How to Check Plug Wires With a Multimeterby Jack Hathcoat
Testing spark plug wires with an multimeter is a good way to verify they are in working condition. When spark plug wires fail the car will hesitate, stumble and jerk, especially going up a hill or under a load. If the car runs especially badly on a rainy day, that can be one sign that plug wires are faulty. Using an multimeter, or ohm meter, will verify if the wires are capable of delivering the required voltage to the spark plug, but a close inspection of the insulation and plug boots is also necessary.
Remove the wire to be tested. Carefully examine it if it is a carbon wire, which is used on most domestic cars. This wire design generates heat due to its resistive qualities and can break down over time. Look for heat marks, cracks or insulation failure. If the wire is an import, it may have a winding design. This creates an inductive field around the wire that contains the interference. Inspect it for similar failures.
Set the ohmmeter to the appropriate scale and measure the wire. For the domestic wire a good reading is no greater than 12,000 ohms per foot. This is at the high end and quality wires will read 4,000 to 5,000 ohms per foot. Refer to the repair manual for import plug wire specifications. Test the wire from end to end with the leads of the meter. The lead colors do not matter for this test: one at one end, one at the other end. Note the reading.
Lubricate inside the plug wire boot before installing it onto the spark plug. Heat sears the boot onto the porcelain insulator and can make it difficult to remove. Boot release can be found at any auto parts store. Replace any defective wires before re-installing. If two or more wires are found to be defective on the vehicle being tested, purchase and install a complete set.
Things You'll Need
- Tape measure
- Spark plug boot release
Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.