I figured out how to get the A/C working on the Miata. I had to do some soldering in two places and added a relay to control the A/C relay.
Now, the A/C works perfectly. Got cold air now. I’m not even using the ECU to control the A/C. The reason is that my engine has so much torque at idle that it doesn’t really drop in RPMs that much when the A/C compressor is running. I just had to increase the idle to about 850-900 RPMs then when the A/C compressor kicks in, then it drops to a nice galloping 750-800 RPMs. But, when I disable the A/C by the button on the dash, the engine revs up a bit more to 850-900 RPMs. Not too bad.
When winter comes around, I’ll just leave the A/C off and adjust the throttle accordingly. Even with the large cam that my engine has, even if it dropped down to 650 RPMs, it’d still gallop nicely with the A/C enabled. The reason why I increased it is for throttle response time. If the RPMs are too low, the engine responds slowly at acceleration.
I’ll be putting up pics of how the wiring goes in on the A/C switch behind the dash. The rest is straightforward. If the A/C button is enabled, the A/C clutch and fan both disable when in START mode (cranking) and activates once again when the engine’s started. You might want to add a relay to disable the whole circuit when the engine is off so the clutch and fan are not energized when in the RUN mode while the engine is off. That’s the next thing I’m going to do. But for the meantime, I’ll just keep the A/C button off when the engine is off. The whole circuit de-energizes when in ACC or OFF mode along with the entire blower fan/cooling/heating circuit as it should.