CDI Diagram

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A CDI diagram shows the capacitor discharge ignition circuit used in small engines and motorcycles, illustrating how a stored high-voltage charge is released through the ignition coil to fire the spark plug at the precise moment determined by the trigger sensor.

Capacitor Discharge Ignition (CDI) is an electronic ignition system used extensively in small two-stroke and four-stroke engines including motorcycles, ATVs, scooters, chainsaws, lawnmowers, outboard marine engines, and small generator sets. It replaced contact-breaker (points) ignition in most applications from the 1970s onward and offers faster rise time, consistent spark energy, and no mechanical wear.

In a CDI system, a small alternator or exciter coil on the engine's flywheel generates AC voltage as the flywheel magnets pass the pickup coil. This AC is rectified by a diode and used to charge a storage capacitor typically to 200–400 V DC. When the trigger (pickup) coil detects the flywheel position reference mark — usually a metal tab or magnet on the flywheel rim passing a stationary trigger coil — it sends a low-voltage trigger pulse to the CDI module's SCR (silicon controlled rectifier) or thyristor circuit. The SCR fires, discharging the capacitor voltage through the primary winding of the ignition coil. This sudden high-voltage, high-current pulse in the primary winding induces a very high voltage (typically 20 000–40 000 V) in the secondary winding of the ignition coil, which fires the spark plug.

There are two main CDI variants. An AC-CDI (also called magneto CDI) derives its charging voltage directly from the flywheel exciter coil — it is self-contained and requires no battery. It is commonly found in motorcycles and small off-road equipment. A DC-CDI draws its charging voltage from the battery via a DC-to-DC boost converter internal to the CDI box, providing consistent spark energy independent of engine speed — commonly used in larger motorcycles, ATVs, and marine engines where battery-dependent ignition is acceptable.

The ignition timing in a CDI system is controlled by the angular position of the trigger reference relative to the flywheel. In a fixed-advance CDI system, this angle is mechanically fixed. In a variable-advance or programmable CDI system, the CDI module electronically delays or advances the trigger pulse based on engine speed (RPM), mapped in an internal lookup table.

CDI (Capacitor Discharge Ignition) wiring diagrams vary by motorcycle brand and displacement, but the core connections are consistent: magneto charge coil to CDI input, pulse/trigger coil to CDI trigger input, CDI output to ignition coil primary, and a common ground. The kill switch interrupts the trigger or ground path to stop the engine. Models like the YTX 125 use a 5-pin CDI connector with specific pin assignments. You can map any CDI pinout and harness layout free online with Circuit Diagram Maker.

How to wire cdi diagram

Identify the CDI system components on the engine Locate and identify: the flywheel/magneto with its exciter/charge coil and trigger/pickup coil; the CDI module (electronic box, usually sealed in epoxy); the ignition coil (with primary and secondary windings); the spark plug and cap; and the kill switch circuit. Refer to the engine's service manual for coil resistance values and connector pin-out.
Test the exciter (charging) coil resistance Disconnect the CDI wiring connector. Use a multimeter on the resistance (Ω) setting to measure the resistance between the exciter coil terminals. Compare to the service manual specification — typically 100–400 Ω for most small engine exciter coils. A very low reading indicates a shorted coil; an open circuit (OL) indicates a broken winding.
Test the trigger (pickup) coil resistance Measure the resistance between the trigger coil terminals (typically a separate wire pair, often white and black). Refer to the service manual for the specified resistance range — typically 50–200 Ω. Also check the air gap between the trigger coil and flywheel trigger tab — excessive gap reduces trigger pulse amplitude.
Test the CDI module output Reconnect the CDI. Use an ignition spark tester (inline tester that shows spark without requiring the engine to be running under compression) connected between the ignition coil high-tension (HT) lead and ground. Crank the engine. If spark is present, the CDI and ignition coil are functioning. No spark indicates a fault in the CDI, ignition coil, or their interconnecting wiring.
Test the ignition coil Disconnect the ignition coil. Measure the primary winding resistance between the primary terminals (typically < 1 Ω to a few ohms — check service manual). Measure the secondary resistance between the HT lead cap and ground (typically 5–15 kΩ for most small engine ignition coils). Values outside specification indicate a faulty coil.
Check the kill switch circuit The kill switch interrupts the ignition by shorting the CDI output or the trigger circuit to ground. A failed kill switch that is permanently shorted will prevent the engine from starting. Disconnect the kill switch wire from the CDI and attempt to start the engine — if it now starts, the kill switch or its wiring has a short to ground.

Specifications

CDI capacitor charge voltage (typical)	200–400 V DC (AC-CDI); 200–350 V DC (DC-CDI via internal boost converter)
Ignition coil secondary voltage (typical output)	20 000 – 40 000 V (20–40 kV) across spark plug gap
Spark duration (CDI system)	50–100 µs (much shorter than inductive ignition systems)
Spark plug gap (typical small engine)	0.6–0.8 mm (engine/plug-specific — always check service manual)
Trigger coil air gap (typical)	0.3–0.5 mm between coil face and flywheel trigger tab
Ignition coil primary resistance (typical)	0.1–3 Ω (engine-specific)
Ignition coil secondary resistance (typical)	5–15 kΩ (engine-specific)

Safety warnings

The CDI system generates high voltage across the ignition coil secondary winding — typically 20 000 to 40 000 volts. Do not touch the ignition coil high-tension lead, spark plug cap, or spark plug terminal when the engine is cranking or running. A shock at this voltage and current level, though typically brief, can cause involuntary muscle contraction and secondary injury from falling or recoiling from the engine.
The storage capacitor inside the CDI module can retain a significant charge after the engine stops. Do not disassemble or probe inside a CDI module. CDI modules are sealed units and are replaced as a whole assembly, not repaired by probing internal components.
Always disconnect the spark plug lead from the spark plug before working on any other part of the engine, to prevent accidental starting. Pull the lead by the cap, not by the wire.
Fuel vapour presents an ignition risk in the vicinity of spark testing. Conduct spark tests outdoors or in a well-ventilated area, away from fuel, and wipe up any fuel spills before cranking.

Tools needed

Digital multimeter (resistance and AC voltage modes)
Inline ignition spark tester (gap tester rated for CDI high-voltage output)
Feeler gauge (for trigger coil air gap adjustment)
Engine service manual (for coil resistance specifications, connector pin-outs, ignition timing)
Spark plug gap tool (wire-type feeler gauge)
Basic hand tools (socket set, screwdrivers) for access to stator and ignition coil

Common mistakes

Testing the HT output by holding the lead near a ground point with bare fingers — the high voltage can cause a painful shock and the involuntary reaction may cause injury.
Replacing the CDI module without first testing the charging coil and trigger coil — a faulty coil that is destroying CDI modules will damage the replacement as well.
Setting the spark plug gap outside specification — too wide a gap can prevent CDI from firing through the gap; too narrow a gap reduces spark energy and causes fouling.
Connecting the kill switch wire permanently to ground (trying to simplify wiring) without a proper switch, leaving no way to stop the engine safely in an emergency.
Ignoring the trigger coil air gap — if the gap between the trigger coil and the flywheel tab is excessive, the trigger pulse is too weak to reliably fire the SCR in the CDI module, causing intermittent ignition or failure to start.

Troubleshooting

Engine cranks but will not start; no spark at plug: Cause: CDI module failure, failed exciter or trigger coil, broken kill switch circuit permanently grounding the trigger, or failed ignition coil Fix: Disconnect the kill switch wire from the CDI connector — if spark appears, the kill switch circuit is faulty. Test exciter and trigger coil resistances against service manual specification. If coils test correctly and kill switch is eliminated, substitute a known-good CDI module. If still no spark, test the ignition coil primary and secondary resistance.
Engine starts and runs but misfires at high RPM: Cause: CDI capacitor beginning to fail (reduced capacitance), dirty or worn spark plug, cracked HT lead or cap, or a trigger coil with a borderline air gap that loses signal amplitude at high RPM Fix: Replace the spark plug and cap first (lowest cost intervention). Check the trigger coil air gap and reduce if excessive. If misfire continues, substitute a new CDI module. Test the HT lead with a high-voltage insulation tester if available, or visually inspect for cracking.
Engine will not stop with kill switch: Cause: Kill switch circuit is open (broken wire, faulty switch), or the kill switch is wired to the wrong terminal on the CDI Fix: Test the kill switch with a multimeter — verify it makes continuity to ground when actuated. Trace the kill switch wire to the CDI connector and verify it connects to the designated kill or ground terminal per the wiring diagram. Repair any broken conductor or replace the kill switch.

Frequently asked questions

What does CDI stand for in ignition systems?

CDI stands for Capacitor Discharge Ignition (sometimes written as Capacitive Discharge Ignition). It refers to the method of generating the ignition spark: electrical energy is stored in a capacitor, then discharged through the primary of an ignition coil to produce the high-voltage secondary pulse that fires the spark plug.

What is the difference between AC-CDI and DC-CDI?

An AC-CDI charges its capacitor directly from the engine's magneto/exciter coil AC output — it works without a battery. A DC-CDI draws charging voltage from the battery and uses an internal DC-DC converter to boost it to the capacitor charging voltage. AC-CDI spark energy varies with engine speed; DC-CDI provides consistent energy at all speeds.

Why does a CDI system produce a much shorter spark than inductive ignition?

CDI stores energy in a capacitor, which discharges extremely rapidly (in microseconds). The resulting spark has a very fast rise time and high peak voltage but a shorter duration than the long, slow spark of an inductive ignition coil system. CDI spark duration is typically 50–100 µs; inductive ignition produces 1–2 ms. This short duration can cause ignition issues with lean mixtures.

What causes a CDI box to fail?

Common failure causes include: failed SCR (thyristor) due to voltage spikes from a defective charging coil, failed internal capacitor (loss of capacitance or dielectric breakdown), failed diode in the rectifier section, moisture ingress causing corrosion or shorts, and vibration-induced cracking of internal solder joints. CDI modules are sealed and generally not field-repairable — they are replaced as a unit.

How do I test whether the CDI box or the ignition coil is faulty?

Substitute testing is most reliable. First verify the trigger coil and charging coil outputs with a multimeter (measure resistance within spec per the service manual). Then substitute a known-good CDI box — if spark is restored, the original CDI is faulty. If no spark with a known-good CDI, test the ignition coil primary resistance, secondary resistance, and the kill switch circuit.

What is the wiring diagram for a CDI YTX 125?

The YTX 125 (a common Chinese-manufactured 125 cc motorcycle) typically uses a 5-pin CDI unit where the pins carry: DC power in from the rectifier/battery, the trigger pulse from the pickup coil, the output to the ignition coil primary, chassis ground, and in some variants a lighting supply or kill input. Exact pin order varies between manufacturers and CDI revisions, so always verify with the factory wiring diagram or use a multimeter to identify the charge and pulse coil wires before connecting. Third-party CDI replacement units for this class of bike often include a colour-coded wiring guide.

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