Adjustable Fuel Cut Mod by Bill Wotshack

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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 3SGTE ECU 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 set point 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 Board.

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 unnecessary, 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.

PARTS LIST:

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 271-343 $1.49

PC Board Terminals ( 2 required) 276-1388 $2.39 (for 4)

Project Enclosure with PC Board 270-283A $3.99

15k ohm ｼ w Fixed Resistor (optional ) 271- 1337 $ .49 (for 10)

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.

SCHEMATIC

T2 l--------->PIM to ECU l T1 l T3 PIM from ECU <----/\/\/\/\/\/\------o------/\/\/\/\/\/\----> E2 to ECU 10k ohm pot 15k ohm fixed resistor

ASSEMBLY

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 alignment.

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 which way it goes.

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 IN SERIOUS 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 ECU) and Black for T3 (circuit ground).)

Drill about a 5/16 hole in the plastic part of the enclosure where you want the 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.

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 set point, it will still activate at a higher boost level.

INSTALLATION:

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 "PIM").

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 Brown(E2) wire.

I prefer soldered connections and heat shrink, 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. Installation complete.

SETUP

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(T#3). (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 set point to match the pressure you have selected.

There may be some variation from vehicle to vehicle regarding the exact fuel cut set point. 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 set point; adjusting it downwards will raise the set point.

DISCLAIMER

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