How To Test Coil On Plug Ignition Coil

Diagnosing a misfire can be a frustrating experience. While many factors contribute to engine performance, a faulty coil on plug (COP) ignition coil is a common culprit. This article will guide you through the process of testing your COP coils, empowering you to identify problems and potentially save money on costly mechanic visits. We'll cover the theory behind how they work, common issues, and best practices for testing. This guide assumes you have a basic understanding of automotive electrical systems and are comfortable using tools like a multimeter.

Background: Ignition Systems and Coil On Plug Technology

To understand how to test a COP coil, it's helpful to understand the evolution of automotive ignition systems. Early systems relied on a single coil and a distributor to route the high-voltage spark to each cylinder. This system had limitations, especially in performance and efficiency. The move towards electronic ignition systems was a game changer, providing precise control over spark timing and duration.

The coil on plug system takes this evolution a step further. Instead of a single coil, each cylinder has its own dedicated ignition coil positioned directly above the spark plug. This eliminates the need for spark plug wires and the energy losses associated with them. This design leads to several advantages:

Improved Spark Energy: Each coil delivers a stronger, more consistent spark directly to the cylinder.
Reduced Radio Frequency Interference (RFI): Spark plug wires are a major source of RFI, which can interfere with sensitive electronic components. Eliminating them reduces this interference.
Enhanced Engine Management: COP systems allow for more precise control over ignition timing, leading to improved fuel efficiency and performance.
Simplified Maintenance: Although a failure might seem more daunting, COP systems actually simplify some aspects of diagnosis, as you can isolate problems to individual cylinders more easily.

Technical Breakdown: How COP Coils Work

A COP coil is essentially a transformer. A transformer consists of two sets of wire windings: a primary winding and a secondary winding. The primary winding has relatively few turns of thick wire, while the secondary winding has many turns of thin wire. Here's the sequence of events:

Power Supply: The car's computer, the Engine Control Unit (ECU), sends a low-voltage (typically 12V) signal to the primary winding of the COP coil. This signal is controlled by a transistor (or other switching device) inside the ECU.
Magnetic Field Generation: When the 12V signal is applied to the primary winding, it creates a magnetic field around the coil.
Field Collapse: The ECU quickly interrupts the current flow to the primary winding. This causes the magnetic field to rapidly collapse.
Voltage Induction: As the magnetic field collapses, it cuts across the secondary winding. This induces a high-voltage (typically 20,000-40,000 volts) in the secondary winding. The voltage is proportional to the ratio of turns between the primary and secondary windings.
Spark Plug Firing: This high-voltage electricity is then discharged across the spark plug gap, creating a spark that ignites the air-fuel mixture in the cylinder.

The rapid switching of the primary current is crucial to generating the high-voltage needed for ignition. The ECU controls the timing and duration of this switching, ensuring that the spark occurs at the precise moment required for optimal combustion.

Common Issues and Maintenance Concerns

COP coils, like any other automotive component, can fail over time. Here are some common issues:

Heat: The intense heat generated by the engine can degrade the coil's insulation, leading to shorts and reduced performance. This is a leading cause of COP coil failure.
Vibration: Constant engine vibration can weaken the coil's internal components and connections.
Moisture: Exposure to moisture can cause corrosion and short circuits within the coil.
Spark Plug Issues: A worn or improperly gapped spark plug can put extra strain on the ignition coil, shortening its lifespan. A larger gap requires more voltage to jump, stressing the coil.
Electrical Overload: Problems with the ECU or the vehicle's electrical system can cause voltage spikes that damage the coil.
Insulation Breakdown: Age and contaminants can degrade the insulation between the windings, leading to arcing and reduced output.

Symptoms of a failing COP coil often include:

Engine Misfire: This is the most common symptom. You might feel a hesitation or jerkiness in the engine's performance.
Rough Idle: The engine might idle roughly or stall.
Reduced Fuel Economy: A misfiring cylinder reduces combustion efficiency, leading to poor fuel economy.
Check Engine Light (CEL): The ECU will often detect a misfire and illuminate the CEL. Diagnostic trouble codes (DTCs) related to misfires (e.g., P0300, P0301, P0302) may be stored.
Lack of Power: The engine might feel sluggish and lack power, especially during acceleration.

Testing COP Coils: A Step-by-Step Guide

Before you begin, remember to always prioritize safety. Disconnect the negative battery terminal to prevent electrical shocks. Wear appropriate safety glasses and gloves.

1. Visual Inspection

Begin with a thorough visual inspection of each coil. Look for:

Cracks or Damage: Check for any visible cracks, chips, or other damage to the coil housing.
Corrosion: Look for signs of corrosion on the electrical connectors or coil body.
Oil Leaks: Check for oil leaks around the spark plug well. Oil can damage the coil and cause misfires.
Burn Marks: Check for signs of burning or discoloration on the coil.

2. Resistance Testing (Primary and Secondary)

This test uses a multimeter to measure the resistance of the primary and secondary windings. This can help identify open circuits or shorted windings within the coil. You'll need your vehicle's service manual or a reliable online source to find the correct resistance specifications for your specific coil.

Set Up the Multimeter: Set your multimeter to the ohms (Ω) setting. Select the appropriate range based on the expected resistance values (usually in the hundreds of ohms for primary, and thousands for secondary).
Primary Winding Test: Disconnect the electrical connector from the coil. Place one multimeter probe on each of the primary winding terminals (the two terminals on the connector side of the coil). Record the resistance reading. Compare this reading to the specification in your service manual. A reading that is significantly higher or lower than the specification indicates a problem.
Secondary Winding Test: Place one multimeter probe inside the spark plug boot (where the spark plug connects). Place the other probe on one of the primary winding terminals. Record the resistance reading. Compare this reading to the specification in your service manual. Again, significant deviations indicate a faulty coil.

Important Note: Resistance testing alone is not always conclusive. A coil can have the correct resistance readings but still fail under load when subjected to high voltage and temperature.

3. Swap Test (The Simplest and Often Most Effective)

The swap test is a simple yet powerful method for diagnosing COP coil issues. If you have multiple cylinders, you can swap the suspect coil with a known good coil from another cylinder. Here's how:

Identify the Suspect Cylinder: If you have a diagnostic code, such as P0301 (misfire in cylinder 1), identify the corresponding coil. If you don't have a code, but suspect a misfire, you can try swapping coils one at a time.
Swap Coils: Swap the suspect coil with a coil from a cylinder that is known to be working correctly (e.g., swap cylinder 1 coil with cylinder 2 coil).
Clear Codes: Clear any diagnostic trouble codes (DTCs) from the ECU using a scan tool.
Test Drive: Start the engine and monitor for misfires. Drive the vehicle and see if the misfire has moved to the cylinder where you installed the suspect coil.

If the misfire follows the coil (e.g., the code changes from P0301 to P0302 after swapping the coils between cylinders 1 and 2), then the coil is likely the problem.

4. Using a Noid Light (Optional, But Recommended)

A noid light is a small test light that plugs into the electrical connector of the COP coil. It's used to verify that the ECU is sending the proper signal to the coil. If the noid light flashes brightly when the engine is running, the ECU is likely sending the correct signal. If the noid light is dim or doesn't flash at all, there may be a problem with the ECU, the wiring harness, or the power supply to the coil.

Disconnect the Coil: Disconnect the electrical connector from the suspect COP coil.
Plug in the Noid Light: Plug the noid light into the electrical connector.
Start the Engine: Start the engine and observe the noid light.

A flashing noid light indicates that the ECU is triggering the coil. The absence of a flash (or a weak flash) points to a problem with the ECU signal, wiring, or power supply to the coil.

Do's and Don'ts / Best Practices

Do consult your vehicle's service manual for specific testing procedures and resistance specifications.
Do disconnect the negative battery terminal before working on the ignition system.
Do use a high-quality multimeter for accurate resistance readings.
Do inspect the spark plugs when diagnosing misfires. Replace worn or damaged spark plugs.
Do use dielectric grease on the electrical connectors to prevent corrosion.
Don't touch the spark plug boot or coil while the engine is running. High voltage can cause serious injury.
Don't assume that a coil is good based solely on resistance testing. Perform other tests as well.
Don't ignore other potential causes of misfires, such as fuel injectors, vacuum leaks, or compression problems.
Don't overtighten the coil mounting bolts.

Conclusion

Testing COP ignition coils is a manageable task for the experienced DIYer. By following these steps, you can effectively diagnose potential coil problems and pinpoint the source of engine misfires. Remember to prioritize safety, consult your vehicle's service manual, and use the right tools for the job. While resistance tests can give you a starting point, the swap test is often the most reliable method for confirming a faulty coil. If you're still unsure after performing these tests, it's best to consult with a qualified mechanic. If you are experiencing engine misfires and have access to a scan tool, start with reading and clearing any codes that may be present. Armed with this knowledge, you'll be well-equipped to tackle COP coil diagnosis and keep your engine running smoothly.