Modern automobile ignition systems would be incomplete without ignition coils. Distributor ignition systems have largely been phased out in favor of electronic-controlled ignition systems in most engines since the 1990s. The ignition is controlled by coil packs or ignition coils in these modern electronic systems.
Most modern cars have a distributor-less electronic ignition system that distributes power to all the spark plugs via a master ignition coil, but older models may still have distributor-based systems.
There are a number of different ignition coil configurations to choose from.
Coil-on-Plug (COP): When using a system known as a coil-on-plug, the ignition coil is mounted directly on top of the spark plug in the cylinder. A separate coil is located in the head of each cylinder and in each spark plug.
Coil-near-Plug (CNP): The ignition coils cannot always be placed directly on the spark plug due to space constraints. When this occurs, the spark plug wires are used to connect the ignition coils instead of the plug itself.
Coil-per-Cylinder (CPC): Systems with a single ignition coil per cylinder can be either COP or CNP, but only use one type of ignition coil. One coil can power two cylinders instead of the CPC system in modern ignition systems.
Whenever possible, use Coil-on-Plug systems because they do away with the need for spark plug wires, which add another point of failure to the ignitoning system.
Coil packs are another name for ignition coils. When referring to a group of ignition coils, we will occasionally use the term “coil pack” even though it is incorrect.
What Are Ignition Coils?
Combustion is required to start and run engines. In order for something to burn, you need three different things: air, fuel, and a spark. While spark plugs are responsible for the actual spark, electricity is required to power them.
Thousands of volts of electricity are required to light a spark plug. Car batteries are typically only 12 volts, which is far below the voltage required to light a spark plug.
Electrical transformers, in general, are what ignition coils are. Coils used to ignite spark plugs take in 12-volt battery voltage and turn it into tens of thousands of volt voltages needed for proper operation of the spark plugs.
How Do Ignition Coils Work?
A coil of wire serves as a conductor inside a spark plug. To move electrical currents through the wire coil, a circuit switch is attached to the coil.
When the switch is opened, the coil’s windings are filled with current. In this way, a magnetic field is created around the coil, amplifying the current’s voltage. The magnetic field collapses back into the coil when the switch is closed, allowing it to be transferred to the spark plug.
Nowadays, the majority of ignition coils use a two-coil wire setup. Using a 12-volt battery, the first coil generates a magnetic field with a voltage that can be 10x-20x greater than the initial voltage. As a result of the second wire’s compactness, it generates 100 times more voltage through the magnetic field, allowing for voltage levels of up to 20,000 volts.
Denso’s technical explanation of how ignition coils work can be found here if you’re interested in learning more.
Bad Ignition Coil Symptoms
- Cylinder misfires
- Hard starts
- Rough idling
- Hesitation or stutter under acceleration
- Exhaust backfires
- Check engine light (codes for misfires)
- Worse fuel economy
Misfires are the most obvious sign of a bad ignition coil, especially when driving fast. When an ignition coil fails, the spark plug is unable to ignite because it lacks sufficient voltage. A buildup of gasoline in the cylinder can lead to a misfire if it detonates due to heat at the wrong time.
When the engine misfires, you’ll notice a jerk or hesitancy in the driving experience. The coil will begin to degrade and you will start to notice misfires more frequently. Start-up issues are more likely if multiple ignition coils fail at the same time, but even when only one fails, it can still be a problem.
Engine codes P0300-P0312 are typically set for misfires in most vehicles. The specific error code will let you know which cylinder is having trouble firing properly.. Codes P0350-P0362, associated with the ignition coils, will appear on some vehicles.
Bad ignition coil symptoms are very similar to bad spark plug symptoms. Before purchasing and installing a new set of ignition coils, make sure you remove your spark plugs and inspect them. When replacing ignition coils, many people will also replace the spark plugs because the symptoms are identical and they’re relatively inexpensive.
How to Test Ignition Coils
The coils themselves should be inspected first and foremost. If they have cracks, burn marks, are melted, or are smeared with oil, they are most likely faulty. Try these methods for testing ignition coils if the coils appear to be in good condition upon visual inspection:
1) Swapping Coils
Swapping ignition coils is the simplest way to determine whether your problems are caused by faulty spark plugs or ignition coils.
Instead of swapping out the ignition coils for every misfire, try swapping the coils for all of them at once. After you’ve cleared the engine codes, drive the car until another misfire occurs, and then re-check the codes.
A bad ignition coil will cause the misfire code to now say cylinder 1. It’s possible your spark plug is bad if you try this method and the engine code still indicates cylinder 2 as bad.
Using the same methodology, you can swap out the wires in cars that use spark plugs. To avoid confusion, do not swap the ignition coil with the wire. Otherwise, you will not be able to tell if the problem is with the wire or the coil.
2) Using a Multimeter
If swapping coils isn’t an option, the next best thing is to check the resistance of the ignition coil’s coil wires for Ohms. Invest in a multimeter and connect the positive and negative coil wire terminal leads with it. Both the primary and secondary windings’ ohm readings must be checked.
Depending on how it was made, every ignition coil will have a different set of contact points. Verify that all of the coil windings have proper Ohm values by consulting your owner’s manual or a repair manual such as Haynes.
Due to the fact that ignition coils operate differently with and without load, this method has the potential to produce incorrect results.
3) Other Testing Options
Your best bets are the two options listed above. However, if you also have spark plug wires and need to perform multiple swaps in order to diagnose the problem, we recommend methodology
#1. Coils can be tested without removing them from the vehicle using ignition coil testers and inline spark testers. More expensive coil-on-plug testers allow you to test coils without removing them from the vehicle.
Ignition Coil Replacement Cost
While this isn’t true for every car, anyone with the right tools can swap out the spark plugs and ignition coils on the majority of modern cars.
There are two options when it comes to replacing your ignition coils: either replace only the bad ones or replace them all. Most people prefer to replace them all at once, but this can be expensive, especially if you have a V8 or something similar.
The majority of the time it takes to replace them is getting to them in the first place, so if you plan on having them replaced at a repair shop, doing them all at once will save on labor costs.
Single Ignition Coil Replacement Costs
- Parts: $20-$40
- Labor: $80-$150 or approximately 1 hour of labor costs
Ignition Coil Set Replacement Costs
- Parts: $100-$400 (cost increases as engine gets bigger ie. inline-4, V6, V8)
- Labor: $120-$250 or approximately 1.5 hours of labor costs
Ignition Coils vs. Spark Plug Wires
Spark plug wires are used in vehicles that do not have coil-on-plug systems. Voltage is generated by ignition coils and transferred to the spark plugs by the spark plug wires.
We know that bad ignition coils cause the same symptoms as bad wires because the wires carry the voltage. The coils and wires are frequently replaced together, but if you’re only interested in swapping out the problematic one, use the testing methods described above to identify whether the problem is with the coils or wires.
Automobiles without spark plug wires will have a coil-on-plug setup that attaches the ignition coils right on top of the spark plug head, eliminating the need for extra transport wires and lowering the risk of a component failing.
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