AutoTips Spark Plug Wire tips page.
Plug Wires (or Spark Plug Wires, Ignition wires) connect the spark plugs to the distributor or ignition coils. Unlike most wires, plug wires are normally designed to have a lot of resistance. The resistance helps reduce radio static that the ignition system generates. DO NOT replace resistance wires with non-resistance wires unless your ignition system was designed for them. Using non-resistance wires may damage modern ignition systems that were designed with the resistance as part of the circuit. Spark plug wires also have high voltage insulation to carry the 30,000 volt or higher voltage of the ignition system. Newer vehicles often have an ignition coil mounted directly above the spark plug, and do not use traditional spark plug wires.
Tip: Keep them
clean. A thin film of dirt can form on the outside of the plug wire
and around the boots at the ends of the wires can form conductive path
to the high voltage when exposed to moisture. The conductive path can drain
away the current that should go to the spark plug. If your car misses
out or runs rough only in damp weather, you may have this type of wire
problem.
A mechanic sprayed the wires on my car around
the distributor with a misting spray bottle filled with water (like one
used to spray plants). The car died. He repeated the experiment a couple
of times with the same result. I tried the same thing when I got home,
with the same results. I cleaned the wires as well as the inside and outside
of the boots (I used Armor All cleaner - use something rated safe for rubber
and vinyl), and then treated the wires with Armor All "protectant".
This time, spraying the wires had no effect, the car continued to run smoothly.
Testing Plug Wires: If you have access to a factory service manual for your car, you may find a resistance rating for the spark plug wires, (such as 1000 - 4000 ohms per foot). If you know how to use an ohm meter, you can measure the plug wires resistance from end to end and see if they are within specifications. Look at the connectors, check for corrosion. Insulation on plug wires can break down, and they may arc to ground or other wires. Crossed Wires: You may see guides on your car that have the plug wires crossed over each other, rather than in nice parallel lines. This is intentional to cancel out magnetic fields. Wires in parallel may cause one wire to induce a voltage in another wire, causing it to fire when it shouldn't . Plug wire problems can result in miss-firing, hard starting, and poor fuel economy. If you change plug wires, you may want to replace them one at a time to keep from mixing them up to the wrong spark plug.
Testing
spark plug wire resistance with a multi-meter.
What goes bad in spark plug wires:
- Vibration can break the wire inside of the insulation, often at one of the connectors.
- The resistance of the wire can go up with age.
- The insulation can break down, allowing leakage of the energy to the engine body (ground) rather than to the plug. Check for cracks in the wires.
- Corrosion at the connectors.
What makes a good replacement spark plug wire:
- Sometimes after-market replacements may be listed to work for a wide range of vehicles, and are not a good match for your vehicle. Although the specifications are not easy to find, the best place to look is a factory service manual. Measure the new ones with an ohm meter. They don't have to be exact, but if they are 5 or 10 to 1 high or low, try another brand.
- There are other differences not easy to detect -- quality of the insulation, how well the wire will stand up to vibration, etc. The old saying "you get what you pay for" tends to apply when buying replacement wires.