Honda CX500 CDI Schematic ~ Cars and Bikes
Cars and Bikes

Wednesday, 18 June 2008

Honda CX500 CDI Schematic

Somebody asked me how to make the DIY cx500 CDI.
Originally I found this CDI schematic from internet long time ago, written by George in NC, however I can't found the link to his site now.

This is the first diagram:

The Actual Honda CDI Circuit Schematic….

click for larger image (not to large aniway...)

Schematic summary: (written by George in NC)

This schematic was given to me by someone in NC who had removed the resin on a non-working CDI and diagramed it. I had also taken the resin off some boards and have found that this diagram is right on the money so it is the basis for the prototypes. There is no difference in the schematic for either the black or gold box (at least none that I’ve found). The only thing to note below is that the ORN and ORN/WHT leads are swapped. The ORN is the left TDC lead and the ORN/WHT is the ADV lead (this also holds true for the LTBLUE leads).

The 3 leads (Ex…ORN/WHT + ORN/RED + ORN or LTBLUE/WHT + LTBLUE/RED + LTBLUE…you get the idea) trigger the SCR (Ex…SCRR or SCRL), with a pulse, to discharge the capacitors (Ex…CR or CL) through the ignition coils (which are step-up transformers/increase the voltage) thus creating a spark at the spark plugs. The power used to charge the capacitors is supplied by the Blue and White wire and the ignition is turned off by the BLK/WHT wire going to ground.

Lead Descriptions: On the above schematic there is an error and the listings below are correct….




pulse lead for SCRR advance


pulse lead for SCRR gate cutoff


pulse lead for SCRR TDC/idle gate trigger


pulse lead for SCRL ‘full’ advance


pulse lead for SCRL gate cutoff


pulse lead for SCRL TDC/idle gate trigger


positive pulsing power for discharge capacitor


not totally sure, but it is used as the negative half of the alternating current flowing through the positive BLUE.


ground to bike frame/engine.


goes to ground when kill switch is tripped or ignition is turned off


goes to Right coil


goes to Left coil

The first thing to note is the extra SCR (SCRP). I think that this SCRP uses a voltage divider to trigger the gate, so that when the pulse coming from ORN/WHT reaches a certain voltage it sends the trigger pulse to ground. This would basically make it a rev limiter…cute….On the prototypes I’ve put together, I haven’t used this part of the original module yet. Will I? Don’t know…

There is an important note I would like to add about the electrical characteristics of a SCR. An SCR is basically a diode that can be turned on/off like a light switch. The trick is that a SCR cannot be turned “off” until the voltage across it is either zero or a negative value. So how does the SCR in this ignition turn off?? I won’t know for a fact until I get a oscilliscope and do alittle testing.

Basic Component Descriptions:




creates positive pulse

D12 & 13

creates negative pulse

R19, R26 & 27

voltage divider

R22, R23

voltage divider


in general, the capacitors (not including CL & CR) are used for filtering out unwanted ‘noise’. An exception could be C4 & C6, which may be used to smooth out the pulse triggering the SCR (SCRR & SCRL).


used to store the charge that is dissipated through the coils to create the spark


used to ground CL & CR so that charge can be dissipated through the coils

D6 & D7

used to allow negative pulses allowed by the Blue wire resistor/diode network to goto ground?


possibly used to send the triggering pulse to ground so as to keep the rpms below a certain point (rev limiter)

Thanks to George in NC for his important information.

I'm blind about motorcycle electronic, so actually I'm not really understand with the above diagram, however I ordered somebody to make the CDI, and its work.

I'll post how to make my home made CDI (including the list of component needed) in the next few days.

Update: OK, the second post of CX500 have been publish, follow this link.

My other posting related to CX500:
1. Honda cx500 review
2. Honda cx500 wiring diagram


Josh said...

The white wire for this is the low power side of the Stator. It provides a minimum of about 90 volts AC between 4500 and 5000 rpms. The blue wire is the high power souce providing at least 100 volts above 5000 rpms. It is a good idea to check the stator resistance first before assuming that the CDI is bad here is a link for all the steps, charts, and values for resitances as well as color charts and a "get you home" fix.

explorer said...

Thank for the comment Josh,
I just receive my new cx500 stator, bought it from It is G47 type. They said it 20% stronger than the original (honestly I'm not really understand what they mean by 20% stronger). Now I'm waiting for CDI box, I order some used CDI box from ebay, hopefully it works. I prefer to use the original CDI, however I also make the prototype 2 for backup.

josh said...

If you suspect your CDI is bad you need to know that it can be rebuilt. They are not made anymore (at least not OEM, and I have not been able to find an after market one). If you do find one before you buy it make sure you see it so you know it is NOT the TRANSISTORIZED ignition. It is possible to convert them however, To me it seems like too much trouble to change. but if you do have a GL500 motor sitting around, it will bolt to the back of the cx500, and will have the componets for the transitorized ignition. If you get a "new" CDI box and it still fails to work, check your pick ups, as well as your timing circuits (simple resistive test will show shorts but not if the part has failed electrically.) If you have one cylinder that works fine, switch the inputs and out puts (the swith an unknown for one that works) if it changes cylinders you know your box is bad. The MOST LIKELY part to fail in a CDI is the capacitors. Age is an enemy to these since the film will dry up over time. The good news is they are not expensive and should be able to be replaced. I hope that this helps anyone getting their bike back on the road. There is a link that I will try to find and post latter about a TCU unit (yamaha's? equivilent) Its a little different but it has good tips for fixing trouble shooting and splitting the case. Good luck

josh said...

here is the link for the TCI, it also has some information about the CDI, I recomend reading or at least skimming it very carefully as it could give some good advice


explorer said...

Thank you for the TCI information. I'll posh an update on my cx, the prototype 2 CDI was failed to work in high rev, I can't find a good quality component here (no radioshack, my electrician suspect teh scr are bad. However, I got 2 secondhand cdi from ebay, one of them working perfectly. I haven't installed my new stator got from electrex, the old one still good.

paul said...

A Czech company now make a CDI ignition unit for the CX500,
Several owners have fitted them with success.

I would like to adapt two of these to work if possible,

any help appreciated.

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DIpak said...

HI Thx for the incredible information. I need some help from you. I am from India. I own two rd-350 bikes. They run on points which becomes very hectic to set them after every 100-200 miles. So I want to build a cdi which will be usefull in drag racing. Plz provide me a circuit diagram for it. what I think is the rev limiter should be isolated for max top speed for each gear. so plz mail to me the dia on my email Id-
I will be very greatfull to you.

Anonymous said...

My CDI just went bad so, I pulled it and took out the rubber/epoxy solution and found 1 resistor and 1 diode that were not soldered on one end. I've always had intermittent power surges and occasional misses or hiccups. I wonder if others have pulled these apart and found similar lack of quality control. Honda does have a lot of new parts for these bikes but, they're out of these. My CX500 is 32 years old. I wonder how long the CDI would have lasted if it was properly soldered.