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How To Test Spark Plug Wires (4 Methods + 2 FAQs)

March 12, 2022

Like any other ignition system component, spark plug wires can also wear out over time and cause a misfire. 

So, if you plan to check them yourself, you should know how to test spark plug wires the right way. 

To that end, we’ll go through the different methods for testing spark plug wires and answer a couple of related questions

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Let’s begin!

How To Test Spark Plug Wires?

Here are the four methods you could use individually or together to test a spark plug wire set (also known as the ignition wire): 

Important: While troubleshooting these spark plug wire sets, check each wire separately and reconnect it before moving on to the next. This helps maintain the firing order of your ignition system and prevents any misfire. 

Some of these tests require a running engine, and any misstep could lead to a high voltage electric shock. 

So, if you’re not familiar with the engine block, it’s best to call an auto repairs professional to test the spark plug wires for you. 

That said, let’s proceed with how to test spark plug wires. 

Method 1: Do A Visual Inspection 

Visually inspect each spark plug wire for any corrosion or insulation loss signs. 

Here’s how: 

  1. Park your car in a well-lit area.
  1. Pop your car’s hood and locate the spark plug wires on the engine block. Each plug wire comes out of the cylinder head at one end and is attached to a distributor (and ignition coil) or coil pack on the other end.
  1. Remove the spark plug wire using pliers. 
  1. Clean the wire with a cloth and look for any cuts or scorch markings on the silicone insulation. Lack of wire insulation can allow the voltage to jump to ground, and the spark plugs won’t be able to power the ignition.
  1. Next, check the spark plug boot for any discoloration or damage in the area where the connection was made. 

Note: Also known as an ignition boot or plug wire boot, a spark plug boot connects the ignition cable to the spark plug.

  1. If there’s no discoloration, you could apply some dielectric grease around the inner wall of the boot. This’ll protect the electrical connectors from corrosion and dirt.
  1. Next, look for corrosion between the boot, the spark plug, and the coil. 

If you notice any sign of damage, it’s time to replace your spark plug wire with a new wire. 

Did you know?
Many modern vehicles use a coil on plug ignition that doesn’t require a long wire to connect the ignition coil to the spark plug. 

Method 2: Check For Spark Leakage

If there’s no visible damage, use a spark plug tester or the spray test to check for electrical arcs around the plug wires. 

Caution: Since you’ll be doing these tests with the engine running, don’t touch any spark plug wire to prevent high voltage shocks. 

A. Ignition Spark Test

Get hold of a spark plug tester to perform these steps: 

  1. Disconnect your car’s battery
  1. Remove the old wire from a spark plug. 
  1. Attach the tester to the spark plug wire and an engine ground. 
  1. Now, reconnect your battery and crank your engine.
  1. With the engine running, look for a spark in the spark tester gap. 
  1. If there’s no light, the spark plug isn’t receiving an electrical charge, which could be due to a faulty spark plug wire, bad ignition coil, or damaged remote coil pack.

Sometimes the fault could also be with the distributor. 
To check this: 

  1. Remove the coil wire from the distributor cap. 
  1. Attach the tester to the distributor end of the coil wire.
  1. Crank the engine and check for a good spark.
  1. If a spark is present, the problem is probably in the distributor, rotor, or spark plug wires. 

B. Spray Test 

If you don’t have a spark tester, you can perform this DIY spray test to check for spark leakages from a bad wire.

Here’s how you can do it:

  1. Grab a spray bottle filled with water. 
  1. Turn on your engine and lightly spray the water along the spark plug wires, focusing near each plug wire boot.
  1. Look for any arcing near the spark plug. Also, pay attention to any snapping noise from a high voltage leak (that’s about 20,000 volts or more of voltage). 
  1. If you notice any arc, turn off your engine and inspect that particular ignition wire. 
  1. Remove the faulty spark plug wire and check the boot’s inside for carbon tracking (black residue). The black build-up indicates an improper connection that may cause a misfire. 
  1. Replace the defective wire and the bad spark plug, if needed. 

Note: You could use a well-insulated, grounded screwdriver and run its tip along the spark plug wires to check for any electric arc. However, only use an insulated screwdriver, or you could suffer an electric shock. 

Method 3: Run A Resistance Test

If you can’t identify any bad spark plug wire with the above two tests, run each ignition cable through a resistance test. It’ll help you assess the conductor’s condition under the thick insulation of your old wire. 

Tools required: 

Here’s how: 

  1. Set your ohm meter to the appropriate scale (50,000 ohms or above).
  1. Remove the spark plug wire and measure its length with the tape. 
  1. Turn on your ohm meter and place one probe on each end of the wire. Make sure they touch the metal contacts.
  1. Note down the wire’s resistance.
  1. Check the resistance specifications for your spark plug wires in your owner’s manual.
  1. Next, multiply the length of the wire in feet by the ohms-per-foot specification for your vehicle. 
  1. Compare the value to the reading on your ohm meter. 
  1. If you get a higher reading than specified, replace the defective wire. 

The Society of Automotive Engineers suggests the maximum spark plug wire resistance should be 12,000 ohms/foot. However, some OEMs recommend different maximum resistances. 

For example, the resistance of the wire for a high-output ignition system can be in the 5000 ohms/foot range, whereas a low resistance wire could do with double-digit ohms per foot.

Method 4: Spark Plug Route Inspection 

You need to ensure that the spark plug wire is routed properly as per your owners’ manual. 

Wonder why? 
Cross-coupling of spark plug wires can cause an energy drain and lower the engine performance. 

It’s also important that the wires don’t come in contact with the hot engine parts, as this could lead to an insulation loss. 

Now you know how to test bad spark plug wires. 
Next, we’ll look at some of the frequently asked questions. 

2 FAQs On Spark Plug Wires

Here are answers to some spark plug wires questions: 

1. What Damages The Spark Plug Wires?

A spark plug wire can get damaged due to: 

  1. Engine Vibration: Constant vibration can loosen the electrical connectors at the spark plugs. The spark plugs will then require more voltage for ignition, which can damage the ignition coil and the spark plug wires. 
  1. Engine Block Heat:  High engine heat can burn the wire insulation, allowing the voltage to jump to ground instead of reaching the spark plugs. 

2. What Happens If My Spark Plug Wire Fails? 

If you have bad spark plug wires, you could experience: 

However, these symptoms could also indicate other engine component failures. So, it’s recommended to get your spark plug wires tested to ascertain if a bad wire is causing an ignition problem. 

Final Thoughts

Failing spark plug wires isn’t something that you should ignore. It’s best to test and replace them as a part of your regular vehicle maintenance. 

But, if you’re not comfortable testing spark plug wires yourself, contact RepairSmith!

We’re a convenient mobile auto repair and maintenance solution offering competitive, upfront pricing. Our ASE-certified mechanics can quickly test and replace your car’s spark plug wires right in your driveway and fix any other ignition problem. 

Fill out this form for an accurate cost estimate for spark plug wire replacement or any other engine repair!