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
Multimeter Testing Procedure for the Stator
- Disconnect the stator connector at the harness so you're testing the coil in isolation, not through the CDI or ignition switch.
- Set the multimeter to resistance (Ω) and touch the probes to the two pickup coil leads (or one lead and ground, if the coil uses the frame as a return). Check for continuity as a baseline even if you don't have the exact spec for that engine.
- Check for shorts to ground. With one probe on a stator lead and the other on bare chassis metal, a low-resistance reading indicates a winding shorted to the frame -- a common stator failure mode, separate from an open winding.
- Set the meter to AC voltage and crank or kick the engine over with the plug grounded to the engine block (so it can't fire loose). A charge coil with no AC output while cranking, but correct resistance at rest, points to a magnet or flywheel timing problem rather than a winding fault.
- Test the CDI unit itself by elimination. A standard multimeter cannot verify a CDI's internal switching under load. If the stator and coil test good but there's still no spark, and pin resistances at the CDI connector aren't shorted to ground, the CDI unit is the most likely remaining cause.
How CDI Wiring Differs Across Vehicle Types
- Scooters: Often run the lighting coil off the same stator winding used for CDI charging, so a failing charge coil can dim the headlight as well as weaken the spark. Most CVT scooters don't have a separate "run" vs. "start" ignition position the way a motorcycle does, which simplifies the kill circuit.
- ATVs: Frequently add a safety interlock into the kill circuit -- a reverse-limiter switch or a parking-brake/neutral switch wired in addition to the handlebar kill switch on some models. This adds extra ground paths that need to be checked when a no-spark condition only happens in certain gear or brake positions.
- Motorcycles: More likely to have a separate battery-charging coil independent from the ignition charge coil, especially on larger-displacement bikes, plus a sidestand or clutch safety switch wired in series with the kill circuit.
Across all three, the charge coil, pickup coil, and CDI-to-coil wiring follow the same basic principles described above -- the differences are mostly in what else shares the kill or ground circuit.
Common Mistakes When Swapping in an Aftermarket CDI
- Assuming pin position instead of function. Aftermarket CDI connectors don't always use the same pin order as the OEM unit, even with the same pin count. Match wires by color and function against a wiring diagram, not by which slot they plug into.
- Leaving the pickup coil polarity reversed. Some pickup coils are symmetrical and run either way, but others are polarity-sensitive and produce a weak or mistimed spark if reversed. If timing seems off after a swap, try reversing the two pickup wires.
- Grounding the CDI case poorly. Many CDI units use their metal case, or a dedicated pin, as a reference ground separate from the ignition coil ground. A poor case ground is a common cause of intermittent misfire that mimics a failing pickup coil.
- Ignoring the advance curve mismatch. A "universal" aftermarket CDI has a fixed timing curve that may not match the original unit, which can cause pinging or reduced power even though the engine starts and runs.
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.
Frequently asked questions
Can an engine run without a CDI unit?
No. The CDI is what actually fires the spark -- it stores charge from the stator and dumps it into the ignition coil at the right moment based on the pickup coil's signal. Without a functioning CDI, the coil never receives its firing pulse, so the engine cannot produce spark and will not start or run.
What happens if a CDI unit's wires are connected backwards?
Swapping the charge coil and ignition coil primary wires, or reversing the pickup coil leads on a polarity-sensitive unit, typically prevents the CDI from charging or firing correctly -- you'll get no spark or a weak, mistimed one. It rarely damages the CDI itself, but always match wires by color and function rather than guessing.
Can a 5-pin CDI be used on a 4-pin harness?
Not directly. A 5-pin CDI expects a signal on its fifth pin, commonly a kill switch input or a second AC reference, and leaving that pin unconnected can cause the unit to not fire at all if the design requires that pin grounded or tied off to complete the circuit. Check the specific unit's diagram first.
What causes a CDI unit to fail?
Common causes include a shorted kill switch or kill wire repeatedly dumping the charge to ground, moisture corroding the internal circuit board, vibration-related solder cracks, and voltage spikes from disconnecting the battery while the engine runs. CDI boxes are solid-state and generally reliable, so confirm the stator and coil test good before replacing one.
Can a bad ground cause a no-spark condition?
Yes. The CDI, ignition coil, and often the pickup coil rely on a clean chassis or engine-block ground to complete their circuits. A corroded or loose ground connection can mimic a failed CDI, weak coil, or bad pickup, so checking ground continuity is a quick step worth doing before replacing any ignition component.
Is it normal for a CDI unit to feel warm during operation?
A CDI unit can run slightly warm to the touch during normal operation since it's switching high voltage many times per second, but it should never be hot enough to be uncomfortable to hold or show discoloration on the case. Excessive heat usually points to a wiring fault forcing the unit to work harder than designed.