Automotive ignition systems rely heavily on spark plug coils to generate the high voltage necessary to ignite the air-fuel mixture in an engine. A faulty spark plug coil can lead to rough engine idle, misfires, reduced power, and even engine failure. Early detection and replacement of a failing coil are crucial to avoid costly repairs and ensure optimal engine performance. Utilizing a multimeter, a versatile electrical measuring device, enables you to quickly and accurately diagnose the condition of your spark plug coil.
Before embarking on the testing procedure, ensure that the engine is turned off and has cooled down completely. Disconnect the negative terminal of the battery to prevent electrical shock and damage to the multimeter. Locate the spark plug coil, which is typically situated on the engine’s valve cover or cylinder head. Identify the terminals on the coil; one terminal is connected to the ignition module, while the other is connected to the spark plug.
Set the multimeter to the “ohms” or “resistance” setting. Connect one probe of the multimeter to the primary terminal of the coil, and the other probe to the secondary terminal. The primary terminal is usually thicker and connected to the ignition module, while the secondary terminal is thinner and connected to the spark plug. A normal spark plug coil should exhibit a resistance value within the manufacturer’s specified range, typically between 0.5 to 2 ohms for the primary winding and several thousand ohms for the secondary winding. Deviations from these values may indicate an open or shorted coil and warrant replacement.
Understanding the Function of a Spark Plug Coil
The spark plug coil is a crucial component of an internal combustion engine, playing a vital role in the ignition process. Its primary function is to convert low-voltage electricity from the battery or charging system into high-voltage electricity required to create a spark across the spark plug gap. This spark ignites the air-fuel mixture within the engine cylinder, initiating the combustion process.
The spark plug coil comprises a primary winding, made of thick wire with a few turns, and a secondary winding, consisting of thousands of thin wire turns. When an electrical current flows through the primary winding, it creates a magnetic field. The collapsing magnetic field induces an electric current in the secondary winding due to electromagnetic induction. The voltage induced in the secondary winding is much higher than that in the primary winding, providing the necessary voltage to create a spark across the spark plug gap.
The spark plug coil’s output voltage is typically in the range of 20,000 to 30,000 volts, which is sufficient to jump the gap between the spark plug electrodes and ignite the air-fuel mixture. The timing of the spark is precisely controlled by the engine’s electronic control unit (ECU) to ensure optimal engine performance and fuel efficiency.
Spark plug coils can vary in design and construction depending on the engine application, but they all serve the same essential function of generating high-voltage electricity for spark plug ignition.
Types of Spark Plug Coils
Spark plug coils come in different designs, including:
| Type | Description |
|---|---|
| Pencil Coils | Individual coils mounted directly on each spark plug |
| Coil-on-Plug Coils | A single coil that sits atop all spark plugs |
| Distributorless Ignition Coils | A single coil that distributes high-voltage electricity to multiple spark plugs through a distributor cap |
Selecting the Correct Multimeter Setting
When testing a spark plug coil with a multimeter, it is crucial to select the correct multimeter setting to obtain accurate readings. Here are the steps involved in selecting the appropriate setting:
1. Determine the Resistance Range
The first step is to determine the resistance range of the spark plug coil you are testing. This information can typically be found in the manufacturer’s specifications or in the vehicle’s repair manual.
2. Set the Multimeter to Ohms
Once you know the resistance range, set the multimeter to the ohms setting. This setting measures the electrical resistance of a component.
3. Select the Appropriate Resistance Scale
Multimeters typically offer multiple resistance scales, such as 200 ohms, 2k ohms, 20k ohms, and so on. The resistance scale you choose should be appropriate for the resistance range of the spark plug coil.
For example, if the spark plug coil has a resistance range of 5k ohms to 10k ohms, you should select the 20k ohms scale. This scale will provide you with the most accurate readings within the specified range.
The following table provides a general guideline for selecting the appropriate resistance scale based on the resistance range of the spark plug coil:
| Spark Plug Coil Resistance Range | Multimeter Resistance Scale |
|---|---|
| Less than 1k ohms | 200 ohms |
| 1k ohms to 10k ohms | 2k ohms |
| 10k ohms to 100k ohms | 20k ohms |
| 100k ohms to 1M ohms | 200k ohms |
Measuring Primary Resistance
Measuring the primary resistance of a spark plug coil with a multimeter is a straightforward process. Here’s a step-by-step guide:
1. Set the Multimeter to Resistance Mode
Locate the “Ω” symbol on the multimeter dial and turn it to the appropriate resistance range. For most spark plug coils, the 200-ohm range will suffice.
2. Identify the Coil Terminals
The spark plug coil typically has two terminals: a primary terminal and a secondary terminal. The primary terminal is usually connected to the ignition module, while the secondary terminal connects to the spark plug.
3. Connect the Multimeter Leads
Attach the black lead of the multimeter to the primary terminal of the coil. Then, connect the red lead to the other primary terminal.
4. Record the Resistance Reading
The multimeter will display the resistance value in ohms (Ω). A typical spark plug coil has a primary resistance of 0.5-2 ohms. If the reading is significantly lower or higher than this range, it may indicate a problem with the coil.
Here are some additional tips for measuring the primary resistance of a spark plug coil:
- Make sure the coil is disconnected from the ignition module and spark plug.
- Clean the coil terminals before connecting the multimeter leads.
- If the multimeter displays an “OL” reading, it means the resistance is too high to measure. In this case, the coil may be open.
Measuring Secondary Resistance
The secondary winding of the ignition coil is where the high voltage required to create a spark is generated. To measure the secondary resistance, you will need a multimeter with a high-voltage range of at least 20,000 volts. The following steps provide a detailed guide to measuring secondary resistance:
1. Safety Precautions
Before proceeding, ensure you follow proper safety precautions. Wear insulated gloves and safety glasses, and disconnect the battery to prevent any electrical shock.
2. Locate the Coil Terminals
Identify the secondary coil terminals. Typically, the secondary terminals are marked with a “+” or “S” and a “-” or “G” symbol. The positive terminal connects to the spark plug wire, while the negative terminal connects to the distributor.
3. Set Multimeter to High-Voltage Range
Set the multimeter to the highest voltage range available, usually 20,000 volts. This ensures accurate measurement of the high voltage generated by the ignition coil.
4. Connect Multimeter Leads
Connect the multimeter leads to the secondary coil terminals. The positive lead should be connected to the positive terminal, and the negative lead to the negative terminal.
5. Measure and Interpret Results
Hold the multimeter probes against the secondary coil terminals. The multimeter will display the secondary resistance measurement in kilo-ohms (kΩ). The typical secondary resistance range for ignition coils varies from 5 kΩ to 15 kΩ.
Refer to the table below for interpretation of the measurement results:
| Resistance Range | Assessment |
|---|---|
| 5 kΩ – 15 kΩ | Coil is within normal resistance range |
| Less than 5 kΩ | Coil may have a short circuit |
| Greater than 15 kΩ | Coil may have an open circuit |
Testing the Coil Voltage Output
The ignition coil is responsible for creating the high voltage that is necessary to ignite the air and fuel mixture in the engine. To test the coil voltage output, you will need a multimeter and a spark plug.
1. Set the multimeter to the volts AC setting.
2. Connect the black lead of the multimeter to the negative terminal of the battery.
3. Connect the red lead of the multimeter to the positive terminal of the ignition coil.
4. Start the engine and let it idle.
5. Read the voltage on the multimeter. The voltage should be between 10,000 and 20,000 volts.
6. If the voltage is not within this range, the ignition coil may be faulty and will need to be replaced.
7. To further test the ignition coil, you can remove the spark plug from the engine and connect it to the ignition coil. Hold the spark plug away from the engine and have someone crank the engine over. You should see a spark between the spark plug and the ignition coil. If you do not see a spark, the ignition coil or the spark plug may be faulty.
Interpreting Voltage Waveforms
The voltage waveforms captured from the ignition coil can provide valuable insights into the coil’s performance. Here’s a more detailed explanation of each waveform type:
Normal Waveform
A normal voltage waveform should exhibit a distinct pattern. During the “dwell” period, when the transistor is on, the voltage should remain relatively constant around battery voltage. When the transistor turns off, the voltage should rapidly collapse, generating a sharp negative spike. This spike represents the inductive discharge of the coil, and it’s followed by a gradual return to battery voltage. The “dwell time” (the duration of the on period) should be approximately two to three milliseconds, while the collapse time (the duration of the negative spike) should be around one millisecond.
| Characteristic | Value |
|---|---|
| Dwell Time | 2-3 milliseconds |
| Collapse Time | ~1 millisecond |
Open Circuit Waveform
An open circuit waveform occurs when there’s a break in the coil’s circuit, preventing the magnetic field from building up. In this case, the voltage waveform will show a flat line at battery voltage, indicating that no current is flowing through the coil.
Short Circuit Waveform
A short circuit waveform occurs when the coil’s windings have shorted together, creating a low-resistance path for the current. This results in a waveform with a very short dwell time and an elongated collapse time. The negative spike will also have a lower amplitude due to the reduced inductance.
Weak Coil Waveform
A weak coil waveform can indicate that the coil is not producing enough voltage to generate a strong spark. This can be caused by a weak magnetic field due to aging or weakened windings. The waveform will exhibit a reduced amplitude of the negative spike, indicating a lower inductive discharge.
Using a Multimeter to Test a Spark Plug Coil
1. Safety Precautions
Ensure the engine is off, the key is removed, and the battery terminals are disconnected for safety.
2. Locate the Spark Plug Coil
Identify the spark plug coil, typically located on top of the engine near the spark plugs.
3. Disconnect the Harness
Remove the electrical harness connector from the spark plug coil.
4. Check Resistance (Primary Winding)
Set the multimeter to the ohms setting. Touch the probes to the negative side terminal and the connector terminal on the coil.
5. Check Resistance (Secondary Winding)
Switch the probes to the positive side terminal and the center coil terminal. The resistance should be higher than the primary winding.
6. Check for Open Circuit
Set the multimeter to continuity mode. Touch one probe to the negative side terminal and the other probe to the center coil terminal or the connector terminal. There should be no reading.
7. Check for Short Circuit
Touch the probes to the positive and negative side terminals. A low reading indicates a short circuit.
8. Check for Voltage Supply
Reconnect the harness connector. Set the multimeter to voltage mode. Touch one probe to the positive side terminal and the other probe to ground. With the engine running, you should see a voltage reading.
9. Troubleshooting a Faulty Spark Plug Coil
If any of the tests fail, the spark plug coil may be faulty. Replace it by following these detailed steps:
- Disconnect the negative battery terminal.
- Locate the mounting bolts or screws securing the coil to the engine.
- Carefully remove the bolts or screws and lift the coil assembly off.
- Inspect the coil for any damage or cracks.
- Disconnect the spark plug wires from the coil.
- Remove the old coil and discard it.
- Install the new coil and secure it with the bolts or screws.
- Reattach the spark plug wires to the coil.
- Reconnect the battery terminal.
- Test the engine to ensure it is running properly.
10. Additional Tips
Use the manufacturer’s specifications for the correct resistance values for your specific coil.
Always wear safety glasses and gloves when working with electrical components.
Verifying the Ignition System Function After Coil Replacement
1. Start the Engine and Listen for Smooth Idle
After installing the new spark plug coil, start the engine and listen for a smooth idle. If the engine idles smoothly, it indicates the coil is functioning correctly.
2. Check Engine Performance Under Load
Drive the vehicle and observe its performance under acceleration and load. If the engine runs smoothly and responds to throttle inputs without hesitation, the coil is likely to be working properly.
3. Check for Spark at the Spark Plugs
Remove the spark plug wires from the spark plugs and use a spark plug tester to check for spark. Ensure a strong, blue spark at each spark plug. If there is no spark or a weak spark, it may indicate a problem with the coil.
4. Measure Primary Coil Resistance
Using a multimeter, measure the resistance between the positive and negative terminals of the coil. The resistance should be within the manufacturer’s specifications. Refer to the vehicle’s service manual for the specific resistance value.
5. Measure Secondary Coil Resistance
Measure the resistance between the coil tower and the spark plug terminal. The resistance should be within the manufacturer’s specifications. If the resistance is too high or low, it may indicate a faulty coil.
6. Check for Continuity
Use a multimeter to check for continuity between the primary and secondary windings of the coil. There should be no continuity between the windings. Continuity indicates a short circuit, which can cause coil failure.
7. Check for Grounding
Measure the resistance between the negative terminal of the coil and a good ground. The resistance should be very low, close to 0 ohms. High resistance indicates a poor ground connection, which can affect coil performance.
8. Monitor Engine Diagnostics
Observe the vehicle’s engine diagnostics system for any trouble codes related to the ignition system. If any error codes are present, refer to the vehicle’s service manual to troubleshoot the issue.
9. Consult a Mechanic if Necessary
If you encounter any difficulties or suspect a problem with the spark plug coil, do not hesitate to consult a qualified mechanic. They can provide expert diagnosis and repairs to ensure the proper functioning of your vehicle’s ignition system.
10. Additional Troubleshooting Tips
| Symptom | Possible Cause |
|---|---|
| No spark | – Faulty coil – Faulty ignition module – Bad wiring |
| Weak spark | – Worn-out spark plugs – Fouled spark plugs – Weak coil – Bad wiring |
| Engine misfires | – Faulty coil – Faulty ignition module – Bad spark plug wires – Worn-out spark plugs – Vacuum leak |
How To Test Spark Plug Coil With Multimeter
A spark plug coil is a vital component of an ignition system, responsible for generating the high voltage needed to create a spark at the spark plugs. A faulty coil can lead to misfires, poor engine performance, and even engine damage. Testing the spark plug coil with a multimeter is a simple and effective way to diagnose any issues with the coil.
To test the spark plug coil, follow these steps:
- Set the multimeter to the ohms setting.
- Disconnect the spark plug wire from the spark plug.
- Connect one probe of the multimeter to the positive terminal of the spark plug coil.
- Connect the other probe of the multimeter to the negative terminal of the spark plug coil.
- The multimeter should read between 0.5 and 2 ohms.
- If the multimeter reads infinity, the coil is open and needs to be replaced.
- If the multimeter reads 0 ohms, the coil is shorted and needs to be replaced.
People Also Ask About How To Test Spark Plug Coil With Multimeter
What is the difference between a primary and secondary coil?
The primary coil is the coil that receives the low voltage from the ignition switch. The secondary coil is the coil that generates the high voltage that is sent to the spark plugs.
How do I know if my spark plug coil is bad?
There are a few signs that may indicate a faulty spark plug coil, including: misfires, poor engine performance, and difficulty starting the engine.
What are the different types of spark plug coils?
There are two main types of spark plug coils: internal and external. Internal coils are mounted inside the distributor, while external coils are mounted on the outside of the engine.
