How to Make a Magneto Tester
Using tightly wrapped wiring around a rotating magnet, a magneto uses movement, usually provided by an engine, to create an electric current. This current is often used to provide electricity to spark plugs in gasoline engines, and because of the prevalence of gas-fueled engines, magnetos are common in everything from automobiles to lawn mowers. The wire winding makes magnetos difficult to visually diagnose when engines have spark-plug issues, so test a magneto to determine if it still creates current.
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Instructions
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1
Determine the amount of current the magneto to be tested should produce using your operator’s manual or another resource. Damaged magnetos may be able to spark, though not produce enough current necessary to power the plugs. Determine if the magneto’s spark is adequate to perform in the engine.
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2
Determine the amperage of the current the magneto produces when operating properly by research in an operator’s manual or repair guide. Apply the amperage figure and voltage using Ohm’s law to determine the resistance of the spark plug assembly, using the formula R = voltage/amperage.
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3
Purchase a resistor with an impedance value as close to the value of R determined in Step 3 without exceeding the value. If necessary, purchase multiple resistors to approximate the total load of the spark plug assembly.
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4
Connect one of the resistor’s terminals to a LED, using insulated copper wire and wire stripper. Attach a separate piece of insulated copper wire to the other contact on the LED, and a third piece of wire from the resistor’s open terminal, creating an open circuit that runs as wire, resistor, wire, LED, wire.
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5
Strip the final inch of insulation off each unattached end of wire. Insert wire into both ends of a magento’s terminal, and rotate the magneto by grasping its central arm and turning. If the LED lights while the magneto spins, that terminal operates properly and produces enough current to meet the engine’s needs. If the LED fails to light, the magneto needs to be repaired or replaced.
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