CDI Module Mod for Megasquirt

There are two options for the ignition on this EFI conversion: CDI or igniter. Personally, coil & igniter, or inductive ignition, is preferable as it outputs bigger spark energy. Besides, I still have my 4A-GE igniter & coil that i can use after last year i converted to EDIS. However, i'm a bit concerned with the power consumption of inductive system. I'm not sure after i add the fuel pump, oxygen sensor heater, injector and ECU, the alternator will still have enough juice to retain charging and supply the ignition coil and the rest of the electrical. I'm even considering of converting to LED for the lighting just to save power. With the same consideration, i will first try if CDI will work.

If i can somehow successfully trigger the stock CDI with Megasquirt, the alternator will have less load. Unlike DC CDI which is supplied by the battery and alternator, Honda Tiger GL200 uses AC CDI which has a separate charging coil. This means it will even spark without battery and alternator.

There are two problems with using the stock CDI on Megasquirt. First is that the stock CDI is triggered by a pulse generator which is basically a VR sensor inside the left crankcase cover which means that the CDI works with sine wave (analog signal). If i want Megasquirt to trigger the CDI directly, i have to somehow make the CDI digitally triggered. The other problem is that there is already a timing advance curve inside the CDI. How can the ignition timing be controlled with Megasquirt if the CDI is already in control of it? The stock CDI and pulse generator system will run the ignition at 10 deg BTDC during idle (approx. 1400 RPM) and slowly advance the timing until 32 deg BTDC at 5000 RPM. To fully control the ignition with Megasquirt, i need to bypass the timing advance system inside the CDI.

Both of the problems i pointed above will require fiddling with the CDI's electronics to solve. This means dissecting the CDI case until i can reach the components. I bought another Honda Tiger CDI for this experimental purpose.

After hours of agony, pain and swearing.. i could only get this far. The manufacturer has really gone all the way in making sure it's a PITA to get to the CDI circuit. That's rock-solid resin covering the components.

Luckily, the bottom resin layer can still be pried off with a knife so that the soldering is accessible.

Before i go explaining what i did to the CDI module, i'd like to share my findings when i was researching about CDI systems especially AC CDI on bikes such as the one on mine. Based on my research, here's a block diagram of AC CDI system.

I won't discuss in detail the function of each block. There's plenty of info for that on the internet. I just want to point out that i need to bypass the Time Delay and Trigger Signal Conditioning so that i can trigger the SCR directly from Megasquirt. The Time Delay is what allows the CDI to control the ignition timing. It's usually done using an RC (Resistor-Capacitor) network which delays the input pulse from reaching its SCR triggering threshold. Trigger Signal Conditioning is sometimes omitted but it's there to get more precise timing control. With Megasquirt, Signal Conditioning is not necessary as the ignition trigger signal is already a good square wave. Keep in mind that my ability to read electronic schematics is barely adequate so take my words with a grain of salt.

So, with that knowledge, i was able to ID and guess some of the parts on the CDI module and made the mod as below.

With just one wire, i made a direct connection from the pulse generator input to the SCR. I utilized the 470 ohm resistor just to be safe and cut off the track going to the caps. If everything goes according to plan, this CDI is now triggerable from Megasquirt using a 5V pullup resistor from LED D17.