SpeedyEFI - Adding Flyback Diode to Improve Idle PWM Duty Cycle
 

I recently installed a SpeedyEFI PNP into my NB1 (2000 precisely). One of the main issues I had was getting the car to idle properly. The car was idling OK, but with values around 89 duty cycle cold and 86 duty cycle warm. My research (and roommate who has a 10AE running on MS3) said that the duty cycle should be much lower and have a much greater range.

After searching for a while it seemed like a flyback diode might need to be added to the IAC valve wiring. It seems like some ECU's have this diode installed on the PCB and some don't. I assume all SpeedyEFI modules do not have this diode.

Today I installed a 1N4001 diode to my wiring harness. I was unable to find both male and female plugs for sale, otherwise I would have made an extension for the stock wiring with the diode installed there. What I ended up doing was stripping the insulation and doing a military splice. I intend for this to be temporary.

EDIT : Knowing what I know now, I would recommend opening the ECU and installing the diode onto the ECU or the ECU's adapter harness/board. Mainly to avoid having to hack up the stock wiring harness, but also to minimize changes in case the stock ECU is reinstalled.
https://cimg0.ibsrv.net/gimg/www.mia...49c2d56964.jpg

Research told me that the orange wire should get the banded end of the diode, however that did not work for me. I'm not sure if it's how the wiring is handled in the SpeedyEFI unit. Regardless, when I reversed the diode direction I was able to get the car started and idling with a duty cycle ranging from 25-40 (corresponding to ~1500-800 RPM). This is roughly the same RPM range that I was getting before with only 3-4 steps of duty cycle.

I'm hoping the additional resolution will help with cold starts and closed loop idle tuning.

I didn't find anything specifically written up about this ECU and issue, so I figured I'd try to help out others who might be having the same issue.

TLDR - If you have a tiny range of duty cycle values that work for your IAC valve, try adding a diode between the wires.

SpeedyEFI - Adding Flyback Diode to Improve Idle PWM Duty Cycle
 

Connector should be https://www.aliexpress.com/item/1005002879764593.html

https://www.miataturbo.net/electroni...r-guide-99970/

Thanks for that link. I suppose I should have specified. I was able to find the female connector from multiple sources. However, I was unable to find any sources for the male connector, which would prevent me from creating an extension.

I could have depinned the factory connector and added on an aftermarket pigtail extension, but I figured if I was going to modify the factory harness it didn't matter which way I did it.

Right - I think the male chinese sealed AMP connectors might be compatible then.

https://www.corsa-technic.com/catego...ategory_id=103

I think you're right actually. I suppose I'll leave it the way I have it for now, but I'll keep that in mind if I need to do the job again. Also a good bit of info for anyone else who needs to do this.

I'm not proud of the current install (military splicing the diode in) but I also don't think it would fail anytime soon. The wires shouldn't move around too much and aren't too close to exhaust heat.
I'm planning on adding a bit of solder to the connections today and wrapping it up with some electrical tape for insulation.

Pos cats awarded, this is a great find.

FYI (to all): for those experiencing unusually high VVT dutycycle with MS3 and increased spring seat pressures while using a high current output, a diode across the VVT actuator should bring the DCs back down to a somewhat normal range. The 36V clamp on the HC outputs turn the valve off much more quickly than outputs with a flyback to the 12V rail. The diode doesn't really change the effective on-time of the valve, but it does increase the off time which forces a reduced DC. This may advantageous from a control system perspective, I am testing now.

I'll add something for clarity. SpeedyEFI is not speeduino. SpeedyEFI is the result of someone creating their own hardware and using the open source speeduino code to control it. The schematic for the official speeduino PNP ECU for the NA chassis has a flyback diode on the boost control output.

Correct. I think the two (SpeedyEFI and Speeduino) get used interchangeably by a lot of people, however that's incorrect. I do think the Speeduino section would be the place for this thread considering it is a Speeduino based board.

This post is related to Idle control though, not boost control. I do not know if the official Speeduino layout has a flyback for Idle Control.

I misspoke. However, both the boost and idle outputs have flyback diodes.

If you have the pnp kit from speedyefi for a 99-00 miata it should be based on what speedyefi calls their "fishdog" ecu according to their website. Their "fishdog" ecu's incorporate the ua4c board made by wmtronics. You can open up your ecu to confirm if it uses the ua4c board and confirm which generation it is (va3 through vc3). Once you know the generation you can go on github for the board schematics. I have two of their fishdog ecu's and they both have the ua4c board made by wmtronics.
This is the schematic for the vc3 board
https://github.com/turboedge/SpeedyB...M/Underdog.pdf

you are a fucking GOD!

Glad you found it useful. I'm honestly surprised that the SpeedyEFI guys haven't added this diode or posted their own write up. I guess some people don't need it but it seems like something they'd want to let people know about.

i'm jumping off the speeduino wagon and onto the rusEFI.....new issues, new adventures.