V8 Miata Wiring, Electrical, and ECUs v8 conversions on a Miata can be tricky. Post your comments or questions within with regards to swap wiring and electrical.

LS Air Conditioning Question

Old 07-18-2016, 07:16 AM
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Default LS Air Conditioning Question

Hey guys - I'm hoping someone can help me make some sense of this, as I think this is a really simple question.

I've read through this thread (that has a lot of great information) but I have one quick question regarding this quote from Jim (hopefully he sees this):

Originally Posted by charchri4
On the wiring you have to put a relay in it because Mazda uses ground to close the AC relay and GM uses hot. So the button on the dash closes the relay using the ground side to switch it and sends B+ to the computer telling it you want AC. The other way you can do it is to tap into the signal from the little blue AC light in the dash and just jumper that off to the ECM. You have to take the controls apart a little but that's the way Shannon did his and it's still working. I used the relay method and but with no hoses have no idea if it works.
I'm using a BP Automotive Harness to run the A/C, which requires a 12V+.


Based on what Jim wrote, the little blue A/C light wire gets a 12V+ when the A/C is on, and it isn't a switched ground. Is that correct? I hope I'm asking that question correctly, as wiring is the most difficult part of this build for me.

If that's true, I can splice directly into this wire and it will send a 12V+ source to my BP Automotive Harness:


Does that make any sense?

Thanks guys!
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Old 07-18-2016, 09:42 AM
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I believe the light is activated by it being grounded. The +12V is there with key on and blower motor relay picked.
Old 07-19-2016, 04:59 PM
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So what wire do I splice into for the relay? Since the BP Automotive harness needs 12v+, I'm thinking the Blue-Orange wire that sends the signal to the Miata ECU?
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Old 07-20-2016, 08:56 AM
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You may be able to use a relay that is picked by the ground side on that blue/red wire and send the relay switched +12V out from there.

Basically you are using a relay to convert from ground switching to +12V switching.
Old 09-20-2016, 11:18 AM
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If you still have the Miata wiring in, and the original AC control in...just use the stock relay to turn on the compressor. It is one of the 2 relays on the right side of the car in the engine bay. The Stock button will trip the relay and allow the 12V signal to exit the relay and turn on the compressor.

The only unfortunate part is the LS2 computer does not have an AC input (that was done by a separate body module). This means you can not tell the car to idle up. I never solved that issue, and just dealt with low idle when I stopped at light, or turned off AC before stopping to prevent a stall.
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