Hi all,

Running a MS3pnp, and I've been fighting with it for a while. Well, turns out, my IACV isn't doing anything. I set to the duty to 0, tighten down the throttle screw, and it idles fine. Same with if I set it to say, 100. No change in RPM. I also checked my I/O in TunerStudio and set the IACV on each free I/O pin. Nothing changed.
I did some diagnosis and found the following. Looking at the OEM wiring diagram, the IACV has 2 connections, a 12v supply from the +B on the diagnostics port (white/red), and a blue/orange wire to pin W of the stock ECU which I assume is the signal.



The pinout at https://www.miata.net/garage/ECU%20P...90-93%20MT.pdf says this blue/orange wire is for idle speed control, and should read around 7 volts at engine off, and 9 volts with the engine on.
The pinout at MegaSquirtPNP by DIYAutoTune.com for the MS3 shows this pin as 4W, IAC PWM1.

I've probed the connectors and these are the results.

12v on white/red at IACV connector and ecu with connector disconnected
12v on blue/orange at ecu and connector when plugged in
2-3v at blue/orange when not plugged in

I also measured resistance across the two pins of the IACV and got 13ohms, which is within spec.
So from what I can tell here, for some reason this blue/orange wire is getting 12 volts, and this is why the IACV isn't playing nice. I assumed its getting it from the white/red wire, which leads me to believe it's a faulty IACV shorted closed.
But when I pulled it and bench tested it with 9 volts, it opens and closes fine.

Anyone have a clue what could be the case here?

 

Normally this would be one I don't respond to because it has been discussed multiple times over the years. The content of your post indicates you did your own real research instead of being a complete eggplant. Be aware that we have been around for decades, and most miata related questions have been answered. Use Google to search <topic> miataturbo.net instead of the IB engine for most things.

i suspect that you will find if you set your valve to 25% and sweep up, that your valve works as expected. The NA6 valve defaults to an open state at 0% duty-cycle, presumably as a fail-functional design.

MS3 to valve interface including MS3 HW and tune settings can be confirmed by using a DMM to measure the DC voltage across the valve while sweeping dutycycle in test mode. Please note that the valve or some other representative load needs to be in-circuit for this test, the MS3 input is open drain.

 

I solved it. The I/O in tuner studio was incorrect. I checked this at first and set the PWM to each available pin, but it didn't change anything. Turns out after a power cycle it did change things, and now my iacv works. Score!

I also checked the pin with a multimeter and didn't see a change, which is how I got the idea to re-check the pinout.