The five CDI wires
| Usual color | Connects to | What it does |
|---|---|---|
| Black/red | Stator black/red | AC power from the source (exciter) coil that charges the CDI’s capacitor |
| Blue/white | Stator blue/white | Pulse from the pickup coil that times the spark |
| Black/yellow | Ignition coil | The output pulse that fires the coil |
| Green | Frame ground | Ground |
| Black/white | Key switch and/or kill switch | Kill: grounding it stops the spark |
Pin positions are not standard. Two CDIs that look identical can put the wires in different places in the plug, and published pinouts disagree with each other. Match wires by color and by what they connect to, and if a replacement CDI’s plug doesn’t match your harness, re-pin it by function rather than forcing it.
AC or DC CDI?
This page is for AC CDIs, which are powered by the stator’s black/red wire. A DC CDI is powered from the battery through the key switch and usually has six pins (typical on GY6 scooter engines). They can look the same from outside. If your CDI’s power wire is red or red/black coming from the key switch rather than the stator, it is a DC unit – don’t use this diagram.
Testing a suspected bad CDI
There’s no reliable way to test a CDI with a basic meter. Rule out everything around it instead:
- Unplug the black/white kill wire. Spark now? The fault is in the kill circuit, not the CDI.
- Check the source and pickup wires (steps 1 and 2 in the checklist) and every ground.
- Check the coil’s primary resistance and the plug cap.
- If all of that is good, swap in a known-good CDI of the same type.