How to Check Spark Plug Wires

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It is very important to make spark plug wires part of your car inspection and maintenance schedule. The insulation and conductor inside the wires wear down due to high engine temperatures and constant use. Eventually, plug wire failure makes it difficult for the ignition coil high voltage to reach the combustion chamber. In turn, fuel consumption increases and engine performance and drivability suffer. But you can prevent these problems following these a few steps that make your regular inspection easier, help you determine the condition of the spark plug wires and help you decide whether it is time for a change.

Things You'll Need

  • Spark plug wire removal tool
  • Clean rag
  • Ohmmeter
  • White dielectric grease
  • Standard screwdriver
  • 3- to 4-foot jumper wire
  • Make sure the engine is cold or you may burn your hands while trying to unplug the wires.

  • Open the hood of your vehicle and locate the spark plug wires. Depending on the number of cylinders, you should see the same number of thick wires--about 1/4 inch in diameter--coming out of the engine block and going to a round distributor cap or square component coil pack, depending on your particular ignition system.

  • Grab one of the spark plug wires by the boot at either end of the wire. With a gentle twisting motion. carefully unplug it. Unplug the other end of the wire in exactly the same way. This is the best way to remove a spark plug wire without damaging the connectors inside. If it is hard for you to reach the wire’s boot, use a spark plug wire removal tool. If the metal connector inside the boot comes loose, replace the wire.

  • Wipe clean the entire wire and check the insulation for cuts, cracks or burns. Look inside the wire boot and check for corrosion or damage. If you see any signs of deterioration, replace the wire.

  • Check the plug wire resistance using an ohmmeter. Touch the metal connectors inside the wire boots with the meter leads to get a resistance value. You should read at least 12,000 ohms per foot of plug wire but no more than 50,000 ohms total for a long wire; otherwise its resistance is worn down and you should replace it.

  • Apply a small amount of white dielectric grease to the metal connectors at both ends of the spark plug wire to prevent corrosion build up. Use a standard screwdriver to apply the grease if necessary. Install the wire back in its original location, making sure both ends snap into place.

  • Check the rest of the spark plug wires, one by one, repeating steps 3 through 6.

  • Start the engine. Connect one end of the long jumper wire to the engine ground and the other end to the metal shaft of a standard screwdriver. Slowly move the tip of the screwdriver along each spark plug wire, from boot to boot, about 1 inch from the insulation. If you see a thin line of light or hear a series of soft clicks--a spark arcing from the wire insulation to the tip of the screwdriver--the insulation is bad and the wire needs to be replaced.

Tips & Warnings

  • Spark plug wires should be replaced at recommended manufacturer intervals since the wire resistance wears down after a period of time. Check your vehicle service manual for manufacturer recommendations.
  • It is best to replace spark plug wires as a whole set to make sure all will provide a good conduit for the high voltage required by the combustion process.
  • Be very careful when working on a running engine. Use short sleeve shirts and do not wear any jewelry or wristwatches as these can get caught between moving engine parts and cause severe injuries. Also keep tools and wires away from moving engine parts.

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