How to build a Digital Gear Position Indicator for a ZX-12R  (ZX1200-A1, A2)

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(NOTE: you can click on the drawings if you want to print them out separately.)

The Kawasaki ZX-12r's Engine Control Unit (ECU) uses a number of sensors including a gear position indicator to calculate the correct fuel/ignition settings for the engine.  The gear position sensor is essentially a rotary switch which connects one of seven wires, gears 1-6 or Neutral, to ground depending on what gear the transmission is currently in. 

Converting the electrical gear signals to a number readable by the rider can be done using a 7 segment LED display. This device can display any number by turning on a specific combination of  segments in the display. All that is needed is a circuit to turn on the segments to display the number that corresponds with the gear selected or 0 in the case of neutral. 

Below is a schematic of a simple circuit of 7 resistors and 16 diodes that will convert the gear signals to a displayed number.

Construction:

Here is a printable list of the parts needed to build and install the circuit. Most of the parts can be found at you local radio shack store. The 7 segment display I purchased there however is not really bright enough for use in direct sunlight and is only visible about 80% of the time I test rode the bike.  When I find and test more suitable displays I will list them here. 

Step 1: Circuit Board Construction.

The construction technique I used to build the board is what I call 'solder bridging'. You take a perforated circuit board with solder pads on one side and solder the parts in place. In this case the resistors and diodes. To interconnect the different parts you place some solder between the soldered connections until it bridges across them making a connection. Normally solder bridges are undesirable objects caused by using too much solder but here we will take advantage of them. They are much easier to produce than an etched circuit board or connecting all the parts together with wire.

Below is a graphic that shows how to place the parts on a piece of perf board and how to 'bridge' them together. For step 1 do not worry about the wire. 

NOTE: The right hand or bottom view (parts down) is the view when you flip the board over to solder the parts.

Step 2: Wiring the board

While this project is fairly simple is does have a bit of wiring associated with it. You will need to run 8 wires from the board to the display, 7 wires from the board to the ECU wiring harness, and 2 wires to a power source. I'm fortunate in that I have about 3000 feet of multi-colored, 12 conductor, 22 gauge cable in my garage. It makes the wiring much easier when each wire can be a different color.  It's not impossible to do with a big spool of same colored wire (18 - 22 ga.) but a lot of extra care needs to be taken to not mistakenly miswire a connection.

Another issue is connectors. It is possible to wire the board, take it out to the bike and install it. However as this was a prototype I elected to wire the board to some molex plugs so that I could remove the board and reconfigure the wiring with out having to cut and then have to reconnect a lot of wires. I will describe how to build it using connectors. If you want to skip this step then you need measure out how much wire you will need to make it from the ECU wire harness to the board and from the board to the instrument panel. Use these wires instead of the 1 foot wires mentioned below.

If you don't have access to some multi-conductor cable you will need to cut 15 pieces of wire 1 foot long and 1 wire just over 2 feet long. You will also need  a number of shorter jumper wires.

Refer to the pictorial above and find the 1st gear red wiring. You need to use a jumper wire to connect the first diode of the A segment group to the first diode of the D segment group (banded ends). This is done by inserting the wire into the hole just to the left of the selected diode, soldering it in place and then bridging it to the diode. Some of the diodes will have more than one connection in which case you just keep adding wires to the next hole to the left and bridging it into the circuit. Refer to the pictorial and/or the table below and install all the jumper wires.

Now connect the long wires for each gear and Neutral. These are the wires that go off board to the left in the pictorial. In addition connect a wire to the common resistor bus on the non diode side of the resistors. This is your 12V line. You may want to mark the wires at this point so they don't get confused later. Mark them 1 - 6 , N, and 12V The two foot wire is your ground and will go directly from the gear plug to the display plug. Mark one end Gnd and the other CAT.

Next connect the 7 segment wires. Insert a wire next to a diode - resistor junction. Solder and bridge it into the group. Mark these wires A - F. 

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