CDI Wiring Diagram: Capacitor Discharge Ignition (4-Pin and 5-Pin)
Capacitor Discharge Ignition is the dominant ignition system on small motorcycles, ATVs, scooters, and outboard engines. Unlike a points-based or inductive system, CDI charges a capacitor from the alternator and dumps that stored charge into the ignition coil primary in a few microseconds, producing a fast-rising high-voltage spark. That rapid rise time gives CDI good fouling resistance compared to inductive systems.
Understanding the wiring is not complicated once you know which coils do what and whether you have an AC-CDI or DC-CDI unit.
AC-CDI vs. DC-CDI: The Critical Difference
This distinction determines everything about how the unit is powered and which pinout it uses.
AC-CDI draws its charging voltage directly from a dedicated high-voltage AC winding on the stator (alternator). This winding produces roughly 100--200V AC at higher RPM. The CDI unit internally rectifies and charges its internal capacitor from this winding. AC-CDI boxes do not need the battery to fire -- engines with dead batteries can still be kick-started.
DC-CDI draws from the 12V battery rail (through an internal boost converter that steps up to ~200V DC to charge the capacitor). It requires a functioning battery. DC-CDI units are common on modern bikes that no longer have a separate charge coil, and on any engine that's been converted to a battery-dependent system.
Mixing up an AC-CDI with a DC-CDI unit on the same engine is one of the most common mistakes -- the pinouts may look similar but the internal circuits are completely different.
Key Components in a CDI System
- Charge coil (exciter coil): A dedicated stator winding that provides the charging voltage for AC-CDI. Typically produces 80--200V AC.
- Pickup coil (trigger/pulse coil): A small inductive coil positioned near a magnet on the flywheel. Produces a narrow AC pulse that tells the CDI when to fire. Output is usually 0.5--3V peak.
- CDI unit (box): Charges the capacitor from the charge coil or battery, then discharges it into the ignition coil primary when it receives the trigger pulse.
- Ignition coil: A step-up transformer. Primary winding receives the CDI's capacitor dump (typically 200--400V, lasting ~10µs); secondary produces 20--40kV for the spark plug.
- Kill switch: Shorts the trigger signal or the CDI output to ground, preventing firing.
4-Pin CDI Wiring Diagram
4-pin CDI units are common on older and simpler single-cylinder engines. The connector carries:
| Pin | Color (typical) | Function |
|---|---|---|
| 1 | Black/White | Pickup coil signal (trigger) |
| 2 | White or Yellow | Charge coil / DC power in |
| 3 | Orange or Black/Yellow | Ignition coil primary |
| 4 | Black (Ground) | Ground |
Wiring Steps (AC-CDI, 4-Pin)
- Connect the charge coil wire (White/Yellow) from the stator to pin 2. This is the high-voltage AC charging supply.
- Connect the pickup coil signal wire to pin 1. The other pickup coil wire goes to ground.
- Connect pin 3 to the ignition coil primary (the input terminal, usually marked "B" or with a smaller wire gauge).
- Connect pin 4 to chassis ground.
- Connect the kill switch in series between the pickup signal wire and pin 1, or wired so that closing the switch shorts pin 1 to ground.
For DC-CDI 4-pin units the charge coil wire (pin 2) is replaced by 12V battery positive, typically fused at 5--10A. The rest of the circuit is the same.
5-Pin CDI Wiring Diagram
5-pin units add either a separate ground for the charging circuit or an additional kill switch input. On Honda and many Chinese clones, the fifth wire is a separate lighting coil or AC power wire that the CDI uses to determine engine speed for an ignition advance curve.
| Pin | Color (typical) | Function |
|---|---|---|
| 1 | Black/White | Pickup coil signal |
| 2 | White | Charge coil / DC power |
| 3 | Orange | Ignition coil primary |
| 4 | Black | Ground |
| 5 | Green | Kill switch input |
Wiring Steps (AC-CDI, 5-Pin)
- Charge coil wire to pin 2 (same as 4-pin).
- Pickup coil to pin 1.
- Ignition coil primary to pin 3.
- Ground to pin 4.
- Kill switch: Wire one terminal to pin 5 and the other to ground. The CDI stops firing when this pin is pulled low.
On 5-pin DC-CDI units, pin 2 carries 12V and pin 5 may carry an ignition switch feed rather than a kill input -- check the specific service manual.
Identifying the Pickup Coil vs. the Charge Coil
Both wires come from the stator and look similar. To tell them apart:
- Resistance: Pickup coils read very low resistance, typically 50--150Ω. Charge coils read 100--500Ω depending on engine size. A $10 multimeter resolves this.
- Output voltage: Kick the engine over with a multimeter on AC Volts. The charge coil produces a large AC voltage (30--150V). The pickup coil produces a small pulse (0.5--3V AC).
- Wire gauge: Charge coils usually run heavier wire than pickup coils.
Diagnosing a No-Spark Condition
- Confirm 12V at the CDI power pin (DC-CDI) or measure the charge coil AC output (AC-CDI). No voltage means the problem is upstream of the CDI.
- Measure pickup coil resistance. Should be within 10% of spec. An open circuit or near-zero reading indicates a failed coil.
- Test the ignition coil primary resistance -- typically 0.4--1Ω. Secondary resistance is 5--20kΩ.
- Bypass the kill switch by disconnecting the kill wire from the CDI. If spark returns, the switch or wiring has a fault.
- Swap the CDI unit last. CDI boxes rarely fail on their own, but a shorted kill switch can destroy one.
Safety Note
The charge coil in an AC-CDI system produces 100--200V AC. Do not probe it with bare fingers or let the wires arc against the frame while the engine is cranking. The capacitor inside the CDI can hold charge after the engine stops -- wait a few seconds before disconnecting connectors.
Create Your Own CDI Wiring Diagram
Documenting your engine's ignition wiring before modifying anything prevents hours of troubleshooting later. With CircuitDiagramMaker you can:
- Place CDI, pickup coil, charge coil, ignition coil, and kill switch symbols on a canvas
- Label each wire with its color code and function
- Add the ignition coil secondary and spark plug for a complete system view
- Export a PDF to keep in the workshop next to the bike
Create your own CDI wiring diagram -- free
Key Takeaways
- AC-CDI powers its internal capacitor from a high-voltage stator winding; DC-CDI uses 12V battery plus an internal boost converter -- they are not interchangeable.
- 4-pin CDI connectors carry: pickup signal, charge/power, ignition coil output, ground.
- 5-pin units add a dedicated kill switch input or a second AC reference wire for advance curves.
- Identify stator wires by resistance and AC output voltage -- pickup coils read 50--150Ω; charge coils read higher resistance and much higher AC output.
- A faulty kill switch shorting to ground is a common no-spark cause and can destroy the CDI unit over time.
- Always check upstream components (charge coil voltage, pickup coil resistance, ignition coil resistance) before condemning the CDI box.