Experiencing engine misfires or rough idling? A faulty ignition coil could be the culprit. The ignition coil plays a vital role in your car’s engine, providing the high-voltage spark needed to ignite the fuel-air mixture. Knowing How To Check A Coil is a valuable skill for any car owner, potentially saving you time and money on unnecessary repairs. This guide will walk you through the steps to test your ignition coil effectively, helping you pinpoint ignition problems and get your engine running smoothly again.
Understanding Ignition Coils and Their Importance
Before diving into testing, let’s understand what an ignition coil does. In simple terms, it transforms your car’s 12-volt battery power into the thousands of volts needed to create a spark at the spark plugs. This spark ignites the air-fuel mixture in the combustion chamber, powering your engine. Modern vehicles often use individual coils for each cylinder (coil-on-plug), while older systems might use a single coil and distributor. Regardless of the system, a failing coil can lead to significant engine performance issues.
Symptoms of a Bad Ignition Coil
Recognizing the signs of a failing coil can help you diagnose the problem quickly. Common symptoms include:
- Engine Misfires: This is perhaps the most noticeable symptom. A misfire occurs when one or more cylinders fail to fire correctly, resulting in a jerky or sputtering engine. You might feel this especially during acceleration.
- Rough Idling: A bad coil can cause the engine to idle unevenly, feeling rough or shaky when you’re stopped.
- Reduced Fuel Efficiency: Misfires lead to incomplete combustion, wasting fuel and decreasing your car’s gas mileage.
- Check Engine Light: Often, a misfire will trigger the check engine light. Diagnostic trouble codes (DTCs) related to misfires (like P0300, P0301, P0302, etc.) can be read with an OBD-II scanner.
- Tachometer Problems: In some cases, a faulty coil can cause erratic readings on your tachometer, as mentioned in the original forum. The tachometer might bounce around or give inconsistent readings.
- Difficulty Starting: In severe cases, a completely failed coil can prevent the engine from starting at all.
If you’re experiencing any of these symptoms, testing your ignition coil is a logical next step.
Tools You’ll Need to Check a Coil
Fortunately, checking an ignition coil doesn’t require a vast array of specialized tools. Here’s what you’ll generally need:
- Multimeter: A digital multimeter is essential for testing coil resistance. These are readily available and relatively inexpensive.
- Basic Hand Tools: Depending on your vehicle, you might need wrenches or sockets to access the ignition coil. Consult your car’s repair manual for specific instructions.
- Spark Plug Tester (Optional): While not strictly necessary for basic coil testing, a spark plug tester can visually confirm if the coil is producing a spark.
- Safety Gear: Wear safety glasses and gloves for protection.
Step-by-Step Guide: How to Check a Coil
Here are several methods to test your ignition coil, ranging from a simple visual inspection to more detailed electrical tests. Always ensure your engine is off and cool before starting any tests.
1. Visual Inspection
Begin with a visual check of the ignition coil. Look for any obvious signs of damage:
- Cracks or Damage to the Coil Housing: Inspect the plastic casing for cracks, chips, or physical damage.
- Leaking Oil: If your spark plugs are located in wells, check for oil leaks around the coil. Oil contamination can damage coils.
- Corrosion on Terminals: Examine the electrical terminals for corrosion or loose connections. Clean any corrosion with a wire brush if found.
- Damaged Wiring: Check the wiring harness and connectors leading to the coil for frayed or broken wires.
Visual inspection is the first step in checking an ignition coil. Look for physical damage like cracks in the housing, which can indicate a coil failure.
While a visual inspection can reveal obvious problems, it won’t catch internal coil failures. For more thorough testing, you’ll need a multimeter.
2. Resistance Testing with a Multimeter
Testing the resistance of the ignition coil is a common method to check its internal windings. You’ll measure two types of resistance: primary and secondary.
a) Primary Resistance Test:
- Set up your multimeter: Set your multimeter to measure Ohms (Ω) on a low resistance scale (e.g., 200Ω or the lowest setting above the expected primary resistance).
- Locate the terminals: Identify the primary terminals on your ignition coil. These are typically the smaller terminals that connect to the car’s wiring harness (often labeled “+” and “-“). Refer to your vehicle’s service manual or coil markings if unsure.
- Connect the multimeter: Touch one multimeter probe to one primary terminal and the other probe to the other primary terminal.
- Read the resistance: Note the resistance reading on your multimeter.
- Compare to specifications: Consult your vehicle’s repair manual or the coil manufacturer’s specifications for the correct primary resistance range. A typical range for many coils is around 0.5 to 3 Ohms. Readings significantly outside this range indicate a potential problem. For example, the forum mentioned values around 3 Ohms for LT (low tension) leads, which refers to the primary side.
b) Secondary Resistance Test:
- Set up your multimeter: Keep your multimeter set to measure Ohms (Ω), but you might need to increase the range to a higher setting (e.g., kΩ or MΩ) as secondary resistance is much higher.
- Locate the terminals: Identify the secondary terminals. One probe will go on one of the primary terminals (either one) and the other probe will go on the high-tension (HT) terminal where the spark plug wire connects (or the spring contact in coil-on-plug systems).
- Connect the multimeter: Touch one probe to a primary terminal and the other probe to the secondary (HT) terminal.
- Read the resistance: Note the resistance reading.
- Compare to specifications: Again, refer to your vehicle’s service manual or coil specifications for the correct secondary resistance range. Typical values are in the kilo-Ohms (kΩ) range, often between 6,000 to 15,000 Ohms or higher. The forum mentioned 7500+ Ohms from LT to HT. An open circuit (infinite resistance) or a reading far outside the specified range suggests a faulty coil.
Using a multimeter to test the primary resistance of an ignition coil. Ensure you have the correct resistance specifications for your coil type.
Testing the secondary resistance of an ignition coil. This test checks the high-voltage winding within the coil.
Important Note: Resistance tests are helpful in identifying open circuits or short circuits in the coil windings. However, they may not always detect subtle insulation breakdowns that can cause misfires, especially under high temperature and load conditions, as ErnieY pointed out in the forum.
3. Spark Test (Substitution Method)
As Terry Ingoldsby mentioned in the forum, the most definitive test for a coil is often substitution. This involves replacing the suspect coil with a known good one and seeing if the problem resolves.
- Acquire a known good coil: Ideally, use a new coil or one you know is working correctly. As Jim (LENZ) considered in the forum, a temporary replacement from an auto parts store can be useful for testing.
- Replace the suspect coil: Carefully remove the suspected faulty coil and install the known good coil in its place. Ensure all connections are secure.
- Start the engine and observe: Start your engine and see if the misfire or other symptoms are gone.
If the problem disappears after replacing the coil, it’s highly likely the original coil was faulty. If the issue persists, the problem might lie elsewhere in the ignition system or engine.
Alternative Spark Test (Use with Caution):
Warning: This method involves working with high voltage. Exercise extreme caution to avoid electrical shock.
- Disconnect the spark plug wire: Carefully disconnect the spark plug wire from the spark plug of the cylinder you suspect has a bad coil (or from the coil if it’s a coil-on-plug system).
- Insert a spark plug tester or spare spark plug: Connect a spark plug tester in line between the spark plug wire and the spark plug, or carefully ground the base of a spare spark plug against a metal part of the engine while connected to the spark plug wire.
- Crank the engine: Have someone crank the engine while you observe the spark plug tester or the spare spark plug gap.
- Check for spark: You should see a bright blue spark jumping across the gap if the coil is functioning correctly. A weak, yellow spark or no spark indicates a potential coil problem.
Important: This spark test is less conclusive for modern coil-on-plug systems and can be risky. Substitution is generally a safer and more reliable method.
Beyond the Coil: Other Potential Causes of Misfires
While a faulty ignition coil is a common cause of misfires, it’s not the only one. As the forum discussion touched upon, other components can also lead to similar symptoms. Consider checking these areas if coil testing doesn’t solve your problem:
- Spark Plugs: Worn, fouled, or incorrectly gapped spark plugs are frequent culprits.
- Spark Plug Wires: Damaged or deteriorated spark plug wires can prevent proper spark delivery.
- Distributor Cap and Rotor (if applicable): Cracks or carbon tracking in the distributor cap or a faulty rotor can cause misfires. Mac Townsend described a simple rotor test in the forum.
- Fuel System Issues: Problems with fuel injectors, fuel pump, or fuel filter can lead to lean conditions and misfires.
- Vacuum Leaks: Vacuum leaks can disrupt the air-fuel mixture, causing misfires.
- Engine Mechanical Problems: In more complex cases, issues like low compression due to valve problems (though ErnieY correctly ruled out valve adjustment as a direct cause in this context) or worn piston rings can cause misfires.
Conclusion: Pinpointing Ignition Coil Problems
Knowing how to check a coil is a valuable diagnostic skill for any car owner. By following these steps – visual inspection, resistance testing, and substitution – you can effectively determine if a faulty ignition coil is causing your engine performance issues. Remember to consult your vehicle’s service manual for specific procedures and resistance specifications.
If you’ve tested your coil and are still experiencing problems, or if you’re uncomfortable performing these tests yourself, it’s always best to consult a qualified mechanic. They can perform more advanced diagnostics and ensure your vehicle is running reliably.
