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Old 07-03-2014, 04:30 PM   #1
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Default AC Questions - No ECU pass through

So I have a Rev built MS2 basic, which I don't believe has the ability to control the AC. What I'm looking to do is bypass the ECU and have the AC button click on the AC relay. I've been looking over the wiring diagrams here:

http://www.miataforumz.com/1990-200-...ring/96sys.pdf

I'm thinking I can just hop the Light Green/Black wire at 1K to the Blue/Black wire at 1G. That should click on the AC relay (assuming the temp and pressure switch are satisfied).

Is that correct? Am I missing anything?
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Old 07-03-2014, 04:38 PM   #2
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Originally Posted by Onyxyth View Post
So I have a Rev built MS2 basic, which I don't believe has the ability to control the AC. What I'm looking to do is bypass the ECU and have the AC button click on the AC relay. I've been looking over the wiring diagrams here:

http://www.miataforumz.com/1990-200-...ring/96sys.pdf

I'm thinking I can just hop the Light Green/Black wire at 1K to the Blue/Black wire at 1G. That should click on the AC relay (assuming the temp and pressure switch are satisfied).

Is that correct? Am I missing anything?
The main issue is your car needs to increase its idle duty when A/C is on, and as far as I am aware this is not a native function in anything pre MS3.
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Old 07-03-2014, 05:08 PM   #3
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Closed loop idle should be able to compensate for that, though. Even it it's a bit slow to react, or idles low. It should be decent.
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Old 07-03-2014, 05:12 PM   #4
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Closed loop idle should be able to compensate for that, though. Even it it's a bit slow to react, or idles low. It should be decent.
What will be responsible for cycling the compressor?
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Old 07-03-2014, 07:15 PM   #5
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Maybe I'm confused - I thought all the ECU did was ground the clutch relay (and both fan relays..) and allow the compressor to turn. The cycling should be handled by the low pressure switch, correct?
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Old 07-03-2014, 07:27 PM   #6
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It looks as though I may be mistaken on this one.

I would suggest reading Joe's post on the matter.

https://www.miataturbo.net/megasquir...t-c-faq-20954/
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Old 07-03-2014, 07:32 PM   #7
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^ Thanks I was looking over that a bit today already, but I think I missed his explanation of the stock config. It does agree with my interpretation, though, so that's good. The pins are just different (92 vs 96). I think I'm on the right track, but I'm gonna keep reading so I don't blow something up.
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Old 07-05-2014, 09:47 PM   #8
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So to follow up on this, I tied together:

1K a/c switch
1G compressor clutch relay
1B condenser fan relay

This causes the a/c clutch and fan to engage when the a/c button is pressed. I think in stock configuration the fan runs even after the compressor has turned off, but I don't see any easy way of wiring that, so this will have to do.

Everything works for the moment. I had to add some 134a to satisfy the low pressure switch. I'll find out this week how well everything holds up, since its not terrible hot out the last few days.
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