How to Test Spark Plug Wires

Updated on March 30, 2016

Testing spark plug wires helps you diagnose common engine problems:

  • cylinder misfires
  • rough idle
  • loss of power
  • hard-to-start issues
  • an increase in gas consumption.

Failing spark plug wires cause all these above issues and will ultimately clog your catalytic converter. Overall, a weak—or lack of—spark due to a bad wire, or another failed ignition component, has a negative effect on engine performance and mechanical components.

Spark plug wires deliver thousands of volts (up to 50,000 or more) to the spark plugs. To carry such large voltages, plug wires need to have some basic physical characteristics. For example, thick insulation to prevent high voltage from jumping off the wire before it reaches the spark plug, and a rubber boot at each end to protect metal connectors from moisture, corrosion, oil, and other contaminants.

Yet, miles of service and the harsh environment in which these wires operate has its toll. The internal conductor breaks or burns, failing to deliver the necessary voltage to them. The insulation deteriorates, leaking voltage to nearby wires or metal surfaces (ground), leaving the plugs with a weak spark or no spark at all.

Still, failing or bad spark plug wires share their symptoms with many other engine components going bad. So you need to verify that you actually have one or more damaged or worn out ones.

To check the wires, apply the next series of tests before deciding to spend money on a new set of wires.

Tools You'll Need:

  • 1- or 2-foot jumper wire
  • Screwdriver with a wooden or plastic handle
  • Spark plug wire pliers, if necessary
  • Clean rag
  • Tape measure
  • Digital or analog multimeter

Checking spark plug wires for better engine performance.
Checking spark plug wires for better engine performance. | Source

Testing for Voltage Leaks

Spark plug wires with bad insulation affect engine performance, especially during damp weather on rainy days. This test will check your wires' insulation for voltage leaks.

  1. Connect one end of a jumper wire to a screwdriver metal shank.
  2. Then, connect the other end of the jumper wire to a metal bracket or screw on the engine to ground the screwdriver.
  3. Set the emergency brakes on your vehicle, start the engine, and let it idle.
  4. Now, run the tip of the screwdriver along the length of each spark plug wire, about an inch away from the insulation.

Pay attention to any spark or electric arc jumping off the insulation toward the tip of the screwdriver. Even if you can't see the arc, you'll hear a cracking sound as the spark jumps to the screwdriver, so pay attention to any sounds as well.

  • Did you detect any sparks or snapping sound during your test? Replace the wires.
  • If your wires appear to have no voltage leaks, go on to the next two tests.

Note: For the next two tests, work on one spark plug wire at a time. That is, disconnect one wire, proceed with the two tests, reconnect the wire, and then test the next wire. This way, you'll plug each wire back to its corresponding cylinder. If you don't, you'll upset the firing order of the ignition system, and the engine won't operate properly, if at all.

Digital Multimeter

Innova 3320 Auto-Ranging Digital Multimeter
Innova 3320 Auto-Ranging Digital Multimeter

UL certified, auto-ranging scales, single-setting resistance function and much more.

 

Testing Spark Plug Wire Insulation

Having a spark plug wire with bad insulation may not be as bad as a wire with a broken conductor. Still, this is enough for your ignition system to cause all kinds of engine performance problems, especially during damp weather. This test will help you check your wires for voltage leaks.

  1. Connect one end of a jumper wire to a screwdriver shaft and the other end to a metal bracket or screw on the engine to ground the screwdriver.
  2. Set the emergency brakes on your vehicle, start the engine, and let it idle.
  3. Run the tip of the screwdriver along the length of each spark plug wire, about an inch away from the insulation. Pay attention to any spark or electric arc jumping off the insulation toward the tip of the screwdriver.

Did you detect any sparks during your test? Replace the wires.

  • Note: Checking them is not a difficult task. But, just as with many other components in your vehicle, spark plug wires don't last forever. For this reason, manufacturers provide a service schedule for them. Check the service interval for your wires and, if necessary, replace them, even if they look in good condition.

When replacing the wires, keep these guidelines in mind:

  • Change them one at a time
  • Replace one wire with another of the same length
  • Follow the same routing path and secure the wires to their clips to keep them away from sensitive circuits and hot surfaces.

How to Test

Checking Insulation

After checking for voltage leaks, you'll need to check the physical condition of the wires' insulation.

  • Work on one wire at a time.

To check:

  1. Disconnect one spark plug wire: Grab the wire's boot and carefully twist the boot back and forth as you pull it off the spark plug. If you can't reach the boot, use a set of spark plug wire pliers.
  2. Do the same at the other end of the wire.
  3. Wipe the wire clean using a rag to remove any dirt, grease. and oil.
  4. Now, carefully inspect the entire wire and boots. Look for any signs of wear and damage like cracks, worn spots, heat marks on the insulation and boots. Look for corroded and loose metal clips or terminals inside the boots.
  • If you see any sign of wear or damage, replace all the spark plug wires.
  • Perform the next test on the same wire before reconnecting it.

Car Ignition System

Old ignition system with mechanical contact breaker points.
Old ignition system with mechanical contact breaker points. | Source

Testing Spark Plug Wires Resistance

With this test, you'll check the condition of the conductor that runs through the thick insulation of your spark plug wires. For this test, use a simple analog or digital ohmmeter.

  1. Set your meter to the ohms scale and to a setting that will let you read 50,000 ohms or above.
  2. Measure the length of the wire you want to read first using the tape measure, and make a note of the length in feet on a piece of paper.
  3. Then, turn on your ohmmeter and connect one lead to a metal connector inside the spark plug wire boot and the other lead to the other connector in the wire.
  4. Make a note of the wire's resistance on the piece of paper as well.
  5. Lastly, you'll want to multiply the length of the wire in feet by the ohms-per-foot specification for your vehicle.
  • Most domestic vehicles use spark plug wires with a fiber glass core covered in latex graphite, which provides between 10,000 and 12,000 ohms of resistance per foot of wire. On the other hand, many import models like Nissan, Honda, and Toyota use spiral-wound stainless steel mag wires with approximately 500 ohms of resistance per foot of wire. Still, other models may use a different type of wire.
  • Check the resistance specifications for your wires in the service manual for your particular vehicle make and model. If you don't have the manual, buy an aftermarket repair manual online or at your local auto parts store. Or check the reference section of your local public library. However, the specifications described in the previous paragraph give you a close approximation of the resistance "health" of your wires.

Compare the result to the reading from your ohmmeter. If you get a higher reading than specified for your wires, replace all spark plug wires. Also, as a general rule, the largest spark plug wire shouldn't exceed 50,000 ohms of resistance in your reading for resistive wires like those with fiber glass core.

Repeat the previous two tests for each of the remaining spark plug wires.

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Tips

When replacing the wires, keep these guidelines in mind:

  • Change them one at a time; replace one wire with another of the same length, follow the same routing path and secure the wires to their clips to keep them away from sensitive circuits and hot surfaces.
  • Follow the service schedule provided in your vehicle service manual by inspecting or replacing the spark plug wires to prevent engine performance issues.

Checking spark plug wires requires a simple procedure, but they're easy to leave out of your maintenance routine. Just like any other component in your vehicle, spark plug wires wear out. That's why many manufacturers provide a service schedule for them. Check the service interval for your wires and, if necessary, replace them, even if they look in good condition.

Questions & Answers

    Comments

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      • Dan Ferrell profile imageAUTHOR

        Dan Ferrell 

        3 years ago

        hardlymoving - yes, that's a very simple way to look for spark leaks. Forgot about it. Thanks.

      • hardlymoving profile image

        hardlymoving 

        3 years ago from Memphis, TN

        Another simple test is to look for spark arcs from the wires while the engine is running at night.

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