How Do You Know If A Ignition Coil Is Bad

Diagnosing a faulty ignition coil is a common troubleshooting task for any seasoned DIY mechanic. Ignition coils are a vital component in your car's ignition system, responsible for transforming the low voltage from your car's battery into the high voltage needed to create a spark at the spark plug, which then ignites the air/fuel mixture in the cylinder. A bad coil can lead to a host of performance issues. This article will provide you with a comprehensive guide on how to determine if your ignition coil is failing, using a combination of observation, testing, and deduction.

Understanding Ignition Coil Function and Failure Modes

Before diving into the diagnosis, it's crucial to understand how an ignition coil works and the common reasons they fail. Modern cars use coil-on-plug (COP) systems, where each cylinder has its own dedicated coil mounted directly above the spark plug. Older vehicles may use a distributor system with a single coil. The principle remains the same: the coil is essentially a transformer consisting of two windings – a primary winding and a secondary winding – wrapped around a laminated iron core.

When the engine control unit (ECU) commands the coil to fire, it interrupts the current flowing through the primary winding. This sudden interruption causes a magnetic field to collapse, inducing a very high voltage in the secondary winding. This high voltage, typically between 20,000 and 40,000 volts, is then sent to the spark plug to create the spark.

Common reasons for ignition coil failure include:

Overheating: Excessive heat can damage the coil's internal components, especially the insulation. This can be caused by factors like poor engine cooling, excessive idling, or prolonged high-RPM driving.
Vibration: Constant vibration can weaken the coil's internal connections and cause them to break.
Moisture/Contamination: Exposure to moisture, oil, or other contaminants can corrode the coil's terminals and windings, leading to short circuits or open circuits.
Spark Plug Issues: Worn or fouled spark plugs require the ignition coil to work harder to generate a spark. This increased stress can shorten the coil's lifespan. A wide spark plug gap demands more voltage to jump, straining the coil.
Voltage Spikes: Electrical surges in the car's electrical system can damage the coil's internal components.
Insulation Breakdown: Over time, the insulation around the coil's windings can deteriorate, leading to short circuits between the windings.

Recognizing the Symptoms of a Bad Ignition Coil

The first step in diagnosing a bad ignition coil is recognizing the symptoms. Here are some of the most common indicators:

Misfiring: This is perhaps the most common symptom. A misfire occurs when one or more cylinders fail to fire properly, resulting in a rough idle, poor acceleration, and a noticeable loss of power. The ECU usually detects the misfire and stores a diagnostic trouble code (DTC).
Rough Idling: A misfiring cylinder will cause the engine to idle unevenly and roughly.
Loss of Power: When a cylinder isn't firing, the engine loses power. This is most noticeable during acceleration or when climbing hills.
Poor Fuel Economy: A misfiring engine burns fuel less efficiently, leading to a decrease in fuel economy.
Check Engine Light (CEL): The CEL will often illuminate when a misfire occurs. The ECU stores a DTC that can be read with an OBD-II scanner. Common codes related to ignition coil issues include P0300 (Random/Multiple Cylinder Misfire Detected), P0301 (Cylinder 1 Misfire Detected), P0302 (Cylinder 2 Misfire Detected), and so on.
Engine Stalling: In severe cases, a bad ignition coil can cause the engine to stall, especially at low speeds or when idling.
Hesitation During Acceleration: The engine might hesitate or stumble when you press the accelerator pedal.

Diagnostic Procedures: Pinpointing the Culprit

Once you suspect a bad ignition coil, you can use several methods to pinpoint the culprit.

1. Reading Diagnostic Trouble Codes (DTCs)

The first step should always be to read the DTCs stored in the ECU using an OBD-II scanner. This will provide you with valuable information about which cylinder(s) are misfiring. If you have a code like P0301, you know to focus your attention on the ignition coil for cylinder 1. Note that a P0300 code indicates a random misfire, making diagnosis trickier. It could still be a coil, but further investigation is needed.

2. Visual Inspection

Carefully inspect the ignition coils for any signs of physical damage, such as cracks, melting, or corrosion. Check the electrical connectors for loose connections or corrosion. Look for any signs of arcing or burning around the coil or its connector. A visual inspection, while simple, can often reveal obvious problems.

3. Coil Swap Test

This is a simple and effective method for confirming a faulty coil. Swap the suspect coil with a known good coil from another cylinder. For example, if you suspect the coil on cylinder 1 is bad and you have a P0301 code, swap it with the coil from cylinder 2. Clear the DTCs and start the engine. If the misfire moves to cylinder 2 (indicated by a P0302 code), then the original coil from cylinder 1 is indeed bad. This test relies on the process of elimination.

4. Resistance Testing

You can use a multimeter to measure the resistance of the primary and secondary windings of the ignition coil. Refer to your vehicle's service manual for the specified resistance values. A significant deviation from the specified values indicates a faulty coil. To perform this test:

Disconnect the electrical connector from the ignition coil.
Set your multimeter to measure resistance (Ohms).
Primary Winding: Connect the multimeter probes to the two terminals of the primary winding. Record the resistance reading.
Secondary Winding: Connect one probe to the coil's high-tension terminal (the one that connects to the spark plug) and the other probe to one of the primary winding terminals. Record the resistance reading.
Compare your readings to the specifications in your vehicle's service manual.

Important Note: Resistance values are typically very low for the primary winding (often less than 1 ohm) and much higher for the secondary winding (typically several thousand ohms). A reading of infinite resistance (open circuit) or very low resistance (short circuit) indicates a faulty coil. Always disconnect the battery before testing any electrical components to prevent damage or injury.

5. Spark Test

This test confirms if the coil is generating a spark. Caution: This test involves high voltage and should be performed with extreme care. Wear insulated gloves and use insulated tools.

Disconnect the ignition coil from the spark plug.
Insert an old spark plug into the coil's high-tension terminal.
Ground the spark plug's body to the engine block.
Have someone crank the engine while you observe the spark plug.
A healthy coil will produce a strong, blue spark. A weak, yellow spark or no spark indicates a faulty coil.

Alternative Method (using an inline spark tester): An inline spark tester is a safer way to perform a spark test. It connects between the coil and the spark plug, allowing you to observe the spark without directly handling the high-tension terminal.

6. Voltage Drop Test

A voltage drop test can help identify excessive resistance in the coil's power and ground circuits. This involves measuring the voltage drop across the circuit while the coil is operating. Excessive voltage drop indicates a problem with the wiring or connections.

Conclusion

Diagnosing a bad ignition coil requires a systematic approach. By carefully observing the symptoms, reading DTCs, performing visual inspections, and conducting the appropriate tests, you can accurately pinpoint the faulty coil and restore your engine's performance. Remember to consult your vehicle's service manual for specific procedures and specifications. Always prioritize safety when working with electrical components.