4-Pin CDI Diagram

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A 4-pin CDI diagram maps the ignition pulse, charge coil, kill switch, and coil output connections that form the capacitor discharge ignition circuit in small engines and motorcycles.

A Capacitor Discharge Ignition (CDI) unit is an electronic ignition system that stores electrical energy in a capacitor, then discharges it rapidly through an ignition coil's primary winding to generate the high-voltage spark required at the spark plug. CDI systems replaced mechanical breaker-point ignitions in motorcycles, scooters, marine outboards, and small petrol engines from the 1970s onwards due to their reliability, maintenance-free operation, and ability to deliver consistent spark energy across a wide RPM range.

In a typical 4-pin CDI system, the four connections serve the following functions:

Pin 1 — Charge coil input (AC supply from stator). The stator generates an alternating voltage as the flywheel magnets rotate past the charge coil windings. This AC voltage (typically 100–250 V peak) is rectified inside the CDI module and used to charge an internal storage capacitor. The charge coil output is specifically wound to supply the CDI — it is distinct from the lighting coil windings that power the vehicle's lights and accessories.

Pin 2 — Trigger/pulse coil input. The trigger (pulser) coil is a small winding positioned to detect the flywheel's reluctor lug or trigger tab as it passes, generating a short voltage pulse at the precise crankshaft angle corresponding to the desired ignition advance. This pulse triggers the CDI module's thyristor (SCR) to discharge the capacitor through the ignition coil primary.

Pin 3 — Kill switch (ground). This pin connects to the kill switch on the handlebar. When the kill switch is pressed, it shorts this pin to ground (chassis), preventing the trigger pulse from reaching the SCR gate and stopping ignition. The CDI is never directly disconnected from the supply by the kill switch — only the trigger signal is suppressed.

Pin 4 — Ignition coil primary output. This pin delivers the CDI's capacitor discharge pulse to the primary winding of the ignition coil (HT coil). The secondary winding of the ignition coil steps up this pulse to the 15 000–40 000 V required to jump the spark plug gap.

Pin count and terminal assignments vary between manufacturers and engine families. This is a generic illustrative reference only.

How to wire 4 pin cdi diagram

Consult the engine or vehicle service manual for the specific pin assignment CDI pin assignments are not standardised across manufacturers. A 4-pin CDI on a Chinese GY6 scooter engine will have different pin assignments from a 4-pin CDI on a small Japanese motorcycle engine. The service manual or wiring diagram for the specific engine is the authoritative source. Do not rely on generic colour-code assumptions.
Verify charge coil output voltage with a multimeter in AC mode With the kill switch in the run position and the engine being cranked on the kickstarter (or electric start), measure AC voltage between the charge coil wire and ground at the CDI connector. Compare the reading with the specification in the service manual. A reading significantly below specification indicates a failing charge coil, a corroded connector, or an open circuit in the stator winding.
Verify trigger coil output with a multimeter in AC millivolts mode Measure AC millivolts between the trigger coil wire and ground while cranking the engine. The trigger coil produces a short pulse rather than a sustained voltage, so the multimeter reading will be low compared to a steady AC source — but some voltage should be present on each crank revolution. A zero reading indicates no trigger pulse; check the air gap between the trigger coil and the flywheel reluctor tab.
Test the kill switch circuit With the kill switch in the run position, verify no continuity between the kill switch wire and ground (the CDI kill input should be floating, not grounded). With the kill switch in the stop/kill position, verify continuity between the kill switch wire and ground. A kill switch that is stuck in the grounded position causes permanent ignition suppression and a no-start condition.
Test the ignition coil primary and secondary resistance Disconnect the CDI connector from the ignition coil. Measure resistance between the coil's primary terminals — typically 0.5–1.5 Ω. Measure resistance between the high-tension lead and the primary input terminal (or ground) — typically 2 000–15 000 Ω, depending on the coil design. Consult the service manual for the specific resistance specification. An open or short circuit in either winding requires coil replacement.
Reconnect the CDI and verify spark before final assembly Connect an in-line spark tester between the HT lead and the spark plug cap, or remove the plug, reconnect the HT lead, earth the plug body against the engine block, and crank the engine in a shaded environment to observe the spark. Verify a bright blue-white spark on each crank cycle. A weak orange or yellow spark, or no spark, indicates CDI, coil, or charge coil fault. Do not hold the HT lead bare — ignition system voltages exceed 15 000 V and are capable of causing a painful electric shock.

Specifications

CDI operating principle	Charge capacitor from stator; discharge via SCR through primary of ignition coil
Charge coil output voltage (typical AC CDI)	100–250 V AC peak (application-specific)
Trigger coil output (typical)	0.5–5 V AC pulse per crankshaft revolution (application-specific)
Ignition coil primary resistance (typical)	0.5–1.5 Ω
Ignition coil secondary resistance (typical)	2 000–15 000 Ω
Secondary output voltage (typical)	15 000–40 000 V
Trigger coil air gap (typical)	0.3–0.5 mm (verify per engine service manual)
Kill circuit function	Ground CDI kill pin to suppress SCR trigger; normally floating (open) in run condition

Safety warnings

The ignition coil secondary voltage exceeds 15 000 V and can cause a severe electric shock. Never hold the HT lead or spark plug terminal with bare hands while cranking the engine. Use an insulated in-line spark tester for spark checks.
The CDI charge coil generates 100–250 V AC peak. Even with the engine stopped, residual voltage may be present on the charge coil wires immediately after the engine halts. Exercise caution when probing the CDI connector with the engine running.
Before disconnecting any part of the ignition system, ensure the engine is fully stopped and the kill switch is in the stop position. Never disconnect the HT lead from the coil or plug while cranking — the open-circuit coil secondary can produce a higher-than-normal voltage spike.
All ignition system electrical work on vehicles should be performed with the engine stopped and the kill switch in the stop position. On fuel-injected engines, additional precautions apply for depressurising the fuel system before electrical work near fuel components.
This diagram is a generic illustrative reference only. CDI pin assignments, charge coil voltage specifications, and timing curves are application-specific. Always consult the engine manufacturer's service manual before any CDI system work.

Tools needed

Digital multimeter (AC voltage, DC voltage, resistance)
In-line spark tester (for safe spark verification)
Feeler gauges (for trigger coil air gap setting)
Insulated screwdrivers
Combination wrenches and sockets
Flywheel puller (if stator or trigger coil replacement is required)
Wiring diagram for the specific engine/vehicle model

Common mistakes

Assuming CDI pin assignments are universal based on wire colour — colours and assignments vary significantly between manufacturers; always verify with the specific wiring diagram.
Testing CDI function with a multimeter resistance check on the module itself — CDI units contain SCR thyristors that cannot be meaningfully tested with a standard continuity or resistance measurement. Substitution is the reliable test method.
Ignoring the trigger coil air gap — a gap outside specification is a common cause of intermittent or absent spark that is incorrectly diagnosed as CDI module failure.
Grounding the kill switch wire permanently (broken kill switch wire earthed to the chassis), which creates a permanent ignition suppression condition identical to the kill switch being held pressed.
Replacing the CDI without first confirming the charge coil and trigger coil are functioning correctly — a faulty coil will destroy a new CDI module if the root cause is not identified.

Troubleshooting

No spark at all; engine does not start: Cause: CDI module failure; no charge coil output (shorted or open stator winding); no trigger pulse (trigger coil fault or excessive air gap); kill switch stuck in ground position; open ignition coil primary winding Fix: Work through the circuit systematically. First: verify kill switch is not grounded (check continuity between kill wire and chassis — should be open in run position). Second: measure charge coil AC output while cranking — should be >30 V AC for most applications. Third: verify trigger coil produces an AC pulse while cranking. Fourth: check ignition coil primary resistance. Fifth: if all inputs are correct, substitute the CDI module.
Engine starts but misfires or cuts out at high RPM: Cause: CDI module internal failure causing weak spark at high RPM; spark plug with incorrect heat range fouling at speed; ignition coil secondary breakdown at high-voltage levels Fix: Replace the spark plug first — it is the lowest-cost part and a common misfire cause. If misfire continues, check the ignition coil for secondary winding breakdown: under high-voltage conditions that do not appear on a resistance check, the coil insulation may fail. Test by substituting a known-good coil. If coil is healthy, suspect CDI module degradation.
Engine starts with kickstart but kill switch does not stop the engine: Cause: Open circuit in the kill switch wiring between the switch and the CDI kill input pin; failed kill switch (mechanically not making contact); incorrect pin identified as the kill input Fix: Test kill switch continuity with a multimeter — in the kill position, the switch should show continuity (closed). Verify the wire from the kill switch reaches the correct CDI pin by tracing the harness. Check for broken wires, particularly at the handlebar clamp where wires flex and fatigue. A broken kill switch wire that simply goes open-circuit causes this symptom — re-terminating the wire restores kill switch function.

Frequently asked questions

What is a CDI unit and how does it produce a spark?

A CDI (Capacitor Discharge Ignition) unit uses energy stored in an internal capacitor to generate a fast, high-energy primary pulse in the ignition coil. The stator's charge coil supplies AC voltage that is rectified and stored in the capacitor. When the trigger coil detects the correct crankshaft position, it fires a thyristor that discharges the capacitor through the ignition coil primary, inducing the high-voltage secondary pulse (15 000–40 000 V) that fires the spark plug.

What is the difference between AC CDI and DC CDI?

An AC CDI charges its internal capacitor from the stator's dedicated charge coil — it draws AC power from the magneto and does not rely on the battery for ignition. It continues to operate if the battery is flat or removed. A DC CDI charges its capacitor from the vehicle's 12 V DC battery through an internal DC-DC boost converter. DC CDI is simpler (no dedicated charge coil required) but fails if the battery is flat. AC CDI is more common in small single-cylinder engines and scooters; DC CDI in multi-cylinder motorcycles with complex wiring systems.

How do I test a CDI unit to determine if it is faulty?

CDI units cannot be reliably tested with a standard multimeter in-circuit because their internal SCR thyristor requires a specific voltage and trigger pulse to conduct. The most reliable test is substitution — replace the suspect CDI with a known-good unit of the same specification and verify whether the no-start or ignition fault is resolved. Before substituting the CDI, verify that the charge coil output voltage and trigger coil pulse voltage are within specification, and confirm the ignition coil primary and secondary resistance are correct.

What happens if the trigger coil (pulse coil) air gap is incorrect?

The trigger coil must be positioned with a specific air gap between the coil's pickup pole and the flywheel reluctor tab — typically 0.3–0.5 mm (consult the engine service manual). Too large a gap reduces the magnitude of the trigger pulse; if it falls below the CDI module's trigger threshold, the SCR will not fire and ignition will not occur. Too small a gap risks the flywheel striking the coil body. The air gap should be set with feeler gauges to the manufacturer's specification.

Can I use a CDI from a different engine model on my engine?

Substituting a CDI from a different model carries significant risk. CDI units are matched to a specific charge coil voltage, trigger coil polarity and voltage, and ignition timing advance curve. Using an incompatible CDI may produce no spark, incorrect timing (retarded or advanced), or damage the ignition coil. If the original unit is unavailable, consult a specialist who can verify electrical compatibility parameters before fitting an alternative.

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