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HomeTips & GuidesSpark Plug Wires – Everything You Need to Know

Spark Plug Wires – Everything You Need to Know

From the outside, spark plug wires look like basic components that carry sparks to the spark plug from the coil. 

However, the most vital part ensures the proper working of the engine, ignition, and other components. 

With damaged spark plug wires, you risk engine misfiring and stalling, onboard electronic malfunctioning, and low power, among other issues.

 That’s why today we discuss what spark plug wires are, their types, and ways to test their functionality and credibility.

What Is a Spark Plug Wire?

Precisely, they function in old combustion engine vehicles to carry a high voltage current of up to 45000V. 

Very old engine models (1970 and 80s) used coil wires, while modern ones now use coil-on-plug ignition

It’s also important to note that spark plug wires only feature in gasoline engines while diesel ones don’t have them. 

Each spark plug wire features thick insulation, e.g., silicon or EPDM rubber, to carry high voltage successfully without loss. 

This insulator prevents current loss and protects the wire from arcing and heat. 

In addition to the flexible insulator, its terminals feature a rubber boot to enhance insulation while preventing moisture and corrosion. 

Going on, you get a single yet enough wire in each cable to carry power one way to the distribution. 

Remember, no need for two wires as the grounding wire is at the negative battery terminal. 

Finally, its ends come with a metal terminal to clip onto the spark plug and distributor. 

A separated spark plug wire set

A separated spark plug wire set

Types of Spark Plug Wires

To know the type you have, check the printing on yours. 

This printing includes the conductor type, insulator thickness and material, manufacturer name, and cylinder number. 

With that clear, let’s see the main types of spark plug wires available.

Distributed Resistance Wires

Distributed resistance wires types of spark plug wires feature a fiberglass construction, impregnated with carbon. 

They are mostly used as a standard spark plug wire in most ’80s American vehicles. 

Typically, these cables have high resistance, which helps them convey high-voltage currents. 

In addition, the resistivity in this material helps reduce radio frequency interference (RFI) that a normal ignition system produces. 

As you know, unchecked RFI can malfunction radios, sensors, computers, and other car electronics.

 The point to note here is that if your car comes with such a spark plug wire, ensure you buy the same during replacement.

Magnetic (Mag) Resistance Wires

Mag wires began with the high rise of the Asian manufactured cars. They feature a copper-nickel alloy core construction, wound spirally. 

This material has low resistivity, requiring very minimal current to produce a spark. 

While this material makes the wire efficient, it also combines with the spiral winding to reduce RFI.

Fixed Resistor Wires

There are also fixed resistor wires commonly found in European vehicles. Their construction is usually steel or copper for high conductivity.

 These spark plug wires come with an inbuilt resistor for electromagnetic interference reduction. 

Symptoms for Damaged Spark Plug Wires

Although spark plug wires feature high-quality material construction, like all other things, they also age, which reduces efficiency. When this happens, you may notice symptoms such as:

  • High fuel consumption
  • Decreased acceleration
  • Low power
  • Engine check light illumination 
  • Stalling
  • Onboard electronics malfunctioning, such as the GPS and radio. 
  • Rough idle
  • Fuel in the cylinder
  • Engine misfiring
  • Starting issues.

It is important to note that the above symptoms are not native to spark plug wires only. 

Some other engine components show the same symptoms, too. With that said, let’s now see how you can know a faulty spark plug wire.

Old spark plug wires

Old spark plug wires

How to Test Your Spark Plug Wires

Before you can administer these tests, you will need:

  • 30 to 60 cm jumper wire
  • Plier that removes spark plug wires
  • Clean rag
  • Ohm meter or multimeter (digital or analog)
  • Rubber mat
  • Tape measure
  • Manufacturer’s manual

After that, you must disconnect and check each wire and reconnect it when done. This procedure ensures the correct connection of each wire to its cylinder.

 Finally, while reconnecting the wires, avoid putting them close to a hot system or other electronics.

Inspecting Visual Damage

As obvious would have it, the first step is to check for signs of melting, burning, abrasion, and wear and tear.

 Before checking for these visual defects, ensure you are in a well-lit area and inspect each wire. 

Disconnect and wipe it clean with a clean cloth to make the damage visible. Start with the insulation, then check the boot at both ends for visible corrosion. 

Check the spring clips to ensure the wire remains in the right position when plugged. 

Once the wire looks fine, reconnect it to its position before disconnecting and inspecting the next one.

Start the Engine

In this next step, you want to check for electrical arcs on the wires. To do so, you must run your engine and check the wires carefully. 

You may also listen to any sparking or snapping noise which comes as a result of high-voltage escape. While doing so, ensure not to touch wires, lest you get electrocuted.

Testing Resistance of Your Wire

Armed with the multimeter we listed above, it’s now time to check if the resistance of each wire is as inscribed.

 As we said, do not forget to disconnect, test, and return each wire at ago. To conduct the test, disconnect the first wire and clean it to make its readings visible. 

Now, set your multimeter to the appropriate resistance depending on your wire. From there, let the multimeter probes touch the wire metallic terminals. 

Check if the written resistance is the same as on the multimeter.

 If so, reconnect the wire and go to the next. However, if the reading is lower than inscribed, replace it with a new one. 

Inspect the others, and if you find more than two wires have issues, I recommend replacing the whole set.

A yellow digital multimeter at work

A yellow digital multimeter at work

Inspect Spark Plug Wire Routing

In this section, you will require your vehicle manual. Now check if the routing of the spark plug wires is correct as it appears on the manual. 

While here, check for any cross-coupling that may bring about energy drain, hence low spark.

 Additionally, ensure your routing is away from hot parts, such as the exhaust part, as this may cause insulator cracking.

Checking Shorts

You will require a test light (12V) tool to ensure the engine is running. Once you have this tool, ground it appropriately to a suitable engine part. 

Then, inspect each wire with the test light by moving it along it. If the wires are ok you do not expect any results. 

However, if the spark comes to the tester light at one point, the insulator is broken and needs replacing.

Spark Test

In the final test, you need a spark test to check the spark quality. Now unplug the spark plug wire you wish to test and connect it to the spark test. 

From there, ground the gadget to the engine and then start your engine. Check the spark tester gap to see if the spark is blue to indicate it’s strong enough. 

On the other hand, if the spark is red or orange, it’s weak, meaning the wire has an issue.

A blue-white spark

A blue-white spark


Now you know that the spark plug wire insulation is the main key to your car starting and functioning properly. 

I recommend checking it always for deformities, especially after 100,000 miles, as many manufacturers recommend. 

If it’s not causing your car issues, check other problems such as grounding and engine harness.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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