How To Check A Magneto: The Complete Technical Diagnostic And Testing Guide
To accurately check a magneto, you must perform a sequence of electrical resistance tests on the primary and secondary windings while verifying the mechanical integrity of the points, condenser, and flywheel air gap. A healthy magneto typically shows primary resistance between 0.5 and 1.5 ohms and secondary resistance between 2,500 and 15,000 ohms, ensuring it can generate the 10,000 to 20,000 volts required to ionize the spark plug gap under combustion pressure.
Ignition System Diagnostic Preparation and Required Equipment
Before initiating a magneto teardown or electrical test, it is vital to understand that a magneto is a self-contained high-voltage generator. Unlike a standard battery ignition system, the magneto relies on permanent magnets embedded in a flywheel or rotor to induce current into a coil. This means your testing environment must be free of stray magnetic interference and your tools must be calibrated for low-resistance measurements. The primary goal is to determine if the failure is "hard" (a broken wire or shorted coil) or "soft" (weak magnets or improper gaps).
Technical Tool and Equipment Requirements:
- Digital Multimeter (DMM): Must have high impedance and the ability to measure down to 0.1 ohms for primary winding accuracy.
- Adjustable Spark Gap Tester: A standard spark plug is insufficient for testing; you need a tool that can force the magneto to jump a 1/4-inch (6mm) to 5/16-inch (8mm) gap to simulate engine compression.
- Non-Magnetic Feeler Gauges: Necessary for setting the air gap between the coil armature and the flywheel magnets.
- Point File or 400-Grit Sandpaper: For cleaning oxidized contact points in vintage systems.
- Flywheel Puller: Specific to the engine model to avoid damaging the crankshaft or the magnets.
- Analog Ohmmeter (Optional): Highly preferred by some technicians for testing condensers, as the needle sweep provides a visual representation of the capacitor charging.
Comprehensive Step-by-Step Magneto Diagnostic Workflow
The following procedure moves from external, non-invasive testing to internal component-level forensics. This sequence ensures you do not overlook simple mechanical issues before condemning an expensive ignition coil.
Step 1: External High-Tension Lead and Spark Strength Verification
The first step is to isolate the magneto from the rest of the engine's electrical system, including the kill switch wire. A faulty ignition switch or a grounded "kill" lead is the most common cause of a perceived magneto failure. Disconnect the small black wire leading from the magneto to the equipment's harness.
Attach your adjustable spark gap tester to the spark plug boot and ground the other end to a clean, unpainted cooling fin on the cylinder head. Set the gap to 0.250 inches (approx. 6mm). Rotate the engine at normal cranking speed. A healthy magneto must produce a crisp, blue snap of electricity across this gap. A weak, orange, or intermittent spark indicates high internal resistance or a failing condenser. If no spark occurs, proceed to internal electrical testing.
Warning: High-voltage ignition systems can deliver a painful and potentially dangerous electrical shock. Always ensure the spark tester is securely grounded and keep your hands away from the high-tension lead during rotation.
Step 2: Evaluating the Primary and Secondary Coil Windings
Remove the engine shroud to gain access to the magneto coil (armature). Using your digital multimeter, you will perform a resistance test. Every magneto coil consists of two sets of copper wire: a primary winding with few turns of heavy wire and a secondary winding with thousands of turns of very fine wire.
To test the primary winding, place one probe on the small "kill" terminal and the other on the laminated metal core of the coil. The reading should be very low, typically between 0.5 and 2.0 ohms. A reading of "OL" (Open Line) indicates a break in the internal wire, while a reading of 0.0 ohms usually indicates an internal short.
To test the secondary winding, place one probe inside the spark plug boot (ensuring contact with the metal terminal) and the other probe on the metal core of the coil. This reading will be much higher. Most small engine magnetos range from 2,500 to 7,500 ohms, while larger aviation or tractor magnetos may reach 15,000 ohms. If the circuit is open, the internal high-tension wire has likely burned through or corroded.
Step 3: Inspecting and Testing the Contact Points and Condenser
On older engines or vintage motorcycles, the magneto uses mechanical points to break the primary circuit. Remove the flywheel or the points cover. Inspect the faces of the points; they should be smooth and silver. If they are pitted, blackened, or "blued," they are creating excessive resistance. Clean them with a dedicated point file or a piece of cardstock soaked in contact cleaner.
The condenser (capacitor) is wired in parallel with the points to prevent arcing. To test it with an analog meter, set it to the highest ohms scale. Touch the leads to the condenser body and the center wire. The needle should jump toward zero and then slowly drift back toward infinity. If it stays at zero, the condenser is shorted. If it never moves, it is "open" and cannot hold a charge.
Pro-Tip: If you find pitted points, always replace the condenser. A failing condenser is almost always the root cause of point arcing and premature wear.
Step 4: Measuring Magnet Strength and Armature Air Gap
A magneto cannot function if the magnetic flux is insufficient. Test the flywheel magnets by holding a heavy steel screwdriver near the magnetic poles. There should be a strong, "snappy" pull. If the magnets are weak (common in engines that have sat for decades or been dropped), the flywheel must be replaced or re-magnetized.
Finally, check the air gap between the flywheel magnets and the coil legs. Use a non-magnetic feeler gauge (brass or plastic) or a standard business card (which is roughly 0.010 to 0.012 inches thick). Rotate the flywheel until the magnets align with the coil. Loosen the coil mounting bolts, insert the gauge, and allow the magnets to pull the coil onto the gauge. Tighten the bolts and remove the gauge. A gap that is too wide will result in a weak spark, especially at low RPM.
How to Check Ignition Coils - Trusted Testing Methods
Magneto Specification and Resistance Reference Table
The following values are generalized industry standards. Always consult your specific service manual (Briggs & Stratton, Tecumseh, Kohler, Slick, or Bendix) for exact tolerances.
| Component Part | Typical Resistance (Primary) | Typical Resistance (Secondary) | Standard Air Gap / Clearance |
|---|---|---|---|
| Small Engine (Push Mower) | 0.2 – 1.0 Ohms | 2,000 – 5,000 Ohms | 0.010" – 0.014" |
| Chainsaw / String Trimmer | 0.5 – 1.5 Ohms | 6,000 – 10,000 Ohms | 0.008" – 0.012" |
| Vintage Tractor (Mag Type) | 1.0 – 2.0 Ohms | 8,000 – 15,000 Ohms | 0.015" – 0.020" (Points) |
| Aviation Magneto (Slick/Bendix) | 0.5 – 1.2 Ohms | 13,000 – 20,000 Ohms | 0.012" – 0.018" (E-Gap) |
| Outboard Marine Engine | 0.1 – 0.5 Ohms | 3,000 – 7,000 Ohms | 0.010" – 0.015" |
Magneto Failure Scenarios and Field Remedies
Understanding the "why" behind a failure allows for more effective repairs. Below are the most common field failures encountered during magneto diagnostics.
Scenario: The Engine Dies After 15 Minutes of Use
- Root Cause: Thermal expansion in the secondary windings. As the coil heats up, the fine copper wires expand, causing a hairline fracture in the wire to open up, breaking the circuit. Once the engine cools, the wire contracts and contact is restored.
- Actionable Fix: Perform the secondary resistance test while the coil is hot. If the reading is "OL" when hot but 5,000 ohms when cold, the coil is defective and must be replaced.
Scenario: Yellow, Weak Spark at Cranking Speed
- Root Cause: Weak flywheel magnets or excessive air gap. If the magnetic field does not "cut" the windings fast enough or with enough force, the induced voltage will not reach the ionization threshold of the spark plug.
- Actionable Fix: Re-adjust the armature air gap to the minimum factory specification (usually 0.010"). If the spark remains yellow, inspect the flywheel for signs of physical impact or overheating which can de-magnetize the poles.
Scenario: Backfiring and "Wet" Spark Plug
- Root Cause: Sheared flywheel key. The magneto is producing a spark, but the timing is incorrect because the flywheel has shifted on the crankshaft. This often happens if the engine hits a solid object (like a rock or stump).
- Actionable Fix: Remove the flywheel nut and inspect the soft metal key. Even a slight deformation (shearing) of the key will retard or advance the spark enough to prevent combustion. Replace with a factory-spec key; never use a steel substitute.
Frequently Asked Questions
Why does my multimeter show zero resistance on the primary coil?
A reading of exactly 0.0 ohms on a standard digital multimeter often indicates a dead short in the primary winding, meaning the insulation on the copper wire has melted. However, because primary resistance is naturally very low (often under 1 ohm), ensure your meter is "zeroed" by touching the leads together first. If your meter isn't sensitive enough, it may struggle to distinguish between a healthy 0.4-ohm coil and a short.
Can I test a magneto without removing the flywheel?
Yes, you can perform basic spark gap tests and resistance tests on the primary and secondary windings without removing the flywheel. However, if these tests indicate a problem with the points, condenser, or the flywheel key, you will need to remove the flywheel to access those components. Resistance testing through the spark plug wire and kill-wire terminal is the standard non-invasive diagnostic starting point.
What is a "P-Lead" and how does it affect testing?
The P-lead (Primary Lead) is the wire used to ground the magneto to shut off the engine. During testing, the P-lead must be disconnected. If the P-lead is frayed and touching the engine frame, the magneto will be permanently grounded, and you will get a "no spark" result even if the magneto itself is in perfect working condition. Always isolate the magneto from the equipment's wiring before condemning the unit.
Can a magneto be repaired or must it be replaced?
Most modern "solid-state" magnetos (encased in plastic or epoxy) are non-serviceable and must be replaced if the internal windings fail. Older magnetos with external points and condensers are fully rebuildable. You can replace the points, the condenser, and even the high-tension lead separately. If the coil itself is the failure point on an old tractor or motorcycle, "re-winding" services exist, but replacement is generally more cost-effective.
Professional Ignition Component Support
If your diagnostic results fall outside the acceptable resistance ranges outlined in this guide, replacing the ignition module is the most reliable path to restoring engine performance. Ensure you source OEM-certified components to maintain the precise timing and voltage curves required for your specific engine model.
