An ignition coil is a crucial part of a vehicle’s ignition system, responsible for transforming low-voltage electricity into high-voltage pulses necessary to ignite the air-fuel mixture in the engine. Failure of an ignition coil can cause a variety of issues, including engine misfires, difficulty starting, and reduced power output. Testing the ignition coils can help identify potential failures and ensure optimal vehicle performance. Here’s a step-by-step guide to testing an ignition coil using an ohmmeter:
Firstly, safety precautions should be taken before initiating any electrical testing. Disconnect the vehicle’s battery and allow ample time for the engine to cool down. Locate the ignition coil(s) in the engine compartment and carefully disconnect the electrical connectors. Wear gloves and safety glasses to protect yourself from potential electrical hazards. The ohmmeter, a device that measures electrical resistance, is then used for testing. Select the appropriate resistance setting on the ohmmeter, generally ranging between ohms (Ω) and kilo-ohms (kΩ). Proper selection ensures accurate readings.
Connect the ohmmeter probes to the primary terminals of the ignition coil. These terminals are typically labeled with “+” and “-“. Note the resistance reading displayed on the ohmmeter. The expected resistance value can vary between different ignition coil models and vehicle manufacturers, so it’s recommended to refer to the vehicle’s service manual for specific specifications. Generally, a resistance reading within the specified range indicates a healthy primary winding. If the reading is significantly higher or lower than expected, it may indicate a break or short in the primary winding, necessitating further inspection or replacement of the ignition coil. Proceed to test the secondary winding of the ignition coil by connecting the ohmmeter probes to the secondary terminal (typically marked with a “T” or “H”) and one of the primary terminals. The resistance reading should be significantly higher compared to the primary winding, often in the kilo-ohms range. An open or short in the secondary winding will result in abnormal resistance readings and may require replacement of the ignition coil.
Understanding the Purpose of an Ignition Coil
An ignition coil is a crucial component in the ignition system of an internal combustion engine. It is responsible for transforming the low-voltage current supplied by the battery into a high-voltage electrical pulse that ignites the air-fuel mixture within the engine’s cylinders. This process initiates the combustion event, which drives the engine’s pistons and generates power.
The ignition coil operates on the principle of electromagnetic induction, where a changing magnetic field creates an electric current in a conductor. When a current flows through the ignition coil’s primary winding, it generates a magnetic field around the coil. Rapidly interrupting the primary current causes the magnetic field to collapse, inducing a high-voltage pulse in the secondary winding. This high-voltage pulse is then directed to the spark plugs, which create the spark that ignites the air-fuel mixture.
Key Characteristics of an Ignition Coil:
| Characteristic | Explanation |
|---|---|
| Primary Resistance | Measures the resistance of the coil’s primary winding in ohms. Typically ranges from 0.5 to 1.5 ohms. |
| Secondary Resistance | Measures the resistance of the coil’s secondary winding in kilo-ohms. Typically ranges from 5 to 30 kilo-ohms. |
| Spark Plug Wire Resistance | Measures the resistance of the spark plug wire connected to the ignition coil. Typically less than 5 kilo-ohms. |
Selecting the Right Ohmmeter
When choosing an ohmmeter for testing ignition coils, several factors must be considered. The primary and secondary windings of ignition coils have different resistance ranges, so you’ll need an ohmmeter with a wide resistance scale. Typically, a range of 0 ohms to 20 megaohms is sufficient for most coils.
In addition to the resistance range, consider the accuracy and sensitivity of the ohmmeter. Accuracy refers to how close the ohmmeter’s readings are to the actual resistance of the coil. Sensitivity measures how small of a resistance change the ohmmeter can detect. A more accurate and sensitive ohmmeter will provide more precise readings.
It’s also important to check the ohmmeter’s input impedance. Input impedance is the resistance that the ohmmeter presents to the circuit it’s measuring. A higher input impedance will minimize the effect of the ohmmeter on the circuit and ensure more accurate readings.
| Feature | Considerations |
|---|---|
| Resistance range | 0 ohms to 20 megaohms is suitable for most ignition coils |
| Accuracy | Choose an ohmmeter with high accuracy for precise readings |
| Sensitivity | A more sensitive ohmmeter can detect smaller resistance changes |
| Input impedance | A higher input impedance minimizes the effect of the ohmmeter on the circuit |
Safety Precautions Before Testing
Before testing an ignition coil with an ohmmeter, it is crucial to adhere to the following safety precautions to prevent injury and electrical hazards:
-
Disconnect the battery
Disconnect the negative terminal of the battery to isolate the electrical system and prevent any unintentional electrical shocks. Ensure the ignition key is in the “OFF” position.
-
Wear protective gear
Wear insulated gloves and safety glasses to protect yourself from potential electrical sparks and splashes.
-
Allow the engine to cool
Allow the engine to cool down completely to avoid burns. The ignition coil can reach high temperatures during operation, so it’s essential to let it cool before handling it.
-
Identify the ignition coil
Locate the ignition coil and inspect its appearance. Ensure it is not damaged or cracked. Note its connections and terminal arrangements for proper testing.
-
Use proper tools
Employ an ohmmeter in good working condition. Refer to the ohmmeter’s user manual for accurate measurement techniques.
| Safety Precaution | Reason |
|---|---|
| Disconnect battery | Prevent electrical shocks |
| Wear protective gear | Protect from sparks and splashes |
| Allow engine to cool | Avoid burns |
| Identify ignition coil | Proper testing and connection |
| Use proper tools | Accurate measurements |
Identifying the Ignition Coil’s Terminals
Before using an ohmmeter to test an ignition coil, it’s crucial to identify the terminals.
1. Primary Terminals
Ignition coils typically have two primary terminals. These terminals carry the low-voltage current from the battery to the coil. They are usually labeled with the symbol “+B” or “BAT” and the symbol “-B” or “GND”.
2. Secondary Terminal
Ignition coils also have one secondary terminal, which carries the high-voltage current to the spark plugs. This terminal is usually labeled with the symbol “S” or “Spark”.
3. Identifying Terminal Functions
To identify the function of each terminal, you can use a simple continuity test. Set the ohmmeter to the “Continuity” setting and touch the probes to the different terminals pairs.
The following table shows the expected results:
| Terminal Pair | Expected Result |
|---|---|
| Primary Terminals | Low resistance (typically below 1 ohm) |
| Primary and Secondary Terminals | No continuity (open circuit) |
| Secondary Terminal and Ground | High resistance (typically above 5,000 ohms) |
Setting Up the Ohmmeter for Resistance Measurement
To prepare your ohmmeter for testing the ignition coil’s resistance, follow these steps:
1. Turn On the Ohmmeter
Switch on the ohmmeter by pressing the power button. Select the resistance measurement mode, usually indicated by the Omega (Ω) symbol.
2. Zero the Ohmmeter
Short the ohmmeter’s test leads together. This creates a zero-ohm connection. Adjust the ohmmeter’s dial or adjust the display to read “0.00Ω” or zero.
3. Choose the Correct Scale
Estimate the expected resistance range of the ignition coil (usually around 0.5-10Ω). Select the appropriate ohmmeter scale to accommodate that range. For instance, a 20Ω scale would be suitable for coils with expected resistance below 20Ω.
4. Attach the Test Leads
Connect the ohmmeter’s test leads to the appropriate terminals on the ignition coil. The polarity of the leads doesn’t matter for resistance measurements.
5. Measure the Resistance
Once the ohmmeter is connected and zeroed, observe the reading on the display. A coil with resistance within the manufacturer’s specified range indicates good electrical continuity. However, if the reading is significantly different or zero, it suggests a problem with the ignition coil.
| Expected Resistance Range | Recommended Ohmmeter Scale |
|---|---|
| 0.5Ω – 1Ω | 2Ω or 5Ω |
| 2Ω – 5Ω | 10Ω or 20Ω |
| 6Ω – 10Ω | 20Ω or 50Ω |
Performing the Resistance Test on the Primary Winding
The primary winding consists of a coil of thicker wire with fewer turns. Its resistance is typically lower than that of the secondary winding. To test its resistance, follow these steps:
6. Measuring the Resistance
Connect the positive (+) probe of the ohmmeter to the positive (+) terminal of the ignition coil and the negative (-) probe to the negative (-) terminal. The ohmmeter should display a reading within the specified range, typically between 0.5 and 2 ohms. A reading outside this range indicates a potential problem with the primary winding.
Refer to the manufacturer’s specifications for the exact resistance value of your specific ignition coil. If the reading is significantly different from the specified value, the coil may have a damaged or open primary winding.
If the ohmmeter displays infinity (OL), it means that the primary winding is open and not conducting electricity. This could be due to a break in the wire or a loose connection.
If the ohmmeter displays zero or a very low resistance, it means that the primary winding has a short circuit. This could be due to the wire being damaged or the insulation breaking down.
| Reading | Possible Problem |
|---|---|
| Within specified range | Normal |
| Outside specified range | Damaged or open primary winding |
| Infinity (OL) | Open primary winding |
| Zero or very low resistance | Short circuit in primary winding |
Interpreting the Primary Winding Resistance Readings
Once you’ve obtained the resistance readings for the primary winding, you need to interpret them to determine if the coil is functioning properly. Here’s how to do that:
1. Compare to Reference Values
Refer to the manufacturer’s specifications for the specific ignition coil you’re testing. The resistance values should be within the specified range. If the reading is significantly higher or lower, it indicates a problem.
2. Check for Open Circuit
If the resistance reading is extremely high (e.g., infinity), it means there’s an open circuit in the primary winding. This prevents current from flowing through the coil, rendering it non-functional.
3. Check for Short Circuit
If the resistance reading is extremely low (e.g., close to zero), there might be a short circuit within the primary winding. This allows excessive current to flow, damaging the coil and potentially causing other electrical problems.
4. Consider Other Factors
Temperature can affect resistance readings. Make sure the coil is at room temperature before testing. Also, ensure the ohmmeter is functioning correctly and the connections are secure.
5. Multiple Readings
Take multiple resistance readings from different points of the primary winding. This can help identify any intermittent issues or localized faults.
6. Check the Continuity
Verify the continuity of the primary winding using the ohmmeter. Set the ohmmeter to continuity mode and check for a low-resistance reading between both terminals of the coil.
7. Reference Table for Resistance Values
Here’s a general reference table for primary winding resistance values:
| Coil Type | Resistance Range (ohms) |
|---|---|
| High-energy coils (HEI) | 0.5 – 1.5 |
| Standard coils | 1.0 – 3.0 |
| Ballast resistor coils | 2.0 – 5.0 |
Testing the Secondary Winding for Continuity
The secondary winding has a much higher number of turns than the primary winding, so it has a much higher resistance. To test the secondary winding for continuity, you will need to use a higher resistance setting on your ohmmeter.
Resistance readings for secondary windings vary depending on the make and model of the vehicle. There are generally three different resistance readings you can encounter, which are listed in the table below.
| Secondary Resistance | Open Circuit | Short Circuit |
|---|---|---|
| Likely Reading | > 20K ohms | 0 ohms |
To test the secondary winding for continuity, follow these steps:
- Set your ohmmeter to the highest resistance setting (usually 20k ohms or higher).
- Touch one probe of the ohmmeter to the positive terminal of the ignition coil.
- Touch the other probe of the ohmmeter to the negative terminal of the ignition coil.
- The ohmmeter should read between 10k ohms and 20k ohms.
- If the ohmmeter reads 0 ohms, the secondary winding is shorted.
- If the ohmmeter reads >20k ohms or infinity, the secondary winding is open.
- If the ohmmeter reads within the specified range for your ignition coil, then the secondary winding is good.
- Repeat steps 2-7 for the remaining ignition coils.
- Disconnect the negative battery terminal.
- Locate the ignition coil. It is usually located on the top of the engine, near the spark plugs.
- Disconnect the electrical connectors from the ignition coil.
- Set the ohmmeter to the lowest resistance scale.
- Touch one probe of the ohmmeter to one terminal of the ignition coil and the other probe to the other terminal.
- The ohmmeter should read between 0.5 and 2 ohms. If the reading is significantly different from this, the ignition coil is faulty and should be replaced.
- Hard starting
- Misfiring
- Reduced engine power
- Stalling
- Backfiring
If you find that any of the ignition coils have a shorted or open secondary winding, you will need to replace the ignition coil.
Troubleshooting Common Ignition Coil Issues
Measuring Resistance
Using an ohmmeter, measure the resistance between the primary terminals (usually marked with a “+” and “-“). The resistance should be within the range specified in the manufacturer’s specifications. A higher resistance may indicate a broken or damaged primary winding.
Checking Secondary Winding Resistance
Measure the resistance between the secondary terminal and the primary terminal. The resistance should be several kilo-ohms. If the resistance is too low, it may indicate a shorted secondary winding, while a high resistance may indicate an open circuit.
Checking for Continuity
Use an ohmmeter to check for continuity between the primary terminals and the secondary terminal. There should be no continuity, as the primary and secondary windings are isolated.
Inspecting the Coil
Visually inspect the ignition coil for any cracks, burns, or other signs of damage. Ensure that the connections are tight and the coil is properly mounted.
Testing the Secondary Voltage
Connect a high-voltage probe to the secondary terminal. Start the engine and measure the voltage while revving the engine. The voltage should rise to several thousand volts when the engine is running properly.
Repeat the test with a spark plug connected to the secondary terminal. The spark should jump the gap easily if the coil is functioning correctly.
Checking for Carbon Tracking
Carbon tracking is caused by the buildup of carbon dust on the insulation between the primary and secondary windings. This can lead to shorts and ignition problems. Inspect the coil for any signs of carbon tracking and clean it if necessary.
Testing the Power Supply
Ensure that the ignition coil is receiving an adequate power supply. Measure the voltage at the primary terminals with the engine running. The voltage should be within the specified range.
Checking Ground Connection
Ensure that the ignition coil has a good ground connection. Clean the terminals where the coil connects to the ground and check for any loose or damaged wires.
Advanced Troubleshooting
If the basic tests do not reveal any problems, additional troubleshooting may be necessary. This may involve using a scan tool or oscilloscope to analyze the signals and waveforms associated with the ignition coil. It is recommended to consult a qualified mechanic for advanced troubleshooting.
Replacement Considerations for a Faulty Ignition Coil
When replacing a faulty ignition coil, car owners and mechanics must take various factors into account to ensure a successful repair. The following considerations should be noted when purchasing a replacement ignition coil:
10. Compatibility with Vehicle Make and Model
It is crucial to ensure that the replacement ignition coil is specifically designed for the make and model of the vehicle. Incompatible coils may not fit properly, resulting in performance issues or potential damage to the engine. To avoid any compatibility issues, car owners should consult the vehicle’s owner’s manual or a trusted mechanic to identify the correct ignition coil.
| Factor | Consideration |
|---|---|
| Engine Type | Gasoline, Diesel, or Hybrid |
| Number of Cylinders | Inline-4, V6, or V8 |
| Year of Manufacture | Specific model year and make |
How to Test an Ignition Coil with an Ohmmeter
An ignition coil is a transformer that converts the low voltage from the vehicle’s battery into the high voltage required to create a spark at the spark plugs. A faulty ignition coil can cause a variety of problems, including hard starting, misfiring, and reduced engine power. Testing the ignition coil with an ohmmeter is a simple way to diagnose whether it is functioning properly.
To test an ignition coil with an ohmmeter, follow these steps:
People Also Ask
How do I know if my ignition coil is bad?
There are a few signs that can indicate a faulty ignition coil, including:
Can I test an ignition coil without an ohmmeter?
Yes, you can test an ignition coil without an ohmmeter by using a spark tester. A spark tester is a device that connects to the ignition coil and produces a spark when the ignition coil is functioning properly.
How often do ignition coils need to be replaced?
Ignition coils typically last for many years, but they can fail prematurely due to a variety of factors, such as excessive heat, vibration, or electrical damage. It is a good idea to have your ignition coils inspected by a qualified mechanic every few years.
