Magneto Ignition System Diagram

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A magneto ignition system diagram shows how a self-contained permanent-magnet AC generator produces and times a high-voltage spark at the spark plug without needing a battery or external power supply.

A magneto ignition system generates its own electricity internally using a permanent magnet rotating past a stationary coil (the primary winding). Unlike a battery-coil ignition system that relies on an external battery, a magneto is entirely self-powered — essential for aircraft piston engines, small two-stroke and four-stroke engines in chainsaws, lawnmowers, and outboard motors, and for any application where a battery is absent or unreliable.

The operating principle begins with the rotating permanent magnet (mounted on the flywheel or as part of a separate rotor) inducing a current in the primary winding of the ignition coil. At the moment of maximum primary current, a set of breaker points (in older systems) or an electronic trigger (Hall-effect sensor or optical pickup in modern systems) interrupts the primary circuit. The rapid collapse of the primary magnetic field induces a very high voltage — typically 15 000–30 000 V — in the secondary winding of the coil through transformer action. This high voltage travels through the high-tension (HT) lead to the spark plug, jumps the electrode gap, and ignites the compressed fuel-air mixture.

Main variants include the single-ignition magneto (one plug per cylinder, used on small engines), the dual-magneto system (two independent magnetos on a single drive shaft, each firing one of two plugs per cylinder — mandatory on most certificated aircraft piston engines for redundancy), and the breakerless magneto using solid-state electronic triggering in place of mechanical contact points. Magneto timing — the crankshaft angle at which the spark occurs before top dead centre (BTDC) — is critical to engine performance and must be set precisely.

The kill switch grounds the primary circuit to stop the engine; a broken connection between the kill switch and the magneto means the switch cannot stop the engine — a common and dangerous maintenance misunderstanding.

How to wire magneto ignition system diagram

Identify system type: points-type or breakerless Older magnetos use mechanical breaker points that open and close as the engine turns; newer and most modern small-engine magnetos use a solid-state trigger (Hall-effect sensor or optical pickup). Points-type magnetos require periodic contact adjustment and replacement. Breakerless systems require no adjustment but the trigger module is replaced as a unit when faulty.
Locate the ignition timing specification Obtain the manufacturer's specified timing: degrees BTDC and the method of verification. For small engines this is usually fixed by the coil-to-flywheel air gap and flywheel key position. For aircraft magnetos, timing is adjustable and is set with a timing light or timing disc on the engine.
Set the coil-to-flywheel air gap (small engine magnetos) The air gap between the ignition coil laminations and the flywheel magnets determines primary coil induction and therefore spark energy. Typical gaps are 0.2–0.4 mm. Set with non-magnetic feeler gauges: position the gauge between the magnet and the coil, loosen the coil mounting screws, allow the magnet to pull the coil in, then tighten the screws. Verify the gap on both sides of the coil if applicable.
Check and adjust points gap (points-type systems only) Rotate the engine to the point of maximum points opening. Measure the gap with a feeler gauge. Compare to the specified gap (typically 0.3–0.5 mm for small engines). Adjust by loosening the points plate screw, moving the fixed contact plate, and retightening. Incorrect points gap shifts the ignition timing.
Verify the kill circuit continuity With the ignition switch in OFF, measure resistance between the kill wire terminal on the magneto and a known chassis earth. It should be near zero (the switch is grounding the terminal). With the switch in ON (RUN), resistance should be very high (open circuit). An open circuit in OFF position means the engine cannot be stopped electrically.
Inspect HT lead and plug cap A cracked, oil-soaked, or arcing HT lead dissipates energy and produces weak sparks. Measure HT lead resistance — most carbon-core suppressor leads measure 5–10 kΩ total. Inspect the plug cap for corrosion and secure seating on the plug. Check spark plug electrode gap against specification and replace if worn or fouled.
Verify spark with a spark tester Connect a spark tester between the HT lead and earth. Crank the engine and confirm a strong, consistent blue-white spark. A weak orange or intermittent spark indicates a coil, points, trigger, or HT lead fault. No spark: follow the diagnostic procedure starting from the primary circuit.

Specifications

Secondary output voltage (typical)	15 000–30 000 V peak
Primary coil resistance (typical)	0.5–2 Ω
Secondary coil resistance (typical)	3–10 kΩ
Condenser capacitance (points-type)	0.10–0.25 µF
Coil-to-flywheel air gap (small engines)	0.20–0.40 mm (engine-specific)
Breaker points gap (points-type)	0.30–0.50 mm (engine-specific)
Spark plug electrode gap (typical)	0.50–0.80 mm (engine/plug specific)
Ignition timing (typical small engine)	20°–30° BTDC (engine-specific)

Safety warnings

A magneto ignition system generates lethal high voltage — up to 30 000 V — at the spark plug and HT lead. Never touch the HT lead, plug cap, or secondary coil terminals while the engine is cranking or running. Disconnect the HT lead from the spark plug and ground it before working inside the cylinder.
Do not assume the engine is safe because the ignition switch is OFF. A broken kill wire means the magneto is not grounded and can fire whenever the flywheel rotates. Always remove the spark plug or ground the HT lead AND prevent flywheel rotation before working on the engine.
On aircraft piston engines, any magneto work must be carried out by a licensed aircraft maintenance engineer (AME/A&P) in accordance with the applicable airworthiness regulations and the engine manufacturer's maintenance manual. Incorrect timing or wiring on an aircraft engine is a flight safety hazard.
The flywheel magnets are powerful permanent magnets that attract tools and create a pinch risk. Keep hands and fingers clear of the air gap between the flywheel and coil when rotating the engine by hand.
Ground all HT output leads before removing the magneto from the engine. A magneto can generate a shock hazard simply by being turned over in the hand.

Tools needed

Non-magnetic feeler gauge set (coil air gap and points gap)
Timing light or timing disc (for adjustable-timing magnetos)
Spark tester
Digital multimeter (resistance measurement for HT lead, kill circuit continuity)
Screwdrivers (flat blade and cross-head)
Flywheel puller (if flywheel removal required)

Common mistakes

Setting the air gap between the ignition coil and flywheel with a steel feeler gauge — the steel is attracted to the flywheel magnet, giving a false reading. Always use a non-magnetic (brass or plastic) gauge.
Forgetting that a broken kill wire leaves the magneto permanently live — the ignition switch cannot stop the engine, and the blades or moving parts remain dangerous even with the switch off.
Replacing the spark plug without checking the electrode gap. Even new plugs may have incorrect gaps from the factory for a specific engine.
Misidentifying the condenser as a capacitor to be removed in a 'performance upgrade.' The condenser is essential — without it, points arc severely, erode rapidly, and the spark weakens within hours of operation.
Overtightening the ignition coil mounting screws after setting the air gap with the feeler gauge, pulling the coil against the flywheel and eliminating the gap.
Ignoring the flywheel key. A sheared or displaced flywheel key shifts the flywheel angular position relative to the crankshaft, advancing or retarding timing — a common cause of hard starting after an engine impact event.

Troubleshooting

No spark at all: Cause: Open HT lead, faulty coil, kill circuit permanently grounding primary, or incorrect air gap Fix: First check the kill circuit: disconnect the kill wire from the magneto and try again — if spark appears, the kill switch or wiring is shorting to earth. Check air gap. Measure coil primary resistance (typically 0.5–2 Ω) and secondary resistance (typically 3–10 kΩ) — open circuit on either winding confirms coil failure.
Weak or intermittent spark: Cause: Excessive air gap, worn points (insufficient points gap), degraded HT lead, or failing condenser Fix: Reset air gap to specification. On points-type magnetos, set points gap and check condenser with a capacitance meter (typical value 0.1–0.25 µF). Replace HT lead if resistance is outside specification or insulation is cracked. Confirm spark plug is correct type and gap.
Engine will not stop with ignition switch OFF: Cause: Open circuit in kill wire between switch and magneto Fix: With the switch in OFF, measure continuity from the magneto kill terminal to chassis earth. Infinite resistance indicates an open wire or failed switch. Trace the wiring and repair the break. Confirm the switch grounds the magneto terminal when OFF.
Hard starting, poor power, or overheating: Cause: Timing incorrect due to sheared flywheel key or incorrect coil/points setting Fix: Verify flywheel key is intact and correctly seated. For adjustable magnetos, verify timing to specification using a timing disc. Retarded timing causes overheating; advanced timing causes hard starting and potential detonation.

Frequently asked questions

Why do aircraft use dual magnetos instead of a single ignition system?

Certificated aircraft piston engines require two completely independent magneto ignition systems — each firing one of two spark plugs per cylinder — so that failure of either magneto does not cause engine stoppage. Each magneto operates independently of the other and of the aircraft electrical system (battery/alternator), providing redundancy critical to flight safety.

How does the magneto kill switch actually stop the engine?

The kill switch (ignition switch in OFF position) grounds the magneto primary circuit, preventing the primary voltage from collapsing sharply enough to induce secondary high voltage. The switch does not disconnect the magneto — it shorts it. A broken wire between switch and magneto means the switch cannot ground the primary, and the engine cannot be switched off by the ignition switch.

What is magneto timing and why does it matter?

Magneto timing is the crankshaft angle — expressed as degrees before top dead centre (BTDC) — at which the spark plug fires. Correct timing ensures combustion pressure peaks just after TDC, maximising power. Retarded timing causes overheating and power loss; advanced timing risks detonation (pre-ignition), which can destroy pistons within seconds.

What is the difference between a magneto and a CDI system?

Both are self-powered ignition systems. A magneto uses transformer action: it generates AC in a primary winding and induces high voltage in a secondary by interrupting primary current. A capacitor discharge ignition (CDI) system uses the magneto stator to charge a capacitor; a thyristor (SCR) then discharges the capacitor rapidly through a small transformer to generate the HT pulse. CDI produces a shorter, sharper spark.

Can I test a magneto on the bench without running the engine?

A basic test can be done with a spark tester: disconnect the HT lead from the plug, connect a spark tester (or a gap gauge held to earth), and turn the engine over by hand or starter. If the magneto is functional you will see a spark. A more rigorous test uses a magneto tester bench that spins the magneto at specified RPM while measuring output voltage and checking timing.

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