The ECU is basically made up of three parts:

Main ECU Does all the clever stuff
Relay board This gives a central point for connecting everything to, the sensors, the injectors, fuel pump, the incoming power, etc. It houses three relays – one to power the ECU, one for the fuel pump and one for an output signal. It also houses fuses for the two injector pairs, the main power and the fuel pump. You don’t need to have the relay board, but it makes wiring things up easier.
Stimulator This isn’t used on the car at all. It is a simple little board that provides a complete set of inputs to the ECU for testing by means of adjustable potentiometers – so you can change the rpm, temperatures, etc. Again, you can do without it, but without it, you don’t get to test your ECU until it is all wired into the car.

The first item I tackled was the Relay board. This was mainly as I was a little rusty with a soldering iron on a PCB – it had been 10 years since doing any at college. The layout was nice and simple with large spacing between components, so it was good practice.
I ordered a case to house the board from Maplin and had to modify it to allow access to the main connector block. This involved cutting most of one side away but leaving the ends so the end plates still had something to fit over and screw into. I was missing the relays so I had to track down the mini automotive fuse type and order a few. That was it – it was simple, which is why I opted to do it first!

The next thing I built was the Stimulator (known as the Stim). This was built second so that it could be used during the main ECU. It is used to provide power initially and then as you build up more it can be used to test the input and then output sections. It is slightly more complicated than the Relay board, but still easy to put together.
Once the main ECU is built the Stim can still be useful as a test bed code changes or to help with testing some add on features you might add at a later date (like shift lights….).

Last up was the main ECU. The ECU is a lot more tightly populated than the relay board but the instructions are excellent. Each individual circuit is detailed in turn and at the end of each section the method of testing ensures you don’t proceed before fixing any problems with the bit you just built.
The first section you build is the power circuit along with the various IC sockets. The testing is simply to attach the Stim and power it up and check the voltages at various points.

The second section is the communications section. This can then be tested by providing power and connecting it to the PC. It is only a simply test that a signal is passed back via the circuit.

Next up is the clock circuit. This is used for the processor as well as the battery voltage reading circuit. Again, it is a nice simple section consisting of some capacitors, resistors, a crystal and the main CPU (just for testing). The test is to fire up the tuning software (MegaTune) and power the board. A clock reading is shown on the Real-time display page and if everything is working, this can be seen to count up to 255 and then reset to zero. The battery voltage will also be working and will show whatever The Stim is getting power wise.

The penultimate section is the input circuits. These deal with all the incoming inputs from the engine. In my set-up this consisted of temperature sensors for the water/coolant and the air intake/manifold (close to the throttle bodies), manifold air pressure, throttle position, exhaust oxygen (O2) and RPM input from the ignition circuit. There are more parts to this section, but mostly simple resistors, etc. One thing to mention was that I was adding a standard Ford coolant temperature sensor as I had only the gauge sensor fitted. Some research showed that this would have a very high resistance at low temperatures and would therefore go out of the range. The workaround for this is to change the value of the resistor on the PCB (referred to as a bias resistor) to bring the values more in line with those expected. A great feature of the ECU is its ability to use any hardware you have to hand. The default temperature sensors are GM items, but I wasn’t using these for either of mine. To facilitate this you download a small program called EasyTherm into which you enter three temperatures and resistance's (plus a new bias resistor value if necessary) and it will create three new lookup tables for the tuning software to use and importantly a new version of code to download into the ECU. Even though I am using one of the ‘project’ versions of code, this is still easy to do, you just point EasyTherm to your code and it modifies it for you. A bit of research was needed to get the information on my sensors (the Ford coolant and a Rover air temp courtesy of Jason). Once I had found out this I just picked three of the readings and created my new version of code and lookup tables.

Temperature (°C)
Air Temp Sensor Resistance
Water Temp Sensor Resistance
-10
12,460 ohms
183,000 ohms
20
2,814 ohms
38,000 ohms
60
560 ohms
2,200 ohms

And here is a sample of my Air Density Factor lookup file.
Testing the input circuits is just a case of powering up the ECU board with the Stim and connecting the PC. The various potentiometers now affect the readings given on the tuning software and you can twiddle and affect the rpm, temps, O2, throttle, etc. It’s starting to act like an ECU at last! The values need checking to make sure they are sensible, especially the MAP – which is simply showing normal atmospheric pressure (about 100kpa – or 1 bar / 14.7psi).

Last comes the output circuits. This is probably the most involved section as it involves extra things like the LED’s which can show the different operations of the ECU. One flashes which the injector pulsing, another indicates if ‘warm-up’ is active and the last is for the acceleration enrichment function. Fitting these meant drilling the front panel of the case. At the same time the opening for the main connector is cut – so time spent out in the garage again!
Testing is again to connect the Stim and to check things like the injector LED’s flashing in accordance with the rpm, the fuel pump light, the warm-up indication should operate according to the coolant pot position, acceleration should respond to the TPS pot, and so on.

That was it, my ECU was built and ready.

 

 

 
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