Megasquirt 2 Assembly

Building MS2: ADD low-impedance injectors circuit thank you . and what is the black wire running to IAC2A AKA pin 29 form JS7 and its mark fan but fan is on IAC1B pin 27 according to the mod off of LED15 …

I've taken the liberty to remove from the component map what you don't need to install to make this very simple and straight forward.

But please refer to for the step by step instructions and tips on building the megasquirt.

The below components are all you'll need to run the majority of MS installs. I have removed the injector PWM circuit (used to drive low-impedance injectors) as the majority of Miata MegaSquirt users will not be using low impedance injectors. The VR tach input circuit is also not needed for Miata users, so these components have been omitted too. However, a diagram will shortly be posted showing the installation of a second optoisolater for the Miata's timing signals which is needed in most MegaSquirt-II builds.

The differences between a stand-alone and parallel install MegaSquirt

There are several key differences between running a parallel MegaSquirt install and a completely stand-alone MegaSquirt install. In a parallel install, the stock ECU is retained to handle engine idle, radiator fan control, air conditioning and the fuel pump. The ECU also regulates the alternator output in NB cars.

For a parallel install, sensor signals must be shared between both ECUs, requiring a harness to be built to appropriately split the signals as required.

For a stand-alone install, we must modify the MegaSquirt to include circuits to drive the radiator fan, the fuel pump (for 1.6L cars), the air conditioning and the alternator.

NON-Standalone Setup (Excluding Fan Relay Circuit)


Standalone (Including Fan Relay Circuit)

The cathode of LED15 is connected to IAC1B on the V3.0 PCB, which connects to pin 27 on the DB37 connector. This provides a switched ground connection to the fan relay, allowing the MegaSquirt to turn the fan on and off. In Tunerstudio, you need to enable output PM5-Warmup LED. See Mods for more info!


Input/Output Mods

The bottom picture is the better spark output created by Joe P on Just follow the mods in both picture, and your done. You must use two 2N2222 transistors for the spark output. The resistors needed in the spark output are 100-300 ohms. Use the spare 270 ohm resistors from R30-36. Also, JS0 to JS4 will not work at input/outputs unless you jump s12c to JS9. So jump s12c to JS9.



There is a simpler method to achieve an inverted spark output circuit, designed by Reverant it requires only resistors which are left over from the MegaSquirt build. Details will be added shortly

Now Lets Make The Harness!

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