Ignition coils are an essential component of a vehicle’s ignition system. They are responsible for providing the high voltage required to create the spark that ignites the air-fuel mixture in the engine’s cylinders. Unfortunately, ignition coils can and do eventually fail. Identifying the root causes of ignition coil failure and addressing them promptly is key to avoiding performance issues, drivability problems, and engine damage. In this 5000 word article, we will take an in-depth look at the most common reasons ignition coils fail.
What is an Ignition Coil?
An ignition coil is a high voltage transformer that converts the 12 volts of electricity from a vehicle’s battery into the thousands of volts required to jump the spark plug gap and ignite the compressed air-fuel mixture in the engine’s cylinders.
Inside the ignition coil are two coils of wire wrapped around an iron core. The primary coil has a few hundred turns of heavy wire. The secondary coil has many thousands of turns of fine wire. As current passes through the primary coil, a magnetic field builds up. When the current is cut off, the magnetic field collapses rapidly, inducing a high voltage across the secondary coil. This voltage pulse is sent to the spark plug.
The ignition coil serves a vital purpose – without the ignition coil producing the required high voltage, the engine would not run. Ignition coils must produce a spark strong enough to jump the spark plug gap under compression, which can require upwards of 40,000 volts.
Ignition Coil Failure Modes
There are several ways in which ignition coils can fail or become damaged. The main failure modes include:
Worn Insulation
The windings inside an ignition coil can short out if the insulation breaks down from heat, vibration, and normal wear over time. Insulation damage allows voltage to leak or arc between the coil windings, reducing the amount of voltage available to fire the plugs. This often causes misfires and poor performance.
Overheating
Ignition coils can overheat from extended periods of heavy load operation, a clogged cooling system, damaged spark plugs, improper spark plug gaps, bad plug wires, low battery voltage, or failure of the coil itself. Overheating causes the insulation to breakdown.
Vibration Damage
The constant vibrations in a running engine can fatigue the ignition coil windings and connections. Bad engine mounts or rough running can accelerate vibration damage. This can lead to open or shorted windings.
Electrical Damage
Voltage spikes from the charging system can damage the sensitive electrical components inside the coil. Corrosion at the electrical connections can also cause resistance and voltage drops.
Bad Spark Plugs/Plug Wires
Worn or damaged spark plugs and plug wires increase the workload on the ignition coil. The increased electrical resistance forces the coil to produce more voltage output to compensate. This strains the coil windings.
Oil Fouling
Oil leaks from worn valve seals, the valve cover gasket, or plug seals can foul the ignition coil boot and terminals. The oil provides an undesirable conductive path that allows voltage to leak to ground. This decreases the coil’s efficiency.
Manufacturer Defect
Sometimes ignition coils are just defective right out of the box, due to substandard materials or poor quality control during manufacturing. This is uncommon, but can happen.
7 Most Common Causes of Ignition Coil Failure
Now that we’ve covered the ways ignition coils can fail, let’s count down the 7 most common specific causes of ignition coil failure.
1. Age and Mileage
Ignition coils are designed to last the life of a vehicle under normal conditions. However, over time the constant heat cycling, vibration, electrical stress, and normal wear adds up. The insulation slowly breaks down and resistance increases. After 5-10 years and 100,000+ miles, ignition coils often reach the end of their lifespan and fail due to the accumulated effects of aging.
2. Overheating
Heat is the enemy of ignition coils. As covered earlier, excessive heat breaks down the coil’s internal insulation. Overheating can be caused by extended heavy load operation, clogged cooling systems, fouled spark plugs, bad plug gaps or wires, low battery voltage, or coil failure. Replacing old spark plugs and wires and cleaning cooling systems can help prevent overheating.
3. Oil Fouling
Oil leaks saturating the coil boots and terminals is a common cause of misfires and ignition coil failure. The oil provides a path for voltage to leak to ground instead of the spark plugs. Careful inspection, cleaning, and replacement of any oil-soaked components typically fixes the problem. Addressing the source of the oil leak is critical.
4. Electrical Spikes
Spikes in voltage from the charging system or environment can puncture the coil’s insulation and damage the windings. Lightning strikes are one example. Using surge protectors and properly grounding electrical systems helps protect the coils.
5. Loose Wiring Connections
Corrosion and vibration can loosen the wiring connections to the ignition coil terminals. This increases resistance which reduces voltage to the spark plugs. Replacing worn terminals and securing connections repairs the problem.
6. Defective Coil
While uncommon, manufacturing defects do result in some coils that fail prematurely. Usually these defects are discovered and the coils recalled and replaced. But occasionally a defective coil will slip through quality control. Replacing it under warranty or through a recall fixes the issue.
7. Failed Spark Plugs/Wires
Bad spark plugs and worn plug wires place more voltage strain on the ignition coil. The increased resistance forces the coil to boost output voltage to compensate. This strains and heats the coil windings over time, eventually leading to failure. Replacing worn plugs and wires restores normal operating conditions.
Ignition Coil Failure Symptoms
How can you tell if you have an ignition coil that is failing or has failed completely? There are some key symptoms to watch for:
Misfires
As ignition coils start to fail, they may struggle to provide adequate voltage to consistently fire the spark plugs. This results in occasional misfires and sputtering as cylinders skip combustion cycles. Misfires get worse as the coil further deteriorates.
Rough Idling
Misfiring cylinders from a bad coil cause the engine to run unevenly at idle. The engine may sound rough, shake, or cut in and out.
Loss of Power
Failed coils reduce the engine’s power output since cylinders are not properly igniting the air/fuel mixture. Acceleration will suffer.
Decreased Fuel Economy
Misfires waste fuel in the unburned air/fuel mixture, lowering fuel economy.
Flashing Check Engine Light
The onboard computer detects misfires through the crankshaft position sensor. Severe misfires trigger the check engine light to flash. Diagnostic trouble codes pinpoint the faulty cylinder(s).
Failed Emissions Test
Excess hydrocarbons in the exhaust from misfiring cylinders cause emissions test failures.
Catching ignition coil failures early provides the best chance to replace coils before engine performance suffers.
Diagnosing Bad Ignition Coils
Here are some tips for diagnosing bad ignition coil conditions:
Check Diagnostic Trouble Codes
The check engine light will illuminate and sound diagnostic trouble codes when engine misfires are detected. These codes indicate which cylinder’s ignition coil is likely having issues.
Conduct Spark Plug Inspections
Remove each spark plug and check for signs of electrode wear and unusual discoloration from misfires. Compare across cylinders to isolate problem areas.
Electrical System Tests
Use a digital multimeter to check coil primary and secondary resistance. Out of spec readings indicate internal winding damage. Verify battery voltage is stable and not low.
Compression & Leak down Tests
This checks for low compression from worn pistons/rings that place more strain on the coils.
Cylinder Power Balance Test
This confirms a weak cylinder by disabling cylinders one by one to identify power changes.
Coil/Plug Swap
Swapping coils and plugs between cylinders can aid troubleshooting by transferring problems.
| Test | How It Diagnoses Coil Issues |
|---|---|
| Trouble Codes | Pinpoint misfiring cylinder(s) |
| Spark Plug Inspection | Reveal misfire effects |
| Electrical Tests | Check winding and voltage specs |
| Compression/Leak Tests | Check for abnormally high mechanical loads |
| Power Balance Test | Verify weak or dead cylinders |
| Coil/Plug Swap | Transfer problems to isolate bad coils |
Thorough testing can definitively diagnose failing or bad ignition coils.
Ignition Coil Replacement
Once diagnosed, bad ignition coils must be replaced to restore engine performance. Here are some key ignition coil replacement considerations:
Matching Coils
Always replace failed coils with ones matching the original part number and specifications. Mixing coil types risks compatibility issues.
Plug Gap
Confirm the spark plug gap is set correctly after new coils are installed. Incorrect gaps create more work for the coils.
Boot Condition
Inspect the coil boot well for cracks or damage that could lead to arcing. Replace if deteriorated.
Connector Fit
There should be no gap or wiggle between coil electrical connectors and ignition harness. Tight fit ensures good electrical contact.
Coil Mounting
The coils must be securely mounted to avoid vibration damage. Inspect any rubber isolators or brackets.
Clear Oil Leaks
Eliminate any oil leaks that could foul new coils.
Heat Protection
Thermal shields or heat sleeve installs protect replacement coils from heat damage.
Taking care to properly install matched, high quality replacement coils minimizes chances of repeat failures. Don’t ignore or reinstall any questionable old coils.
Ignition Coil Care Tips
Proper maintenance and care extends the lifespan of ignition coils:
– Use only approved replacement coils that match OEM part numbers.
– Ensure good plug wires and correct spark plug gaps.
– Fix any engine oil leaks.
– Clean coils and boots to remove oil and carbon fouling.
– Ensure ignition system connections are clean and tight.
– Replace worn spark plugs.
– Verify charging system voltage is stable.
– Install thermal protection sleeves if exposed to excessive heat.
Well maintained ignition coils often last over 100,000 miles. But inevitably coils do age out and fail. Following a robust maintenance regime reduces chances of premature coil failures.
Conclusion
Ignition coil failure is a common issue once engines accumulate higher mileage. Causes such as age, heat damage, electrical spikes, oil fouling and defective parts are often to blame. Symptoms like misfires, rough idle and power loss alert drivers to failing coils. Validated through testing, bad coils must be promptly replaced and properly installed to restore optimal engine performance. While no maintenance can make coils last forever, careful care and upkeep helps maximize the ignition coil lifespan. With this knowledge of the coil failure process, drivers can be alert to signs of trouble and proactively address any issues.