How to Test an Ignition Coil

Written by derek odom
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While there are several ways to test your vehicle's ignition coil, by far the easiest is the simple arc test, which can be done with only two tools and a pair of gloves. Another way is through the implementation of a multimeter and usually requires removing the coil from the vehicle, which is more in-depth than most of us really need. Be careful any time you test the ignition coil because there is usually high voltage involved.

Skill level:

Things you need

  • Socket set with a spark plug-sized attachment
  • Insulated pliers
  • Gloves (optional, but highly recommended)
  • An extra person to turn the ignition key

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  1. 1

    Disconnect one spark plug temporarily. To do this, remove the spark plug wire from it by pulling on the boot part, closest to the plug. Once the wire is detached, slip on the correct-sized socket (although many plugs are 13/16 in size, they will vary depending on make, model and year) and unscrew the plug from the engine's cylinder head. Once the spark plug is completely removed, reattach the spark plug wire by pushing the boot over the ceramic portion of the plug until you feel it snap on.

  2. 2

    Place the exposed end of the spark plug against a metal portion of the engine or chassis. Many times, the exhaust manifold is the best place to hold the spark plug against because they normally have the least amount of paint. However, many times the valve cover or intake manifold will work, as well. Just be sure the surface you hold the plug against is not caked with grease or fuel, as there will be a live spark when the coil is tested. Also, hold the spark plug wire with rubber-insulated handled pliers and wear thick gloves as well. The normal ignition coil puts out between 20,000 and 40,000 volts.

  3. 3

    Crank the engine and watch for a bright blue spark. For this step, you will need a helper to be inside the automobile, operating the ignition key. Once the spark plug is against the metal, have the person inside the car, turn the key as if to start it. If there is a bright blue spark, the coil is probably fine. If there is a spark, but it is faint, the coil might be working, but not putting out enough volts to operate the vehicle correctly. You should be able to see the spark easily in broad daylight. If nothing happens or the spark is faint, the coil is probably bad and should be replaced immediately.

Tips and warnings

  • The engine only needs to be cranked over for a second or two to notice a spark.
  • Let the engine cool off before working on it.

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