As the story goes

Unfortunately it is not possible to just cut the wire. This causes the ECU to go into a fault mode that not only turns on the Fuel Injection Warning light on the instrument cluster, which is very annoying, it also according to Kawasaki will cause the ECU to restrict the bike to an even slower speed than with it connected. This necessitates that the ECU be feed some kind of signal to keep from entering this fault mode.

The normal speedo signal is a 5 V square wave coming from a sensor that mounts to the case and sits over a gear on the output shaft and pulses every time a gear tooth goes by or 37 times per output shaft rotation. Combined with the final drive ratio and rear tire diameter it works out to approximately 1083 Hz (pulses per second) at 60 mph.  That works out to 3357Hz at 186 mph.  

It seemed to me that if the speedo signal was replaced with a 5V signal that never exceeded say 3000Hz that the ECU could be prevented from going into restriction mode. Obviously a 555 timer type circuit could do the job but I figured out a simpler method that does not require circuit board construction or tapping into the 12v power.

I finally came upon the idea of feeding the Tach signal, which is generated by the ECU, back into the ECU’s speed input. The tach signal is generated by the ECU to drive the Tachometer in the instrument cluster. It pulses twice per crank rotation. This translates to 6000 pulses per MINUTE at 3000 RPM or 100 Hz. This signal would be 383 Hz at engine redline of 11,500 rpm which is well below the 3387 Hz signal that would cause the ECU to go into speed restriction mode. 

While the tach signal is perfect timing wise it is a 12V signal unlike the 5V speedo signal. This necessitates a simple level converter. The following schematic shows a simple 3 part circuit that can be constructed for a couple dollars.  The speedo input to the ECU is pulled high to 5 V by an internal resistor. This means the converter circuit need only ground the input and can be a simple open collector transistor circuit.

Parts List 

Construction

In addition to the parts listed above you will need about a foot of wire. I used some 22AWG wire I had laying around. It will help to prevent wiring confusion later if you can get 3 different colors of wire.

The transistor has 3 leads E, B, and C. Check the back of the transistor packaging for information on how to identify the leads. They may not be the same as shown in this picture.

Step 1. Solder a wire to the C lead. Slide a piece of heat shrink over the lead and shrink it using a heat gun (hair dryer) or other heat source. If you have not used heat shrink before you may want to practice with it first. This is the SPEEDO wire (White in the photo)

Step 2. Solder a wire to one end of the 100k (Brown, Black, Yellow) Resistor. This is the Tach wire. (Light Blue in the photo)

Step 3. Solder one end of the 22k (Red, Red, Orange) Resistor to the E lead of the transistor. Then solder a wire to the solder connection you just made between the 22k resistor and the E lead. This is the GROUND wire (Black with white stripe in the photo) Heat shrink this connection making sure the unused end of the 22k resistor is exposed.

Step 4. Solder the unused end of the 100k, the unused end of the 22k, and the B lead of the transistor together. At this point your circuit should look something like the photo below.

Step 5. Slide a large piece of heat shrink over the entire assembly. If you are using a single color of wire make sure to mark your wires before this final construction step.

Electrical Test

You will need 3 clip leads, an extra 22k resistor, a voltmeter, and a 12 volt DC power source. In a pinch you could use a 9v battery.

For lack of a better term we will refer to the circuit you just created as the device.

Use a clip lead to attach the GROUND wire of the device to the negative ( - ) side of your power source or battery

Clip one end of the 22k resistor to the SPEEDO wire of the device. Clip the other end of the 22k resistor to the positive ( + ) side of your power source.

Attach the black or negative probe of your voltmeter to the GROUND wire.

Use the red probe to measure the end of the SPEEDO wire clipped to the 22k resistor. It should read close to +12 volts.

While holding the probe in place, touch the TACH wire of the device to the positive ( + ) side of your power source or battery.

The voltmeter should read less than 2 volts when ever the TACH lead is attached and return to +12 volts when the TACH lead is removed from the positive side of the power source.

If it does not work as described above go back to the construction section and double check your work.

Installation

Make sure your bike is turned off.

Locate the ECU in your tail section and unplug the wire harness. Carefully cut or unwrap the black tape around the harness starting at the ECU plugs for a length of about 9 to 12 inches.

Use the diagram above to locate the Yellow, Light Blue, and Black w/yellow stripe wires and separate them from the other wires.

CUT the yellow wire at least 4 inches back from the plug. Tape the end that goes forward so it doesn’t short out to anything.

USE one of the wire taps to connect the SPEEDO wire of the device to the yellow wire that leads to the ECU plug.

NOTE: Space the tap connectors so they won’t all be bunched together when you re-tape the wire harness.

Use a wire tap to connect the GROUND wire of the device to the Black w/Yellow stripe wire. DO NOT CUT.

Use a wire tap to connect the TACH wire of the device to the Light Blue wire. DO NOT CUT.

Tape up the wire harness being sure to tape over the tap connectors to keep them dry. Reconnect ECU Plugs.

Performance

I have installed this device on my own ZX-12R and made several high speed passes with in an hour of each other with the device connected and disconnected. An onboard GPS confirmed that the bike was up to 7 mph faster with the device installed.

IMPORTANT NOTE ON FINAL TEST 

The ZX-12R has very good brakes. It will go from 60 to 0 in about 2.3 seconds or about 100 feet. But let me share some simple math with you. At that rate if you double the speed to 120 mph it will take you twice as long to stop, about 4.6 seconds. BUT IN THE TIME YOU WILL TRAVEL 4 TIMES THE DISTANCE or 428 feet. At 3 times 60mph or 180 mph it will take you 3 times as long or about 7 seconds to stop. BUT YOU WILL TRAVEL MORE THAN 900 FEET.

In other words at 180 mph if someone pulls into the road the length of 3 football fields down the road you are still going to be doing 60 mph when you hit them. You’ll be going even faster if it takes you more than 0.37 seconds to recognize the threat and pull in the brake lever.