What Is an Ignition Coil? Coil Packs, Symptoms & What They Do
Does your car have trouble starting? Does it stall, misfire, backfire or get worse gas mileage than it used to? If you’ve noticed any of these symptoms, you may have a failing ignition coil or coil pack. Below, the ASE-certified technicians at ExperTec Automotive explain what an ignition coil is, what it does, how many your car has and how to tell when one is going bad.
What Is an Ignition Coil?
An ignition coil, also known as a spark coil, is an induction coil in your car’s ignition system that transforms the battery’s low voltage into the thousands of volts needed to create a spark at the spark plugs and ignite the fuel. Diesel engines don’t use ignition coils, because they rely on compression to ignite the air/fuel mixture.
On older cars, a single coil sent high voltage through a distributor and spark plug wires (high tension leads) to each spark plug. Distributors started disappearing in the late 1980s. Today, ignition is controlled electronically, and most engines use one coil per cylinder or one coil per pair of cylinders.
What Does an Ignition Coil Do?
The coil builds up energy in a magnetic field and then releases it as a high-voltage pulse at exactly the right moment. That pulse travels to the spark plug, jumps the gap at its tip and lights the fuel in the cylinder. In a running engine this happens thousands of times a minute.
What Is a Coil Pack?
When a car has individual coils for each cylinder or pair of cylinders, they may all be contained in one molded block with multiple high-tension terminals. That block is called a coil pack. Modern coils can also be mounted directly on top of each spark plug, called coil-on-plug or direct ignition. The symptoms of a failing coil pack are the same as a failing ignition coil.
Ignition Coils and Spark Plugs
The ignition coil and the spark plug are two different parts that work together: the coil makes the voltage and the plug delivers the spark. A worn spark plug forces the coil to produce more voltage to fire, which adds heat and stress and is one of the most common reasons coils fail. When we replace a coil, we check the plugs too. See our tune-up service for spark plug replacement.
Signs of a Bad Ignition Coil
- Check engine light, steady or flashing
- Engine misfires, shaking or rough idle
- Hard starting or stalling
- Loss of power and hesitation when accelerating
- Backfiring
- Worse fuel economy (the engine burns more fuel to make up for the lost power)
Your car’s computer will usually set a check engine light when it detects misfires or a problem in the coil circuit. A check engine light can have many other causes, so having the trouble codes read by a professional is highly recommended. If the light is flashing, read what a flashing check engine light means. Continuing to drive with a bad misfire can damage your catalytic converter, so get it taken care of as soon as possible.
Ignition Coil FAQ
What is an ignition coil?
An ignition coil (also called a spark coil) is a small transformer that turns your car battery’s 12 volts into the 20,000 to 45,000+ volts a spark plug needs to fire. Every gasoline engine has at least one. Without a working coil, the spark plug can’t ignite the air and fuel in the cylinder.
What does an ignition coil do?
The coil stores energy in a magnetic field, then releases it all at once as a high-voltage pulse when the engine computer tells it to. That pulse jumps the gap at the spark plug tip and lights the fuel. It happens thousands of times a minute, in perfect timing with the engine.
What is a coil pack on a car?
A coil pack is several ignition coils molded into one block, with a tower or terminal for each spark plug wire. Coil packs are common on engines that use a ‘wasted spark’ system, where one coil fires two cylinders. Many people also use “coil pack” for the individual coil-on-plug coils found on most newer cars.
Coil pack vs. ignition coil: what’s the difference?
They do the same job. “Ignition coil” is the general name for the part. A coil pack is just a group of coils built into one unit. On a coil-on-plug engine each cylinder has its own separate coil, so if one fails you usually replace only that coil. With a one-piece coil pack, the whole pack is replaced.
Are ignition coils the same as spark plugs?
No. The coil creates the high voltage; the spark plug sits inside the cylinder and turns that voltage into a spark. They work as a team, and a worn spark plug makes the coil work harder, which is a common reason coils fail. That’s why we often recommend replacing worn plugs when a coil is replaced.
How many ignition coils does a car have?
It depends on the ignition system. Coil-on-plug engines (most cars built in the last 20 years) have one coil per cylinder, so a 4-cylinder has 4 coils, a V6 has 6 and a V8 has 8. Wasted-spark engines have one coil for every two cylinders, so a 4-cylinder has 2. Older cars with a distributor have just one coil.
Where is the ignition coil located?
On most modern engines the coils sit on top of the engine, plugged directly into each spark plug under a plastic engine cover. On older cars the single coil is usually mounted near the distributor, on the fender or on the firewall, connected to the plugs with spark plug wires.
What does a coil pack look like?
A coil-on-plug coil looks like a plastic tube or boot, a few inches long, with an electrical connector on top. It pushes straight down onto the spark plug. A traditional coil pack is a rectangular plastic block with two or more posts where the spark plug wires attach.
What are the symptoms of a bad ignition coil?
Common signs are a flashing or steady check engine light, engine misfires, rough idle, shaking, hard starting, loss of power, backfiring, stalling and worse gas mileage. Misfire codes such as P0300 to P0308, or coil circuit codes like P0351 to P0358, often point to a coil.
Can I drive with a bad ignition coil?
It’s not a good idea. A misfiring cylinder sends unburned fuel into the exhaust, which can overheat and destroy the catalytic converter, a much more expensive repair. If your check engine light is flashing, reduce speed and get the car checked as soon as possible.
How long do ignition coils last?
Many ignition coils last 100,000 miles or more, but heat, engine vibration, oil leaks into the spark plug wells and worn-out spark plugs can make them fail early.
Think your ignition coil is failing? Our ASE-certified technicians can pinpoint the problem with a full engine diagnostic and fix it right, backed by our 3-year/36,000-mile nationwide warranty. Book at Huntington Beach, Costa Mesa or Anaheim.