Whether you do a complete tune-up or only change the spark plugs, spark plug wires (also known as ignition cables) should always be checked first. This is necessary due to the aging of the spark plug wires from exposure to extreme temperatures (hot and cold), engine chemicals (oil, grease, coolant, etc.) and electrical stresses. Common signs of failing spark plug wires include a decrease in power, acceleration, and fuel efficiency. In addition, the engine light coming on or visible damange to the cables can be signs of a failing wire.
What to Look For
Be sure to first check spark plug wires for visual damage. Look for any signs of melting or cracking. The following situations call for immediate replacement of spark plug wires:
- Vibration damage – Constant wear and tear caused by engine vibration can loosen the electrical connection at the spark plug. This causes an increase in the voltage required to fire the spark plug, which could damage the ignition coil as well as the spark plug wires.
- Heat damage – Engine heat can burn wire insulation and boots. A damaged boot can impede proper spark plug wire seating and performance. Wire insulation that’s damaged by head can allow voltage to jump to the ground rather than jumping the gap at the bottom of the spark plug.
- Abrasion damage – When spark plug wires rub against engine parts, particularly sharp edges, it causes cuts and breaks in the insulation. Once this happens, voltage can jump to ground instead of reaching the spark plug.
Tools Needed: Digital multimeter
Use the digital multimeter, set to Ohms Ω. Simply attach or hold a meter probe on each end of the cable and read the total resistance on the meter. The Society of Automotive Engineers suggests the maximum resistance should be 12,000 ohms-per-foot. However, some OEMs have recommended different maximum resistances. Consult a tune-up manual for exact specifications.
Checking For Shorts
Tools Needed: 12 Volt light test
Attach the test light to a good engine ground. With the engine running, move the test light along the length of each spark plug wire. If a spark jumps from a plug wire to the test light at any point, this is an indication that the insulation has broken down, and the spark plug wire should be replaced.
Note: Try to avoid direct contact with metal parts of the vehicle when checking for shorts.
Tools Needed: Spark tester
Remove a spark plug wire from any plug. Attach a spark tester to the wire and to an engine ground. Crank the engine and check for a good spark at the spark tester gap.
A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system. Check the fuel system and/or stark timing.
Weak sparks are orange or red and may be hard to see in daylight. If you did not see a spark, remove the coil wire from the distributor cap. Attach the spark tester to the distributor end of the coil wire. Crack the engine and check for a good spark at the spark tester. If a spark is present, the problem is probably in the distributor cap, rotor or spark plug wires.
It’s important to keep an eye on your spark plug wires to avoid subsequent engine issues. Here’s a guide for installing new wires if it’s time for replacements.
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