THIS DEVICE ALTERS OR DEFEATS A MANUFACTURER'S SAFETY DEVICE. ALTERING OR
DEFEATING THE OVERBOOST PROTECTION FEATURE MAY RESULT IN SERIOUS DAMAGE TO
YOUR VEHICLE. IF YOU INSTALL THIS OR SIMILAR DEVICES, YOU DO SO AT YOUR OWN RISK
All parts and part numbers are Radio Shack. Similar suitable parts are
available from any electronics supplier.
10k ohm 1.25 w 15-turn Cermet Potentiometer Pic
The only absolutely necessary components are 1) a 10K ohm potentiometer and
2) 3 lengths of wire long enough to connect it into the boost pressure
wiring harness. The additional components used are to make it neat,
professional, and weather-tight.
These are the instructions to build, install, and adjust a simple,
fully-adjustable fuel cut defeat system. It allows you increase the cutoff
point to a higher level, without completely eliminating the protection.
"Fuel-cut" is a safety sequence built into the MR2 turbo as a failsafe to
protect the engine in the event that the boost control system should
somehow fail. A pressure sensor measures boost pressure and translates it
to a voltage signal and sends it to the ECU. By simply controlling the
voltage between the sensor and the ECU, we can adjust the fuel-cut setpoint
to any value we choose.
This unit is simply a10K ohm potentiometer (hereinafter referred to as a
"pot") and an optional15K ohm fixed resistor mounted on a Printed Wiring
The 15 K ohm fixed resistor simply increases the resistance between the
"hot" side of the circuit and ground, minimizing the amount of current
(amperage) that can flow through the circuit, and minimize any voltage drop.
The added resistor (actual cost-5 cents) is probably unneccesary, but is
cheap insurance and doesn't interfere with the operation pf the BlackBox FCD
It may sound difficult, but is really very easy. It will take about 15
minutes to put it together, and maybe 30 minutes to install and adjust.
The assembly process is really simple, but if your completely unfamiliar
with how components are installed on a circuit board and don't feel
comfortable doing it yourself, anybody that does small electrical or
electronic repairs can do it for you. If you don't know anyone, try taking
the instructions and materials to a local tech college. I'm sure the
industrial technology department would do it for you-probably free!
In addition to these materials, you will need a soldering iron (pencil-type
is recommended), and solder.
Start by fitting the pot onto the board, so the pot is on the plain side,
and the pins extend through to the side with the gold circles around the
holes You will notice that the spacing of the terminal pins matches the
hole spacing on the board. Mount the pot toward one end of the board. Now
mount the terminal blocks. Since they come in pairs, you will actually
have 2 pairs, or 4 terminals. Mount the terminal blocks about 1-1 1/2"
from the pot, and so that three of the terminal pins line up with the three
pins on the pot. I had to break them into 2 pairs to get the correct
The pot pin closest to the adjusting screw will be Terminal 1(T1), the
center pin will be Terminal 2 (T2) and the pin at the other end will be
Terminal 3 (T3).
Now you must solder the components in place and connect the 3 pins on the
pot to the 3 pins on the terminal blocks. You can do this with a short
length of small wire , or if you're reasonably skilled, you can just
create a solder trace. I used Radio Shack 24 ga bus wire (pn 278-1314b)
NOTE: If you use the optional (recommended) 15k
ohm fixed resistor, use the resistor to connect the T3 to the terminal
block. Do not create an additional path with the wire or solder trace if you
use the resistor.There is no polarity to a resistor, so it doesn't matter
When you are finished, you should have 3 electrically-sound
connections linking the pot to the terminals and no shorts between the 3
paths. A SHORT COULD RESULT INSERIOUS DAMAGE TO YOUR CAR!
Now attach about 15" of 18 ga stranded wire to the three terminals
(Remember, there is an extra terminal-don't attach a wire to the unused
terminal) You must identify the three wires: You can label them with wire
markers: T1, T2, and T3, or you can color code them. I used Red for T1
("hot" input from the sensor, White for T2 (reduced voltage to the ECU) and
Black for T3 (circuit ground).)
Drill about a 5/16 hole in the plastic part of the enclosure where you want
he wires to come out, and feed the wires through the hole. Do not mount the
board to the enclosure yet. It will need to be accessible for tuning.
When tuning is complete, the board should be mounted to the enclosure, and
and the cover installed. If your enclosure has a metal cover, it would be
a good idea to glue some type of insulating material to the inside of the
cover so it can't short the pins on the underside of the board.
Diagram Of The FCD *Pic* Built And Open *Pic* Built *Pic*
Assembly is complete, you're ready to install and tune!!
HOW IT WORKS
A pot is often referred to as a voltage splitter. While this is not
incorrect, it is, more correctly, a current splitter. As voltage varies
directly with current, changing the current will cause a proportional
change in voltage. Think of it as diverter valve. It will receive flow
from one direction, and direct that flow to two different outlets according
to how you adjust the valve.
When you apply electrical energy to T1 of the pot, current, and therefore
voltage, will divide between the T2 and T3 terminals according relative
resistance of the two paths (T1 to T2, and T1 to T3). If the input
voltage is 5vdc,and you adjust the pot to the middle of its range, you will
have 2.5 vdc at T2, and 2.5vdc at T3). The ratio between the input voltage
and the two output voltages will remain the same at any input voltage;
10vdc would divide to two 5vdc outputs and 4vdc would divide to two 2vdc
outputs. Adjusting the pot adjusts the ratio. We can therefore adjust the
ratio so that,say, a 4.8vdc input can be divided into a 4.4 vdc output and
a .4vdc output.
Now you should start to see how the pot can be used to control fuel cut
without eliminating it.
The pressure sensor is in fact just a special kind of pot called a
transducer. Instead of the sensor's resistance being adjusted by turning a
screw, it is operated by a strain gauge that changes the pot setting as
pressure from the manifold is applied. The ECU sends a fixed 5vdc signal
to the transducer, and the transducer returns a varying 0-5vdc signal back
to the ECU, according to pressure being applied to the transducer. By
installing our pot between the sensor and the ECU, we can change the 0-5
signal from the sensor to ,say, 0-4.5 going back to the ECU. Fuel cut is
initiated at about 4.4 vdc, so while we have raised the fuel cut setpoint,
it will still activate at a higher boost level.
With ignition off, disconnect the plug connecting the harness to the
pressure sensor. Unwrap enough harness tape to give you good access to the
three wires. Red with Blue is the +5vdc from the ECU (terminal "VC").
Brown is the circuit ground ( terminal "E2"). Blue is the reduced voltage
signal used to power the OEM boost gauge and initiate fuel cut. (terminal
Snip the Blue wire (PIM) allowing enough length to make a splice at each
end. Connect the Black Box red wire(T1) to the Blue (PIM) wire coming from
the sensor plug. Connect the BlackBox white wire(T2) to the Blue(PIM) wire
leading to the ECU. Strip about 1/2" of insulation of off the
Brown(E2)wire (do not snip!) Connect the BlackBox black wire(T3) to the
I prefer soldered connections and heatshrink, but as long as your
connections are secure and insulated, you'll be OK
With ignition still off, plug the harness back on to the sensor.
You will need a pressure gauge and some means of applying controlled
pressure to the pressure sensor. A bicycle pump, blood pressure monitor
bulb,or similar hand pump will work. You can use a portable air tank (no
more than 20-30 psi!) or even one of your tires and appropriate hoses and
valves. I would apply only the minimum pressure required, and by no means
would I exceed 20 psi. It may not hurt anything, but why take a chance.
You will need a voltmeter that will accurately read 0-6 vdc.
Turn the ignition switch to "on", but do not start the engine.
Open your BlackBox. The little blue block with the small screw is the pot.
Set your voltmeter to the appropriate scale and connect the "+" lead to
the terminal with the white wire (T2), and the "-" lead to the terminal
with black wire(T3). (Now you see why I use the terminal blocks!)
Set the pressure where you would like fuel-cut to occur. (I used 16 psi).
Look at the voltage reading on your meter. Adjust the screw on the pot
until your voltage across white and black is 4.4 volts. This seems to be
the value at which fuel cut occurs. You have now set the fuel-cut
setpoint to match the pressure you have selected.
There may be some variation from vehicle to vehicle regarding the exact
fuel cut setpoint. While this procedure should get you close to where you
want to be, additional fine tuning based on actual road testing may be
required. Adjusting the voltage between the white and black wire upwards
will lower the setpoint; adjusting it downwards will raise the setpoint.
Here are some resistances values for different FC setpoints
These readings are between T1 and T2, and are made BEFORE INSTALLING,
and with no power or other circuitry connected to the FCD.
14 psi=1140 ohms
15 psi=1730 ohms
16 psi=2340 ohms
17 psi=2950 ohms