Spark Plug Test Cable Harness Wiring Diagram To test a spark plug (car) do the following:

  1. Connect the spark plug to the high tension cable. Ground the outer electrode (main body), and crank the engine. In the atmosphere, only short sparks are produced because of the small discharge gap. If the spark plug is good, however, sparks will occur in the discharge gap (between the electrodes). In a defective spark plug, no sparks will occur because of a leak through the insulation.
  2. Check the cap and outer shell for cracks.
  3. Measure the resistance (If resistance is higher than 10,000 Ohm per foot of cable, replace the cable)

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