How To Test A Magneto With A Multimeter

Magnetos are self-contained ignition systems used primarily in small engines like those found in lawnmowers, chainsaws, and older vehicles. Testing a magneto with a multimeter can help diagnose issues preventing the engine from starting or running correctly. This process involves checking the continuity and resistance of various magneto components.

Understanding Magneto Components

Before testing, familiarize yourself with the key components:

• Primary Coil: Generates the initial low-voltage current.
• Secondary Coil: Steps up the voltage to fire the spark plug.
• Laminated Iron Core: Increases the magnetic field strength.
• Magnets: Create the magnetic field.
• Points (or electronic module): Interrupt the primary circuit, causing the secondary coil to discharge.
• Condenser (Capacitor): Absorbs voltage spikes when the points open, protecting them and improving spark.

Safety Precautions

Always disconnect the spark plug wire before testing. Ensure the engine is cool to avoid burns. Work in a well-ventilated area and avoid fuel spills. Wear safety glasses to protect your eyes.

Tools and Materials

You will need:

• Digital Multimeter
• Small Screwdrivers
• Spark Plug Wrench
• Clean Rags
• Magneto Tester (Optional but Recommended for Comprehensive Testing)

Testing the Primary Coil

The primary coil is responsible for creating the initial low-voltage current that the magneto uses to generate the spark.

Continuity Test

Set the multimeter to the continuity setting (indicated by a sound wave symbol or an Ohm symbol). Touch one probe to the primary coil's terminal (typically connected to the points or electronic module) and the other probe to the engine's ground or the magneto's frame. A beep or a reading close to zero indicates continuity. No continuity (infinite resistance) suggests a broken wire or open circuit.

Resistance Test

Set the multimeter to measure resistance (Ohms). Consult the engine's service manual for the specified resistance range for the primary coil. Touch one probe to the primary coil's terminal and the other to the engine's ground. Compare the reading to the specification. A reading outside the specified range indicates a faulty coil. A very low reading (near zero) suggests a short circuit.

Testing the Secondary Coil

The secondary coil steps up the voltage generated by the primary coil to a level sufficient to create a spark at the spark plug.

Continuity Test

Set the multimeter to the continuity setting. Touch one probe to the spark plug wire terminal and the other probe to the engine's ground or the magneto's frame. A beep or reading close to zero indicates continuity. No continuity signifies a break in the coil windings.

Resistance Test

Set the multimeter to measure resistance (Ohms). Consult the engine's service manual for the specified resistance range for the secondary coil. Touch one probe to the spark plug wire terminal and the other to the engine's ground. Compare the reading to the specification. The secondary coil's resistance is usually much higher than the primary coil's. A reading outside the specified range indicates a faulty coil.

Testing the Points (or Electronic Module)

The points, or the electronic module in newer magnetos, interrupt the primary circuit, which causes the secondary coil to discharge, generating the spark.

Points Test

Disconnect the wire leading from the points to the coil. Set the multimeter to the continuity setting. With the points closed (use a screwdriver to manually close them if needed), touch one probe to one side of the points and the other probe to the other side. You should hear a beep or see a reading close to zero, indicating continuity. With the points open (use a screwdriver to manually open them), you should have no continuity (infinite resistance). If the points show continuity when open, they are likely dirty or worn and need cleaning or replacement.

Electronic Module Test

Testing an electronic module typically requires a specialized magneto tester. A simple multimeter test may not accurately diagnose module failures. However, you can check the wiring and connections for continuity and shorts. Consult the service manual for specific testing procedures.

Testing the Condenser (Capacitor)

The condenser protects the points from excessive arcing and helps to improve the spark.

Continuity Test

Disconnect the condenser from the circuit. Set the multimeter to the continuity setting. Touch one probe to the condenser terminal and the other probe to the condenser's body (ground). You should hear no beep or see infinite resistance. A beep or reading close to zero indicates a shorted condenser, which must be replaced.

Capacitance Test (Requires a Multimeter with Capacitance Measurement)

Set the multimeter to the capacitance setting (Farads). Touch one probe to the condenser terminal and the other probe to the condenser's body (ground). Compare the reading to the condenser's specified capacitance value (usually printed on the condenser). A significantly different reading indicates a faulty condenser.

Note: If your multimeter doesn't have a capacitance setting, this test cannot be performed.

Visual Inspection

Always perform a visual inspection of all components. Look for:

• Cracked or broken insulation on the coils.
• Corroded or loose connections.
• Worn or pitted points.
• Damaged wiring.
• Rust or corrosion on the magnets.

Troubleshooting Tips

• If you suspect a problem with the grounding, clean the magneto's mounting surface and ensure good contact with the engine block.
• Ensure the air gap between the magneto's armature and the flywheel is correct. Consult the service manual for the specified air gap.
• If you replace any components, use high-quality replacements specifically designed for your engine.

Interpreting Results

Carefully compare your test results to the manufacturer's specifications. Deviations from the specified ranges indicate potential problems. Combine your test results with visual inspection to accurately diagnose the issue. In many cases, replacing a faulty component will restore the magneto to proper operation.

If the magneto still doesn't produce a spark after testing and replacing faulty components, consider consulting a qualified mechanic or small engine repair technician. Complex magneto problems may require specialized equipment and expertise.

Summary

Testing a magneto with a multimeter is a valuable skill for diagnosing and resolving engine starting and running problems. By checking the continuity and resistance of the primary and secondary coils, points (or electronic module), and condenser, you can identify faulty components and potentially save time and money on repairs. Regular magneto maintenance and testing can extend the life of your engine and ensure reliable performance.