How to Check an Ignition Coil With a Spare Spark Plug


Before solid state ignition systems and computerized engine management systems, checking ignition coil output was simply a matter holding the coil wire near the engine block while you cranked the engine over. The color of the spark as well as the length of the spark was a good indication of the condition of the ignition coil. This method of checking spark can cause irreparable damage to modern solid state ignitions systems and computerized engine management systems because of the resulting surge of high voltage current. You can buy a special tester that allows you to view the spark safely, or you can make your own from a spare spark plug for a fraction of what the tool would cost you.

Things You'll Need

  • Sparkplug
  • Length of sparkplug wire
  • Spark plug gapping tool
  • Vehicle service manual
  • Select a relatively long spark plug wire to use with your tester. You want a wire long enough so you can position your tester safely away from fuel lines and fuel injector air intake to eliminate the danger of igniting any fuel or fuel vapor.

  • Adjust the spark gap on the spark plug that you will be using to test your ignition coil to the gap specified for the spark plugs in your engine as specified in your vehicle's service manual.

  • Insert one end of the spark plug wire in the tower on the ignition coil and attach the other end to the spark plug.

  • Wedge the spark plug tightly against a clean are of the engine block. A good ground is essential to controlling the high voltage current surge, which is what damages sensitive computers and solid state ignition components. A solid ground is also essential for the test to give you a good reading on the condition of the ignition coil. Make sure that the spark plug/tester is positioned in such a way that you have an unobstructed view of the spark plug's electrodes.

  • Observe the spark as a helper cranks the engine over. A solid bluish-white spark indicates a good ignition coil. If the spark appears weak, check to make sure that the wire is properly seated in the coil and firmly attached to the spark plug before you condemn the coil as being bad. Also, make sure that the spark plug is firmly grounded before you decide that the coil is bad.

Tips & Warnings

  • Use caution when testing the high voltage side of the ignition system. Some solid state ignition systems have voltages that exceed 30 or 40 thousand volts. The current is so minuscule that the chances of electrocution is nil but the nasty shock can cause you to injure yourself by yanking your hand back and striking sharp objects.

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  • Photo Credit old spark plugs image by Rog999 from
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