Introduction: How to Test a Spark Plug Wire

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Video tutorial on how to test a spark plug wires. Over time spark plug wires will wear out, causing various engine operation issues such as hard starting, misfires, rough idling, poor performance, poor fuel economy, and improper combustion burn. Spark plug wires normally last to about 100,000 kilometres or 60,000 miles. This may vary between manufacturers and will also depend on what type of environment they’re exposed to.

Tools/Supplies Needed:

  • spark plug wire
  • multimeter
  • spray bottle with water
  • flashlight

Step 1: Inspection

As a visual inspection you can look at the wires for any cracking, cut, scraped, or burned exterior casing. Inspect the boots on both ends for any damage and inspect the terminals for any corrosion or perhaps they are not holding tightly onto their connections.

Step 2: Spark Leakage

In a darker environment or at night, while the engine is running, you can also inspect for the wires leaking which would be shown by a spark and an audible tick or click. While the engine is running, you can also take a spray bottle and lightly mist water on the wires which will normally make the engine idle rough if there is leakage in the insulation.

Step 3: Multimeter Test With Old Wires

For copper core wires, the resistance will be 1 to 6500 ohms, inductive wires the resistance will be 2200 to 8000 ohms per 1 meter or 650 to 2500 ohms per foot and carbon wires, the resistance is 10000 to 23000 ohms per 1 meter or 3000 to 7000 ohms per foot. Keep in mind these are generic values and may vary between vehicle manufacturers and spark plug wire manufacturers. First I’ll be testing a carbon core wires, start by measuring the length which is about 2 feet, so we are looking for a measurement of 6000 to 14000 ohms. Setting the multimeter to the ohms sitting to measure resistance, on the 20k setting which will show us values in the 10,000 range. Touch the test probes together to determine the existing resistance in the test leads, then hold one test probe on one terminal, the other probe on the other terminal, this wire show’s 10,000 ohms, which by generic specs is good. Even if the plug wires meet within their specifications, it doesn’t necessarily mean they are still in good condition as the insulation could be failing, terminals are corroded, or perhaps the conductor inside is broken and that would be show up if you were bending the wire while testing it creating and open circuit.

Step 4: Multimeter Test With New Wires

Testing an almost brand new wire, these were replaced about 5000 kilometres ago. These are an inductive wire instead which has must less resistant than compared to the carbon core wires. This wire is about 2 feet long so we should be expecting about 1300 to 2500 ohms. We are looking at 940 ohms. While this is below the generic range, these are a higher quality wire so the resistance is less, so as mentioned before, check your vehicle’s specifications or the spark plug wire manufacturer specifications.

Step 5: Bonus (Wire Cross Section)

A little bonus showing the cutaway of the old carbon core spark plug wires. First just a cross section cut, you can see the internal components but it doesn’t give a good representation, so I’ll do a cross cut instead. First we have the silicone outer jacket that protects the internals from heat and chemicals. Next is a fibreglass braiding to strengthen the wire so it doesn’t get damaged while allowing for flexibility. The insulation stopping any voltage leaks. Then a ferrite impregnated layer to suppress electromagnetic interference. And finally the carbon impregnated fibreglass core, where the electricity flows. You can actually see the black residue on my finger is carbon.

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