Test Spark Plug Wires
A spark plug wire is a basic but critical engine component. It carries the high voltage (30k to 50k volts) produced by the ignition coil to the spark plug. When exposed to heat and vibration, the carbon in the wire may loosen and reduce the conduction between the coil and the spark plug. For optimal engine performance, spark plug wires should be replaced periodically. We'll show you how to test spark plug wires so that you may be able to diagnose or circumvent a variety of problems, including engine misfires and radio static.
Steps
- Check for symptoms. Defective spark plug wire leave tell-tale signs of their deterioration, including:
- An erratic idle
- Engine misfire
- Radio interference
- Decreased fuel mileage
- Failed emissions tests due to elevated hydrocarbon emissions or a fault code indicating cylinder misfire.
- Check engine light
- Inspect the wires. Use a flashlight or an area with good lighting to perform a visual inspection of the spark plug wires.
- Look for physical damage to the wires or boots such as cuts and scorch marks.
- Check the spark plug wire, and examine the insulation around the wire.
- Look for damage due to high heat from the engine compartment (scorch marks).
- Check for corrosion between the boot and the spark plug and coil.
- Run the engine. Look for electrical arcs or listen for a snapping noise that may indicate a high-voltage electrical leak.
- Use a jumper wire to ground a well-insulated screwdriver. Drag the screwdriver down the length of each spark plug wire, around the coil, and the boots. If you see an arc from a wire to the screwdriver, you have defective wire.
- Use a spray bottle with water to spray down the wires. You will want to especially spray near the boots that attach to the spark plugs to see if any arcing occurs, and if mist is present. If arcing is found at the boot near the spark plug, turn off the engine. Then remove the boot from the plug and check inside the boot for carbon tracking. If carbon tracking is present, the spark plug will also need to be replaced.
- Perform a spark plug wire resistance test. Locate the spark plug's resistance rating in the vehicle's factory service manual. You can also search online for the resistance of after-market wires.
- Use an ohm meter to determine if the resistance of the wire is in compliance with the recommendations. Place the probes on either end of the wire lead, making sure they touch the metal contacts, and verify that they're within the range called for in the manual.
- Inspect the spring clips that hold the wires in the distributor cap. Damaged clips may allow the wires to slip and not hold their proper position.
- Determine if your spark plug wires are routed properly. Look in the vehicle's manual to help you with this task. Cross coupling can create an energy drain.
Tips
- Don't remove all your wires at once—do each one, one at a time, then replace.
- Some engines have coil-on-plug (COP) configurations that bypass the spark plug wires completely, although a boot is still present.
- Keep the spark plug wires clean to reduce conduction drain.
- Do not assume that crossing spark plug wires is a bad sign. Some manufacturers do this to cancel out magnetic fields.
Warnings
- If you have a pacemaker or a similar device, you should not test spark plug wires.
Things You'll Need
- Flashlight
- Jumper wire
- Screwdriver with insulated handle
- Ohm meter
