How to Test an Ignition Coil: A Comprehensive Guide for DIY Mechanics

The ignition coil is a crucial component in your vehicle’s engine, responsible for converting the battery’s low voltage into the high voltage needed to create a spark at the spark plugs. This spark ignites the air-fuel mixture, powering your engine. A faulty ignition coil can lead to a variety of problems, from rough idling and misfires to poor fuel economy and even a complete engine stall. Fortunately, testing an ignition coil is a relatively straightforward process that can often be done at home with a few basic tools. This comprehensive guide will walk you through the steps, enabling you to diagnose a potential coil issue before resorting to expensive professional repairs.

Understanding the Ignition System

Before diving into the testing procedure, it’s essential to understand the basics of your vehicle’s ignition system. Most modern vehicles use a distributorless ignition system (DIS), where each spark plug has its own dedicated coil or a coil pack that serves two cylinders. Older vehicles might utilize a distributor-based system, with a single coil. Both systems work on the same principle of electromagnetic induction, but their configurations differ. The basic components include:

• Battery: Provides the initial low-voltage power.
• Ignition Switch: Controls power flow to the ignition system.
• Ignition Control Module (ICM) or Engine Control Unit (ECU): The ‘brain’ of the system, triggering the coils.
• Ignition Coil(s): Step up the voltage to thousands of volts.
• Spark Plugs: Initiate combustion in the cylinders.
• Wiring: Connects all the components.

Symptoms of a Failing Ignition Coil

Recognizing the symptoms of a failing ignition coil is the first step in diagnosing the problem. Common signs include:

• Engine Misfires: A misfire occurs when a cylinder doesn’t fire correctly, causing a noticeable jerk or hesitation. This is one of the most prominent indicators.
• Rough Idling: The engine may struggle to maintain a smooth idle, shaking or vibrating excessively.
• Reduced Power: You may notice a lack of acceleration or a decrease in overall engine power.
• Poor Fuel Economy: Misfires and incomplete combustion can lead to increased fuel consumption.
• Check Engine Light: A faulty coil will often trigger the ‘Check Engine Light’ on your dashboard. A diagnostic scanner (OBD-II) can help pinpoint the specific cylinder with the problem.
• Engine Stalling: In severe cases, a faulty coil can lead to the engine cutting out and stalling completely.

Safety Precautions

Working with your vehicle’s electrical system requires caution. Always prioritize safety by observing the following:

• Disconnect the Battery: Before starting any electrical work, disconnect the negative battery terminal using a wrench. This prevents accidental shorts and electrical shocks. Wait a few minutes after disconnecting to allow capacitors to discharge fully.
• Work in a Well-Ventilated Area: Working near an open flame or sources of ignition is a huge no-no.
• Use Proper Tools: Ensure your tools are in good condition and are insulated when necessary.
• Be Aware of Hot Components: The engine can be very hot. Exercise caution when working around hot parts.
• Refer to Your Vehicle’s Manual: Consult your vehicle’s repair manual for specific instructions related to your model.

Tools You’ll Need

Before you begin, gather the following tools:

• Digital Multimeter: A multimeter capable of measuring resistance (ohms) is essential.
• Wrench Set: For disconnecting the battery and removing the ignition coil if needed.
• Pliers: Needle-nose pliers may be helpful for disconnecting connectors.
• Diagnostic Scanner (OBD-II): Optional, but useful for reading trouble codes and determining which cylinder has an issue.
• Vehicle’s Service Manual: A service manual specific to your vehicle will provide helpful diagrams and specific torque specifications.

Testing an Ignition Coil: Step-by-Step Guide

The following steps will help you test an ignition coil. It is important to note that the location of the ignition coil can differ from vehicle to vehicle. It’s always advisable to refer to your vehicle’s repair manual.

Step 1: Preparation

1. Park Safely: Park your vehicle on a level surface and engage the parking brake.
2. Cool Down: Ensure the engine is completely cool before you start working. Working on a hot engine increases risk of burn and inaccurate readings.
3. Locate the Ignition Coils: Consult your vehicle’s repair manual or do some research online to find the specific location of the ignition coil(s). They are generally located under the hood, either directly attached to the spark plug, as a bank, or near the valve cover.
4. Disconnect the Battery: Disconnect the negative terminal of the battery as described in the safety precautions section.

Step 2: Visual Inspection

Before any electrical testing, perform a visual inspection of the coil and its connections:

• Check for Damage: Look for cracks, swelling, or other signs of physical damage on the coil body.
• Examine Connectors: Inspect the electrical connectors for any corrosion, loose pins, or damage. Ensure they fit snugly and lock correctly.
• Inspect Wiring: Check the wires connected to the coil for any signs of fraying, cuts, or breaks in the insulation.
• Look for Oil Leaks: Oil leaks or residue on the coil can indicate a problem, or an area where oil is entering.

If you observe any obvious damage, the coil may need replacement.

Step 3: Resistance Testing (Primary Winding)

The primary winding is the internal coil that receives the low voltage from the ignition control module. To test it:

1. Set the Multimeter: Set your digital multimeter to measure resistance (ohms, Ω). Select a range appropriate for the resistance of the primary side, typically in the 0-20 ohm range.
2. Disconnect the Connector: If the coil has a connector, unplug it from the vehicle’s harness.
3. Connect the Probes: Place one probe of the multimeter on each of the primary winding terminals of the coil. These are often on the connector pins on the end of the coil. If your coil uses wires, you need to find the wire connections as described in your repair manual. Do not touch the probes with your fingers as it may alter the reading.
4. Read the Measurement: Record the resistance reading displayed on the multimeter.
5. Compare to Specifications: Consult your vehicle’s repair manual or the coil manufacturer’s specifications for the acceptable resistance range for the primary winding. If the reading is outside of that range, the coil likely needs to be replaced. A shorted coil will have near-zero resistance, while an open coil will show an infinite reading on your multimeter.

Step 4: Resistance Testing (Secondary Winding)

The secondary winding is responsible for stepping up the voltage to power the spark plug. To test this:

1. Set the Multimeter: Change the multimeter setting to measure resistance (ohms, Ω). This time, you’ll need a much higher range, generally in the kilo-ohm (kΩ) range (1000s of ohms, often 5kΩ – 15kΩ).
2. Connect the Probes: Place one probe of the multimeter on the primary terminal (same one as the primary winding test). Place the other probe on the terminal that connects to the spark plug (this may be an internal contact or a tower where the spark plug boot connects).
3. Read the Measurement: Note the resistance reading.
4. Compare to Specifications: Consult your vehicle’s repair manual or the coil manufacturer’s specifications for the acceptable secondary winding resistance range. A reading outside of this range could indicate a faulty coil.

Step 5: Spark Test (Advanced – Proceed With Caution)

This test requires running the engine briefly and should be done with extreme caution. It involves removing the spark plug and using an external ground to check for spark. It should only be performed if the previous resistance tests gave inconclusive results.

Warning: This test can produce high voltage and is dangerous if not done carefully. Only attempt this if you are confident and understand the risks involved.

1. Reconnect the Battery: Temporarily reconnect the battery for this test.
2. Disconnect the Spark Plug: Remove the spark plug boot and the spark plug from the cylinder you are testing.
3. Connect the Spark Plug: Insert the spark plug back into the boot.
4. Ground the Spark Plug: Carefully touch the metal threads of the spark plug to a clean metal ground point on the engine (like an engine bolt or bracket). Do not touch the metal part of the spark plug with your hand during the test.
5. Start the Engine (Briefly): Briefly crank the engine for a couple of seconds (or have someone crank it for you) while observing the spark plug gap.
6. Check for Spark: Look for a bright, consistent blue spark jumping across the spark plug gap. A weak or intermittent spark, or no spark, indicates a problem with the ignition coil, or other components in the ignition circuit.
7. Stop the Engine: As soon as you observe the spark (or not), immediately stop cranking the engine.
8. Disconnect the Battery: Disconnect the battery again to prevent accidental starts while you are doing the rest of the work.
9. Reinstall Spark Plug: Reinstall the spark plug, the boot and reconnect the wiring harness.

Step 6: Checking the Control Signal (Advanced)

This test requires knowledge of electrical circuits and using a test light or a multimeter. With the wiring harness connected to the coil, you need to check for the trigger signal (12V Pulse) from the ICM or ECU that activates the coil. Do not perform this test if you do not feel confident with vehicle wiring and electronics. If the resistance tests were inconclusive, the issue may not be the coil but the wiring to the coil itself. Refer to your vehicle specific schematics for the proper pin locations.

1. Reconnect the Battery: Reconnect the battery for this test.
2. Connect the Multimeter or Test Light: Connect the ground lead of your multimeter (set to DC Volts) or test light to ground. Connect the positive lead of your multimeter or test light to the trigger signal wire for the coil.
3. Start the Engine (Briefly): Briefly crank the engine for a few seconds (or have someone crank it for you) while observing the multimeter or test light.
4. Check for signal: The multimeter should show a voltage of around 12 volts that pulses as the engine cranks. A test light should flicker with the trigger signal. No voltage, or a steady voltage indicates an issue with the ECU, wiring or an open circuit in the coil control circuit.
5. Stop the Engine: As soon as you observe the results, immediately stop cranking the engine.
6. Disconnect the Battery: Disconnect the battery again to prevent accidental starts while you are doing the rest of the work.

Interpreting the Results

Once you’ve completed the tests, interpret your results:

• Resistance Test: If the resistance readings for either the primary or secondary windings are outside the manufacturer’s specified ranges, the coil is likely faulty and needs to be replaced.
• Spark Test: If there is a weak, intermittent, or no spark during the spark test, the coil is likely faulty.
• Control Signal Test: If there is no control signal, wiring harness problems or a faulty ECU are potential culprits.
• Visual Inspection: Any signs of physical damage, like cracks, corrosion or oil leaks warrant immediate replacement.

Replacing the Ignition Coil

If your tests indicate that a coil is faulty, replacement is usually the best course of action. Here’s a general process:

1. Disconnect the Battery: Disconnect the negative battery terminal.
2. Disconnect the Connector: Disconnect the electrical connector from the faulty coil.
3. Remove the Coil: Remove any mounting bolts or screws securing the coil to the engine.
4. Install the New Coil: Install the new coil in the reverse order, ensuring it’s securely mounted.
5. Connect the Connector: Connect the electrical connector securely to the new coil.
6. Reconnect the Battery: Reconnect the negative battery terminal.
7. Clear Diagnostic Codes: Use an OBD-II scanner to clear any stored trouble codes, if applicable.
8. Test Drive: Test drive your vehicle to ensure the issue is resolved.

Remember that these steps are a general guide. It’s essential to refer to your vehicle’s service manual for specific instructions and torque specifications.

Conclusion

Testing an ignition coil is a manageable task for DIY mechanics. By following the steps outlined in this guide, you can identify a faulty ignition coil and potentially save money on costly repairs. Remember to prioritize safety, use the correct tools, and refer to your vehicle’s repair manual. With careful inspection and testing, you’ll be able to keep your vehicle running smoothly and efficiently for years to come. If you are not comfortable with any of the above tests, seek the advice of a certified mechanic.