Testing Car Coils: Tools and Methods Demystified

by:Haiyan     2024-01-08

Testing Car Coils: Tools and Methods Demystified


Car coils play a crucial role in the ignition system of a vehicle. A properly functioning coil ensures that the spark plugs receive the necessary voltage to ignite the fuel-air mixture in the combustion chambers. However, over time, these coils can deteriorate, affecting engine performance. Therefore, regular testing of car coils is essential to identify any potential issues. In this article, we will delve into the tools and methods used to effectively test car coils, demystifying this process for automotive enthusiasts.

Understanding Car Coils

Before we dive into testing methods, it is essential to understand the basics of car coils. Car coils, also known as ignition coils or spark coils, are responsible for converting the 12-volt power from the battery into the high voltage needed to create sparks in the spark plugs. This voltage, which can range from 20,000 to 100,000 volts, ignites the fuel mixture in the cylinders, propelling the vehicle forward.

Types of Car Coils

There are two main types of car coils—coil-on-plug (COP) and coil pack. COP systems feature individual coils for each spark plug, while coil pack systems use a single coil for multiple spark plugs. Additionally, some vehicles may feature an external ignition coil mounted separately from the spark plugs. Understanding the type of coil your vehicle has is crucial for selecting the appropriate testing method.

Tools for Testing Car Coils

1. Digital Multimeter (DMM): A DMM is an indispensable tool for testing car coils. It allows you to measure electrical properties such as voltage, resistance, and continuity. Make sure your DMM has an appropriate voltage range, preferably at least 40,000 volts, to ensure accurate readings during coil testing.

2. Spark Tester: A spark tester is a handy tool that simulates the conditions inside the combustion chamber, allowing you to check if the coil produces a spark. It consists of a grounded alligator clip and a terminal for connecting to the coil. When connected correctly, the spark tester will emit a visible spark when the coil fires.

3. Oscilloscope: An oscilloscope is a more advanced tool used by professionals to visualize and analyze electrical waveforms. It can provide detailed insights into the coil's performance and help identify any abnormalities. While not necessary for basic coil testing, an oscilloscope can be a valuable addition to a well-equipped automotive shop.

4. Insulation Tester: Also known as a megger, an insulation tester measures the insulation quality of the coil windings. This test is particularly important to identify any internal shorts or faults within the coil. Insulation testers apply a high voltage to the coil, measuring the resistance between windings and the coil's insulation.

5. Screwdriver or Pliers: These tools are necessary for physically inspecting the coil. Use them to check for loose connections, damaged wires, or any signs of wear and tear. Ensure the vehicle is turned off before performing any physical inspections to prevent electric shocks.

Methods for Testing Car Coils

1. Basic Resistance Test

The resistance test is the simplest and most common method for testing car coils. To perform this test, start by disconnecting the coil's electrical connector. Using a DMM, set the range to ohms and connect the meter's leads to the positive and negative terminals of the coil. Compare the measured resistance value with the manufacturer's specifications. If the resistance is significantly higher or lower than recommended, the coil may be faulty.

2. Spark Test

The spark test helps determine if the coil is producing a spark. Begin by removing one spark plug wire from the coil, leaving a small gap between the wire and the coil terminal. Connect the spark tester between the spark plug wire and the coil terminal. Start the engine or crank it using a remote starter switch. Observe the spark tester for a bright, blue spark. If there is no spark or the spark is weak, the coil may require further testing or replacement.

3. Secondary Output Test

An oscilloscope is highly useful for conducting the secondary output test. Connect the oscilloscope's leads to the coil's secondary terminals. Start the engine or crank it using a remote starter switch, while simultaneously monitoring the oscilloscope's waveform. The waveform should exhibit a consistent pattern with sharp peaks. Any irregularities in the waveform may indicate a faulty coil.

4. Insulation Test

To perform an insulation test, disconnect the coil from the vehicle's electrical system. Connect the insulation tester according to its manufacturer's instructions. Apply the recommended voltage and observe the reading. A high resistance reading suggests good insulation, while a low resistance reading indicates potential internal shorts.

5. Physical Inspection

Perform a visual inspection of the coil, looking for visible signs of damage or wear. Examine the wires, connectors, and overall condition of the coil. Pay close attention to any corrosion, loose connections, or melted insulation. If any issues are found, it may be necessary to replace the coil.


Regular testing of car coils is crucial for maintaining optimal engine performance and preventing potential breakdowns. By using the right tools and methods, enthusiasts can effectively diagnose coil issues. Remember to consult the vehicle's service manual for specific testing procedures and refer to professional help when in doubt. With the information provided in this article, the process of testing car coils is demystified, empowering automotive enthusiasts to keep their vehicles running smoothly.

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