How to Test a Single Fire Coil

Save money by testing a single fire ignition coil yourself.

The single fire ignition coil is a critical component of any internal combustion engine's electrical system. The coil is a simple electrical transformer that contains a primary and secondary set of copper windings. By passing a current through the primary windings, a magnetic field is created. When the primary current is turned off by means of mechanical points or electronic controls, a high voltage is created that fires the engine's spark plugs. Testing a single fire coil for proper operation is a simple task that can be done at home and without the need for an expensive professional mechanic.

Things You'll Need

• New or known good spark plug
• Digital multimeter
• Combination wrench set

Instructions

1. Remove the high tension lead from the distributor cap. Connect a new or known good spark plug to the end of the high voltage lead. Place the spark plug base on a good ground such as the engine cylinder head. Position the spark plug so the electrode is visible.

2. Turn the ignition key to the "start" position and crank the motor over. Observe the spark plug and look for a blue or bright yellow spark. If a strong spark is present, the ignition coil is good. If no spark is present, proceed to the next step.

3. Locate the ignition coil positive (+) terminal. Turn the ignition switch to the "run" position. Set the multimeter to the "volts" function. Place the red lead of the multimeter on the positive (+) terminal of the ignition coil. Place the black lead of the multimeter to an engine ground or battery negative (-) terminal. The multimeter should read either six or 12 volts depending on your vehicle specifications. If no voltage at all is present, then there is a problem with the primary ignition circuit to the vehicle that must be corrected.

4. Disconnect the positive (+) and negative (-) lead wires from the ignition coil using a combination wrench. Turn the multimeter to the "ohms" function. Place the red lead of the multimeter on the positive (+) terminal of the coil. Place the black lead of the multimeter on the negative (-) terminal of the coil. The multimeter should display a resistance of about 0.4 to 0.6 ohms.

5. Move the red lead of the multimeter to the coil output terminal while keeping the black lead on the coil negative (-) terminal. The multimeter should show a resistance of about 5000 to 7200 ohms. If the readings from the primary or secondary windings show a zero "0" or infinity "1" then the coil is bad and must be replaced.

Tips & Warnings

• The ignition coil generates very high voltages during operation. When testing the coil, keep hands, metal tools and flammable materials away from the high tension lead.

References

• Ignition System Troubleshooting: Clark's Garage
• How to Diagnose and Repair Automotive Electrical Systems; Tracy Martin