan switch wire?

[MegaMicra]

Nissan have it so my 4 position interior fan switch has to be in any position above off for the AC to work. I just drew it without all the other positions.
This is exactly how Nissan have it wired. All I did was just copy the wiring as Nissan diagram spans many pages.

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[MegaMicra]

So do you think it's OK to wire the pressure switch oem ecu input to one of my MS3X ground input connections such as tableswitch.

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[Matt Cramer]

Looks like the pressure switch is a 12 volt input, not ground.

The fan switch doesn't look like it would work as drawn - that ought to cause a dead short to ground and pop the A/C fuse when the fan is switched on.

[MegaMicra]

It doesn't cause a dead short as it takes the feed after it has passed through the diode and AC on lamp. It did take me a good few times going over it to realise this.
Although there is 12v when the button is pressed and the interior fan switch is not made, there is actually next to no current due to the fact that the diode and lamp are there.

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[Matt Cramer]

Looks like you'd have to use a loop condition or some external AND logic to activate the A/C when the fan button input is low and the A/C pressure switch input is high.

[MegaMicra]

Yes I did think this was the case that if both or either were high then don't output to the AC relay.
So not one to be beaten I've been out to the car with a calibrated micro/milliammeter and measured the current draw of that diode and lamp back to the ecu. It measures at 0.08mA which is effectively nothing. Therefore I'm hoping that's low enough to do no harm to anything if I do try it going to the MS3X.

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[Matt Cramer]

Interesting, looks like Nissan's logic with the pressure switch is backwards from most pressure switches I've seen. Usually when the pressure switch is wired to the ECU, it is set up so that it closes when the pressure is good to go and opens when the A/C should be off. Here, they're using NOR logic instead.

[MegaMicra]

Oh don't I know it. I don't know about the latest Nissans but these older ones have some strange wiring as I've found out through my install.
A lot of the oem wiring is permanently live so it caused me no end of grief with the MS3X running on when the key was off.
I'm hoping to get it all back in this weekend so I'll report back with any issues/findings.
Thanks for the help.

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[MegaMicra]

OK so a little update. I've connected the pressure switch to the tableswitch input and it all works perfectly.
The resistance to gnd of that input is really high so it works well. I now have aircon again. Not that we really need it in the North of England. I just use it for demisting the screen as it's always blooming raining over here lol.

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