How to build a Capacitor Discharge Ignition for a early CX500

These Ignition systems were used on the CX500 from 1978 to about 1982 or so. They can be identified by a single box unit that included a 8 pin connector and a 2 pin connector that supplies power. The more modern ignition used a transistor ignition and was mounted in 2 boxes.

 Note pictures show old style slide pots. new one uses round pots as listed on parts list.


Thge spark is developed by special coils on the alternator charging a capacitor and then other coils trigging a SCR to discharge the capacitor through the primary of the coil.  The spark plug voltage is taken from the secondary of this coil.  

As the engine turns it is connected to the alternator.    This is a set of magnets which rotate inside a  stator.    Three seperate sets of windings on the stator develope a three phase output.   This output is fed to a three phase bridge rectifier and a current regulator.    This is the battery charging system.    

In addition to the charging coils on the stator there is a seperate set of two coils.  their function is to develope two sets of six pulses.    With each revolution of the motor the white wire sends out 6 pulses to charge the capacitor.    The blue wire places a sine  wave ( both positive and negative) on the capacitor and the SCR.    The function of this blue wire seems to be to apply a short negative pulse to be sure the SCR is turned off after the last fireing pluse.    The blue and white wires together  develop a charge of  150 volts on each of two  ( 2.2 uF )  capacitors.  


The SCR's are triggered from the outputs of 3 seperate coils.   These pulses occur at correct times to advance the spark as required.   The first pulse to occur is the orange red wire. This is a negative pulse and prevents the SCR from turning on. It's function is to limit the extent that the advance pulse can fire. This avoids damage to the engine.   At low speeds there is no effect from this pulse.

The second  pulse to occur is the orange/white wire. The pulse on the orange white wire is a triangle waveform.    It is too small to trigger the SCR.  

The next pulse is the orange wire. This pulse triggers the SCR  at low speeds.    As the engine speed increases to 2000 RPM  the amplitude of the orange / white wire increases and it begins to trigger the SCR at a earler time.  

As the engine speed increases even more the triangle waveform of the orange white wire gets bigger and it triggers the SCR even sooner. This causes the spark to advance more and more until at 4500 RPM  it is limited by the negative pulse from the orange / red wire.   When this negative pulse occurs it prevents any more spark advance from taking place.  This is done to avoid engine damage.  

One difficulty is that the trigger voltage of the SCR is dependent upon temperature. This is compensated for by a thermister. It is difficult to duplicate the thermister Honda used.

My design is different from that of Honda. The advance pulse in my design triggers a comparator. The is followed by a transistor and which then triggers the SCR. This design has no temperature dependence but 12 volt power is required for the comparator.  In the event of a power failure the engine will still run but it will have no spark advance.


The following resistance measurements were made:

Orange White Trigger  200 ohms to ground

Orange / Red Negative pluse 93  ohms to ground

Orange / Left Advance pluse 100 ohms to ground

Coil wire = yellow

Blue White Trig 200 Ohms to ground

Blue / Red negative pluse 90  ohms to ground

Blue Right advance pluse 100 ohms to ground 

Coil wire = Peach

Coil secondary high tension ( connector unscrewed from cable cable to ground 7.4 K ohms.
Coil primary resistance to ground 1 ohm


Blue to White = 80 Ohms  

White to Gnd =  350 TO 400 OHMS 

Kill wire = black with white stripe


start @ 15 degrees = 2000 RPM

end @ 37 degrees = 5500 RPM




SCR NTE 5465 # 70215791 X3

C2 = Pulse capacitor 2.2uF 600 volts # 70103271 X2 ( metalized polypropylene )


prototype board board $ 20.77 # 574-169P44WE

523-L77-DE09S connector socket $5.43

523-L77-DE09P connector pins $ 4.25

512-LM7805ACT 5 volt power supply

511- LM339N comparator

72-T7RYB-1K X 4 Potentiometer 1K

72-T7RYB-2K X1 Potentiometer 2K

Capacitors .047 600V # 667-ECQ-E6473KF X8

Box Mouser # 563-CU-3006A


LEDs 2 red 2 green

Q1 Q2 2N2222

Diode N4005 X 16

Resistors 1/4 watt (see schematic )


Connectors 8 pin female = # 8P250-CNA-TL

3.5 mm bullet connectors 3 - 5BCS - 10 need 3 female

The unit was built on two perforated boards. The first one contained the SCRs and the capacitors. The second contained the 6 timing inputs and the adjustment circuits.


The second was also built on a piece of vectorboard and held the timing components.


Here is a drawing of the box



Top Board   

top board

Bottom board

Bottom of lower board


This shown the bottom of one of the boards. all grounds are connected here. 

finished assembly

Above is finished assemply

The above two pictures show the pulser waveform on the bottom. The top waveform is the 5 degree BTDC idle pluse . Only 1/4 of the pulser waveform is used. it is the rising edge in box 9.

This also shows the idle pluse on top and the max advance pulse on the bottom. Note that the large peaks are not used it is the small step on the top trace that is clipped by the input diode which triggers the idle pluse. On the bottom trace it is the first negative pluse that stops the advance. The positive pluse is not used. Magnetic pickups produce both positive and negative pluses.

Here is a test procedure


Materials to test CDI board

clip leads from Radio Shack .
2 push button switches.
5 AA batteries.
Two 9 volt batteries and with two battery clips with leads
radio speaker
Wire and solder and electrical tape.

Connect 5 AA batteries together in series to give + 7.5 Volts. use tape and solder

Add a wire to the first battery to give +1.5 volts. and a wire to the last battery to supply+ 7.5 volts.

Connect leads with alligator clips to the neon indicator light


1) Connect 9 volt battery to 12 volt input.

1A) connect both 9 volt batteries ground to board ground. Input on 8 pin connector

2) check for 5 volts .with voltmeter

3) check for vreff changes with pot. with voltmeter vreff should go from .9 v to 2.9 volt.

3A) check for Vmax should be 2,9 volts with no adjustment.

4) connect 2nd 9 volt battery to blue lead Through push button switch.

7) Connect ground wire to 5 AA battery set.

7A) Connect 1.5 volt lead to blue input through push button switch.

8) connect speaker to right coil output other end of speaker should be ground

9) Push and release first push button switch to charge capacitor. check with meter .

12) Discharge right capacitor by pushing second button . speaker should click.

13) Set both left and right potentiometers to minimum .

14) Connect 7.5 volt battery through push button switch to blue white wire.

15) push and release first switch to charge cap. push 2nd to discharge through speaker speaker should click

16) repeat above for other circuit.

16) Note that the third input, the blue/red and orange/red wires cannot be tested. Review circuit to be sure they are correct.


Note it may be necessary for a 2nd person to maintain the RPM of the engine as it will change with the pot adjustment .

1) Run engine at 4000 RPM.

2) Adjust orange pot to flicker red LED at 4000 RPM

3) Adjust blue pot to flicker red LED at 4000 RPM

4) Run engine at 2000 RPM

5) Adjust green pot to flicker both green LEDs at 2000 RPM.

Repeat if necessary

To check performance of motorcycle find a empty road, place engine in 4th gear and use a stopwatch to measure acceleration from 30 MPH to 50 MPH with no gear changes.

My early data

Stock CDI from honda 30 MPH to 50 MPH = 4 sec

First try with CDI 30 MPH to 60 MPH = 6 sec