Design a site like this with
Get started

Miata Alternator Control


Click here for an interactive BOM

I designed this board because because of 2 reasons.
The 1st reason it that I couldn’t get the software PWM alternator control in the Megasquirt firmware working right. I played with it for a while but it either didn’t charge at all or only charged over 18V. No matter what I tried, I just couldn’t get it right. When the fuse on my MS3 Pro finally blew, I decided to call it quits and design this pcb.

dead-time-150x150The 2nd reason is that the programmed injector dead time is directly related to battery voltage, meaning that if you have voltage swings – something the Megasquirt most definitely has – than your injector dead time changes accordingly. I do not want my mixture going out of wack whenever I open my windows or turn on the headlights.

For the little time it takes to build the circuit, I see no point in spending hours trying to get software pwm right. IIRC, even DIY didn’t get it entirly correct (overshooting in some conditions etc).

The board is proved to be rock solid to within 0.1 over the entire rpm band, no matter what you throw at it. Once installed and adjusted, you don’t have to worry about it ever again. Opening a windows, turning on the headlights or the A/C, not a problem. Your voltage stays rock solid all day long.

The current board is the V3 board. It will work with any make of ecu designed for a 99-05 miata.
It’s 20*100mm and slides into any regular Megasquirt MS2 / MS3 case.  If you have a different type of ecu, you’ll have to get creative to fix it inside the case with some stand-offs.

The board has 5 pins:

  • 12V
  • GND
  • Field: this is the output to the alternator field wire (pin 1O on a ’99-’00 NB and pin 3M on a ’01-’05 NBFL)
  • Disable (optional): When you ground this pin, the alternator stops charging.
    I use a general purpose output to disable the alternator while I’m cranking (<600rpm). If not desired, leave disconnected.
  • LowBatt (optional): This output goes to the charge light (pin 1Q on a ’99-’00 NB and pin 3U on a ’01-’05 NBFL) and triggers when voltage goes below 12.8V. If not desired, leave disconnected.


Assembly is very easy, simply solder all components as indicated on the pcb. If you do not need the low battery warning circuit, then ommit the parts in the white rectangle.


The board simply slides into the case. The exact location in the case depends on what type of Megasquirt you have, but it definitely fits in all cases (MS1, MS2, MS3, MS3+MS3X).


The trimmer is preset at around 13 to 14V.
Once the board is assembled, fine tune the voltage in a running car. Start the car whilst keeping a close eye on battery voltage. Adjust the trimmer until battery voltage is 14V. It might take several turns.
Turning clockwise raises the voltage.

Ordering (worldwide):


complete kit: $29 + $9 shipping

To order, Paypal the correct amount to or contact me if you have any questions.

The kits are in stock.

Create a website or blog at

Up ↑