DIY ECU kit information

Last update: 12/25/09 Return to home page links

Purpose of article:

This article gives a little more information about the engine control unit (ECU) that is the heart of controlling fuel delivery to a fuel injected engine. I am NOT going to give you exact information as to what you should do or buy. Each engine has different requirements which have to be taken into account.

This is not a step by step assembly tutorial but rather an brief article to illustrate what is involved if you want to build your own ECU. As most things involving a project you need to decide what it is you really want to do. This seems obvious, but many people barge ahead and figure they will decide what to do when the time comes. Well this is the time to decide, i.e. before you spend several hundreds of dollars buying kits or a ready to go unit.

Expect to pay 4 or more times than the cost of a kit if you buy one ready to go from most companies.

Considerations you have to take into account before you order an ECU kit:

If you chose to go the do it yourself route you will have to do quite a bit of reading and learning before you even buy a DIY kit if you haven't built one before. If you really don't like to or don't have the skill to solder small parts on a circuit board then you might want to enlist the help of a friend etc to build the kit for you. It you are building your first ECU expect to take several days of building and testing of the unit to check your workmanship.

I choose to use the Megasquirt  ECU. The Megasquirt DIY kit has been on the market for many years now and has an established reputation. The kits are used on many automotive and motorcycle world speed and racing vehicles.  So they do work!

The stock number of the unit I bought is MS230-K. I also bought a "stimulator" kit (MSStim22-K) at the same time. For those of you who would rather not assemble your own unit they also have assembled units, bare PCBs, and various other parts to build/modify the ECUs etc.  These model numbers may have been discontinued by now but I'm sure they have one to meet your needs.  Again, I don't work for this company either. (I'm retired.)

If you are interested in the Megasquirt kit, I would strongly suggest that you go to their website and try to absorb as much information as you can. As they present on the site, this whole process is also meant to be a learning experience.

The Megasquirt website has the manuals for all the ECU kits they offer and are required reading to learn what, why and how the units work. Go to the forums and read the questions/answers that other people have posted.

There is so much to learn that it will take some time to get enough information to even figure out what questions to ask. I spent several -months- reading and downloading articles etc so I could understand what & how the ECU works and what options are available.

The old saying, "The more you know, the more you know that you don't know enough." applies here.  The owners of  diyautotune and the software authors are extremely knowledgeable and are very willing to share their knowledge.

The build instructions are on line for downloading. If you are familiar with electronics you will find that the instructions are complete although they might be cryptic for a beginner. If you understand how to read electrical schematics you will find them on the site also. If you have built an electronic kit(s) before you shouldn't have much trouble putting the ECU together.

I would highly recommend that you also buy one of the "stimulator" kits when you place your ECU order. A stimulator by any other name would be a simulator. This piece of test instrument simulates the signals that you would get from the sensors on your engine. If you buy a stimulator, build it first as you need it to test the ECU as you assembled it.

You build one circuit at a time on the ECU board and then test it. If you find a problem as you go you know exactly which circuit has the problem vs building the whole ECU and then trying to figure out which circuit is not working correctly.

I could have easily built a stimulator from the schematics and using parts I already have but I decided that it would be time better spent building the ECU itself. And if you aren't that good at soldering, building the simple simulator will be good practice.

The stimulator I bought only supplies the crank sensor signal to the ECU. My engine has an 8 tooth crank "wheel" and a camshaft sensor signal that is required by the ECU. I designed a simple divide by 16 circuit to simulate the camshaft signal. (16 teeth of the crank wheel pass by the crank sensor to one pulse from the cam sensor.) My divider is powered by the simulator. There is a newer stimulator that has circuitry to simulate various common car engines timing "wheels". I'm not sure if it also provides a cam signal or not.

Near the end of building the ECU you have to load the firmware into the micro-processor on the ECU board. You use a COM port on your computer to do this. There are several current versions of the free firmware available. The basic version is very complete and can control both the fuel and ignition of your engine. There is another version of the firmware called "Extra" that has even more options that you can use.

There are also two different (free) tuning programs on the site that you use to set the ECU up for your engine. Only one program is required and they are both quite similar to each other. But at least you have a choice to see which one you like best.

Just getting the engine to start is going to be your first big test. And by using the simulator and using one of the tuning programs to set up the ECU with your engine parameters you will have a pretty good chance that the engine will start and run. The idea is to eliminate as many variables as possible before you install the ECU into the car/motorcycle etc. Considering that you have to modify the injector and/or ignition wiring of the vehicle, it's best to have all the settings that the Megasquirt is capable of already set up and or the proper jumpers installed..

OK, let's take a look at the kits themselves.

To see an enlarged view of most pictures, left click on a picture or right click and select "View Image".
Stimulator kit

These are all the parts that you need to build the stimulator. This kit is not difficult at all. Everything is included except the solder. The diyautotune website has a short tutorial on soldering if you need some guidance.

stimulator done

Here is the stimulator that I built. The various potentiometers are used to vary the signals to the ECU that simulate your car sensors. The large connector, a DB-37 [more properly named a DC-37] on the left side plugs into the mating connector on the ECU for testing. The power supply connections are located at the upper right corner.

ECU kit

Everything you need to build the ECU is included in the kit except the solder. I used sockets for all the ICs. This comes in handy for testing or troubleshooting.

A very nice looking case and pre-drilled end plates are also included in the kit (not shown here).

This kit can power the ignition and the fuel injectors of engines from 1 to 16 cylinders. This is done by jumpering various points on the printed circuit board (PCB). The kit was designed to be very adaptable to many different engines.

stimulator done

And here is the completed ECU with the stimulator connected. I built the small adapter with a slide switch and a two pin female connector. It is used while loading the firmware into the micro-processor. The red switch on the upper right is another addition I added for testing to turn the power ON/OFF.

There are several different connectors available to power the units. A two hole terminal block, a wall wart connector and a 9V battery connector can be used for connecting to a power supply. You can input any voltage from ~8 Vdc to <15 Vdc. The both units draw 120 to 180 ma total current with no loads connected to the output pins.

I wouldn't even try to use a 9V battery with the ECU connected since the ECU draws way too much current for the battery to last very long.


1/8/10:  You can find my modification instructions here.   Be aware that I have NOT installed the Megasquirt on my car yet but all my bench tests show that the mod itself works correctly.

12/18/09:  I've made lots of progress in understanding what the various circuits do and setting the ECU up for my engine. I bought 4 high impedance injectors from the junk yard to use in my bench tests. I'm quite surprised how much noise they make when being pulsed (especially at the equivalent of 18,000 RPM!).

The various manuals for the Extra3 firmware (sequential injection) advises you (in affect) to not use the original FETs to drive two of the four injectors my engine has. Instead they recommend using four separate FETs to drive the injectors partially because of the way a modification is made to the ECU to allow sequential injection instead of bank injection. 

After studying the schematics (and redrawing them so all the parts are shown on one sheet I have come up with a way to use the original injector drive circuitry including the two original drive FETs. By removing some of the circuitry that isn't required when using sequential injection I have room on the internal heat sink to mount two additional FETs to drive the remaining injectors.

11/25/09:  Since completing the unit the unit I have been experimenting with the simulator to get familiar with what the various tuner menu selections do. I am working with firmware that allows the Megasquirt to energize the injectors in "sequential mode" as opposed to "bank mode".  Both methods have been used by manufacturers for years so it's not a case of one being completely "better" than the other. My GSXR engine uses sequential firing so I decided that I would too.

Sequential mode means that each injector or ignition event is directed to the cylinder in order of the firing sequence (1, 2, 4, 3 in the case of my engine).  Sequential injection should give smoother low speed performance, slightly better fuel mileage and better emission control.

Bank mode has two drive circuits that fire 1/2 of the fuel injectors that your engine uses at a time in "bank" mode (i.e. 1 & 4, 2 & 3 or what ever combination you'd like, but always one half of the total injectors at a time). If your engine only has two injectors then bank mode is perfect for you although it works fine with 4 injectors.

NOTE:  You have to use the tuning  program "TunerStudio" for sequential mode since it is capable of utilizing the sequential injection features.

The firmware ( is in alpha testing for quite some time now with many people using it on a day to day basis. "Extra" firmware has all the features of the standard firmware and adds several features of it's own (such sequential, launch control etc). 

The program author has been very helpful on the msextra forum with any questions etc..

Stay tuned for more sordid details as they develop.

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