The ECU is basically made up of three parts:
||Does all the clever stuff
||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
dont need to have the relay board, but it makes
wiring things up easier.
||This isnt 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
dont 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
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
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
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 dont 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
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 wasnt 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.
Air Temp Sensor Resistance
Water Temp Sensor Resistance
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. Its 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
Last comes the output circuits. This is probably the most
involved section as it involves extra things like the LEDs
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
Testing is again to connect the Stim and to check things
like the injector LEDs 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.