How To Check Coil Resistance (Chainsaw Ignition Tips)

Ever wondered why your chainsaw, usually a reliable beast, suddenly refuses to roar to life? Could be a simple fuel issue, a clogged air filter, or perhaps a more insidious electrical gremlin lurking within its ignition system. More often than not, the culprit is a faulty ignition coil. Now, I know what you might be thinking: “Ignition coil? Sounds complicated!” But trust me, checking the coil resistance is a task even a novice can tackle with the right guidance and a little bit of patience.

I’ve spent years felling trees, bucking logs, and splitting firewood, and I’ve learned that a well-maintained chainsaw is the cornerstone of any successful wood processing endeavor. Knowing how to troubleshoot common issues like a faulty ignition coil is a crucial skill that can save you time, money, and a whole lot of frustration. So, grab your multimeter, and let’s get started!

Understanding the Chainsaw Ignition System: A Primer

Before we dive into the specifics of checking coil resistance, it’s essential to understand how the ignition system works in your chainsaw. This knowledge will not only help you diagnose problems more effectively but also give you a greater appreciation for the intricate engineering that goes into these powerful tools.

The Role of the Ignition Coil

At its core, the chainsaw’s ignition system is responsible for generating the high-voltage spark that ignites the fuel-air mixture in the cylinder, initiating the combustion process that drives the engine. The ignition coil is the heart of this system, acting as a transformer that steps up the relatively low voltage from the magneto (a small generator driven by the engine’s flywheel) to the thousands of volts needed to create a spark across the spark plug gap.

Think of it like this: the magneto is like a small water pump, and the ignition coil is like a water pressure amplifier. The magneto provides a steady but low-pressure flow of electricity, while the ignition coil boosts that pressure to create a powerful surge that can jump the gap in the spark plug.

Components of the Ignition System

The ignition system consists of several key components working in harmony:

Magneto: Generates a low-voltage electrical current as the flywheel rotates. The flywheel has magnets embedded in it, and as these magnets pass by the magneto coil, they induce a current.
Ignition Coil: Steps up the voltage from the magneto to a level sufficient to create a spark at the spark plug.
Spark Plug: Provides a gap across which the high-voltage spark jumps, igniting the fuel-air mixture.
Flywheel: A rotating component connected to the engine’s crankshaft, containing magnets that interact with the magneto to generate electricity.
Ignition Module (in some models): An electronic component that controls the timing and duration of the spark.

How the Ignition System Works

The process unfolds as follows:

As the engine’s flywheel rotates, the magnets embedded within it pass by the magneto.
This interaction induces a low-voltage electrical current in the magneto’s coil.
The magneto sends this current to the ignition coil.
The ignition coil, acting as a transformer, steps up the voltage to thousands of volts.
This high-voltage surge is then sent to the spark plug.
The high voltage creates a spark across the spark plug gap, igniting the fuel-air mixture in the cylinder.
The combustion process drives the piston, which in turn rotates the crankshaft and powers the chainsaw.

Why Ignition Coils Fail

Ignition coils are subjected to harsh conditions, including high temperatures, vibrations, and exposure to dirt and moisture. Over time, these factors can lead to the degradation of the coil’s internal components, resulting in reduced performance or complete failure.

Common causes of ignition coil failure include:

Overheating: Excessive heat can damage the coil’s insulation, leading to short circuits.
Vibration: Constant vibration can weaken the coil’s internal connections.
Moisture: Moisture can corrode the coil’s windings, causing them to short out.
Physical Damage: Impacts or other forms of physical damage can crack the coil’s housing or damage its internal components.
Normal Wear and Tear: Like any mechanical component, ignition coils have a limited lifespan and will eventually wear out with use.

Preparing to Check Coil Resistance: Tools and Safety

Before you start poking around your chainsaw’s ignition system, it’s crucial to gather the necessary tools and take the appropriate safety precautions. Working with electrical components can be dangerous, so it’s essential to prioritize your safety at all times.

Essential Tools

Multimeter: A multimeter is an essential tool for any electrical troubleshooting task. It allows you to measure voltage, current, and resistance. For checking coil resistance, you’ll need a multimeter that can accurately measure low resistance values (typically in the ohms range). A digital multimeter (DMM) is generally preferred for its accuracy and ease of use.
Screwdrivers: You’ll need a variety of screwdrivers (both flathead and Phillips head) to remove the chainsaw’s covers and access the ignition coil.
Wrench or Socket Set: Depending on the chainsaw model, you may need a wrench or socket set to remove certain components, such as the flywheel cover.
Spark Plug Wrench: A spark plug wrench is necessary to remove the spark plug for inspection.
Feeler Gauges: Feeler gauges are used to measure the air gap between the ignition coil and the flywheel. This gap is critical for proper ignition system operation.
Cleaning Supplies: A clean work area is essential for any repair job. You’ll need some cleaning supplies, such as a brush, rags, and solvent, to remove dirt and debris from the chainsaw.
Safety Glasses: Always wear safety glasses when working on power equipment to protect your eyes from flying debris.
Gloves: Wear gloves to protect your hands from dirt, grease, and sharp edges.
Chainsaw Repair Manual (if available): A repair manual specific to your chainsaw model can provide valuable information and diagrams to guide you through the troubleshooting process.

Safety Precautions

Disconnect the Spark Plug Wire: Before you begin any work on the ignition system, disconnect the spark plug wire from the spark plug. This will prevent the engine from accidentally starting and potentially causing injury.
Allow the Engine to Cool: If the engine has been running recently, allow it to cool down completely before you start working on it. The engine’s components can get very hot and cause burns.
Work in a Well-Ventilated Area: When working with gasoline or other flammable liquids, work in a well-ventilated area to avoid inhaling harmful fumes.
Avoid Contact with Electrical Components: Be careful not to touch any exposed electrical components while the engine is running or the ignition system is energized.
Consult a Qualified Technician: If you are not comfortable working on electrical components or if you are unsure about any aspect of the troubleshooting process, consult a qualified technician.

Understanding Resistance Values

Before we get into the actual testing procedure, it’s important to understand what we’re looking for. The resistance of the ignition coil is measured in ohms (Ω). A healthy ignition coil will have a specific resistance range for both the primary and secondary windings.

Primary Winding: The primary winding is the low-voltage side of the coil, connected to the magneto. Its resistance is typically very low, often less than 1 ohm.
Secondary Winding: The secondary winding is the high-voltage side of the coil, connected to the spark plug wire. Its resistance is typically much higher, ranging from a few thousand ohms to tens of thousands of ohms.

These resistance values vary depending on the chainsaw model and the specific ignition coil. You’ll need to consult your chainsaw’s repair manual or the ignition coil’s specifications to determine the correct resistance range for your particular coil. If you cannot find the exact specifications, you can often find similar coil resistance values online for comparable chainsaw models.

Step-by-Step Guide: Checking Chainsaw Coil Resistance

Now that you have your tools and safety precautions in place, let’s get down to the business of checking the ignition coil’s resistance. This process involves using a multimeter to measure the resistance of both the primary and secondary windings of the coil.

Step 1: Accessing the Ignition Coil

The first step is to access the ignition coil. The location of the coil varies depending on the chainsaw model, but it’s typically located under the engine cover, near the flywheel.

Remove the Engine Cover: Use a screwdriver or wrench to remove the engine cover, exposing the ignition coil and other engine components.
Locate the Ignition Coil: The ignition coil is usually a small, rectangular component with two or three terminals. One terminal is connected to the magneto, another to the spark plug wire, and sometimes there is a ground terminal.
Disconnect the Spark Plug Wire: If you haven’t already, disconnect the spark plug wire from the spark plug.
Disconnect the Wires from the Coil: Carefully disconnect the wires connected to the ignition coil terminals. Make sure to note the position of each wire so you can reconnect them correctly later.

Step 2: Measuring Primary Winding Resistance

The primary winding is the low-voltage side of the coil, connected to the magneto. Its resistance is typically very low, often less than 1 ohm.

Set the Multimeter to Ohms: Turn on your multimeter and set it to the ohms (Ω) setting. Select the lowest resistance range available, typically 200 ohms or less.
Connect the Multimeter Probes: Connect the multimeter probes to the terminals of the primary winding. These terminals are usually marked with symbols or letters, such as “+” and “-“. If there are no markings, it doesn’t matter which probe goes to which terminal.
Read the Resistance Value: Observe the resistance value displayed on the multimeter. A healthy primary winding should have a very low resistance, typically less than 1 ohm.
Interpret the Results:
- Resistance within the specified range: The primary winding is likely good.
- Resistance significantly higher than the specified range: The primary winding may be faulty.
- Resistance of zero ohms (short circuit): The primary winding is shorted and needs replacement.
- Resistance of infinite ohms (open circuit): The primary winding is open and needs replacement.

Step 3: Measuring Secondary Winding Resistance

The secondary winding is the high-voltage side of the coil, connected to the spark plug wire. Its resistance is typically much higher, ranging from a few thousand ohms to tens of thousands of ohms.

Set the Multimeter to Ohms: Change the multimeter setting to a higher ohms range, typically in the thousands of ohms (kΩ) or megaohms (MΩ) range.
Connect the Multimeter Probes: Connect one multimeter probe to the terminal where the spark plug wire connects and the other probe to the coil body (ground).
Read the Resistance Value: Observe the resistance value displayed on the multimeter. A healthy secondary winding should have a resistance within the specified range, typically ranging from a few thousand ohms to tens of thousands of ohms.
Interpret the Results:
- Resistance within the specified range: The secondary winding is likely good.
- Resistance significantly higher than the specified range: The secondary winding may be faulty.
- Resistance of zero ohms (short circuit): The secondary winding is shorted and needs replacement.
- Resistance of infinite ohms (open circuit): The secondary winding is open and needs replacement.

Step 4: Checking for Grounding Issues

In addition to measuring the resistance of the primary and secondary windings, it’s also important to check for grounding issues. A grounded ignition coil can cause a weak spark or no spark at all.

Set the Multimeter to Ohms: Set the multimeter to the highest ohms range available.
Connect the Multimeter Probes: Connect one multimeter probe to the coil’s primary or secondary terminal and the other probe to the engine’s metal frame (ground).
Read the Resistance Value: Observe the resistance value displayed on the multimeter. A healthy ignition coil should have infinite resistance to ground.
Interpret the Results:
- Infinite resistance: The ignition coil is not grounded and is likely good.
- Any resistance reading (less than infinite): The ignition coil is grounded and needs replacement.

Step 5: Reassembling and Testing

Once you have completed the resistance tests, it’s time to reassemble the chainsaw and test the ignition system.

Reconnect the Wires to the Coil: Carefully reconnect the wires to the ignition coil terminals, ensuring that they are connected in the correct positions.
Reconnect the Spark Plug Wire: Reconnect the spark plug wire to the spark plug.
Reinstall the Engine Cover: Reinstall the engine cover and tighten the screws or bolts.
Check the Air Gap: Use feeler gauges to check the air gap between the ignition coil and the flywheel. The correct air gap is typically specified in the chainsaw’s repair manual. Adjust the coil’s position if necessary to achieve the correct air gap.
Test the Chainsaw: Attempt to start the chainsaw. If the ignition coil was the problem, the chainsaw should now start and run normally.

Interpreting Results and Troubleshooting Further

So, you’ve meticulously checked the coil resistance, and now you’re staring at the multimeter readings, scratching your head. What do those numbers actually mean? Let’s break down how to interpret your results and what steps to take next, depending on what you find.

Deciphering the Resistance Readings

As we discussed earlier, a healthy ignition coil will have specific resistance ranges for both the primary and secondary windings. These values can vary depending on the chainsaw model, so it’s essential to consult your repair manual or the coil’s specifications.

Within Range: If your multimeter readings fall within the specified range for both the primary and secondary windings, congratulations! Your ignition coil is likely in good working order. However, this doesn’t necessarily rule out the coil as the source of your chainsaw’s problems. There could be other issues, such as a faulty spark plug, a clogged fuel filter, or a problem with the carburetor.
Out of Range: If your multimeter readings are significantly higher or lower than the specified range, it indicates a problem with the ignition coil.
- High Resistance (Open Circuit): A very high or infinite resistance reading suggests an open circuit in the winding. This means that the wire inside the coil is broken, preventing the flow of electricity. In this case, the ignition coil needs to be replaced.
- Low Resistance (Short Circuit): A very low resistance reading suggests a short circuit in the winding. This means that the insulation between the wires inside the coil has broken down, allowing the electricity to flow directly from one wire to another. In this case, the ignition coil also needs to be replaced.
- Grounded Coil: If you find any resistance between the coil’s terminals and the engine’s metal frame (ground), it indicates that the coil is grounded. This means that the insulation has broken down, allowing the electricity to leak to ground. A grounded coil can cause a weak spark or no spark at all. In this case, the ignition coil needs to be replaced.

Beyond Resistance: Other Checks

Even if the coil resistance checks out, it’s prudent to examine other aspects of the ignition system. Here’s a checklist:

Spark Plug Inspection: Remove the spark plug and inspect it for signs of wear, fouling, or damage. A worn or fouled spark plug can prevent the engine from starting. Clean the spark plug with a wire brush or replace it with a new one if necessary. Also, check the spark plug gap and adjust it to the specified value.
Spark Test: Perform a spark test to verify that the ignition system is producing a strong spark. To do this, remove the spark plug, reconnect it to the spark plug wire, and hold the spark plug against the engine’s metal frame. Pull the starter rope and observe the spark plug gap. A healthy ignition system should produce a strong, blue spark. A weak, yellow spark or no spark indicates a problem with the ignition system.
Air Gap Adjustment: The air gap between the ignition coil and the flywheel is critical for proper ignition system operation. If the air gap is too small, the coil may overheat and fail. If the air gap is too large, the spark may be too weak to ignite the fuel-air mixture. Use feeler gauges to check the air gap and adjust it to the specified value.
Wiring Inspection: Carefully inspect the wiring connected to the ignition coil for signs of damage, such as frayed wires, loose connections, or corrosion. Repair or replace any damaged wiring.
Flywheel Key: The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If the flywheel key is damaged or sheared, it can cause the ignition timing to be off, preventing the engine from starting. Inspect the flywheel key for damage and replace it if necessary.

A Personal Anecdote

I remember one time, I was helping a friend clear some brush on his property. His chainsaw, a trusty old workhorse, suddenly died on him. We checked the usual suspects – fuel, air filter, spark plug – but nothing seemed to work. Finally, we decided to check the ignition coil. The resistance readings were all over the place, indicating a faulty coil. We replaced the coil, and the chainsaw fired right up. It was a satisfying moment, knowing that we had diagnosed and fixed the problem ourselves.

Advanced Troubleshooting and When to Seek Professional Help

While I’ve armed you with a solid foundation for troubleshooting your chainsaw’s ignition system, some problems are more complex and may require advanced diagnostic techniques or specialized tools. Knowing when to throw in the towel and seek professional help is crucial to avoid further damage to your chainsaw or, more importantly, injury to yourself.

Advanced Diagnostic Techniques

Peak Voltage Testing: A peak voltage tester can measure the peak voltage produced by the ignition coil. This test can help identify coils that are producing a weak spark, even if the resistance readings are within the specified range.
Oscilloscope Analysis: An oscilloscope can display the waveform of the ignition signal. This allows you to analyze the timing, duration, and amplitude of the spark, providing valuable insights into the performance of the ignition system.
Timing Light: A timing light can be used to check the ignition timing. Incorrect ignition timing can cause the engine to run poorly or not at all.

Common Scenarios Requiring Professional Assistance

Complex Electrical Problems: If you’ve exhausted all the basic troubleshooting steps and are still unable to diagnose the problem, it’s time to seek professional help. Complex electrical problems may require specialized diagnostic equipment and expertise.
Engine Disassembly: If the problem requires disassembling the engine, it’s best to leave it to a qualified technician. Engine disassembly can be complex and requires specialized tools and knowledge.
Unfamiliar with Electrical Systems: If you are not comfortable working on electrical systems, it’s always best to seek professional help. Working with electricity can be dangerous, and it’s important to prioritize your safety.
Recurring Problems: If you’ve repaired the ignition system multiple times and the problem keeps recurring, it may indicate a more serious underlying issue. A qualified technician can diagnose the root cause of the problem and recommend a permanent solution.

Choosing a Qualified Technician

When seeking professional help, it’s important to choose a qualified technician with experience in chainsaw repair. Look for a technician who is certified by a reputable organization, such as the Equipment & Engine Training Council (EETC). Ask the technician about their experience with chainsaw repair and their diagnostic process. Get a written estimate before authorizing any repairs.

Cost Considerations

Chainsaw repair costs can vary depending on the complexity of the problem and the technician’s labor rate. Before authorizing any repairs, get a written estimate that includes the cost of parts and labor. Compare estimates from multiple technicians to ensure that you are getting a fair price.

In some cases, it may be more cost-effective to replace the chainsaw rather than repair it. Consider the age and condition of the chainsaw, as well as the cost of the repairs, when making this decision.

Preventative Maintenance: Extending Ignition Coil Life

The best way to avoid ignition coil problems is to practice preventative maintenance. By taking a few simple steps, you can extend the life of your ignition coil and keep your chainsaw running smoothly.

Key Preventative Measures

Keep the Chainsaw Clean: Regularly clean the chainsaw to remove dirt, debris, and moisture. Pay particular attention to the engine area, where the ignition coil is located.
Store the Chainsaw Properly: Store the chainsaw in a dry, well-ventilated area. Avoid storing the chainsaw in direct sunlight or in damp conditions.
Use Fresh Fuel: Use fresh, high-quality fuel. Old or contaminated fuel can damage the engine and ignition system.
Maintain the Air Filter: Regularly clean or replace the air filter. A clogged air filter can cause the engine to overheat, which can damage the ignition coil.
Check the Spark Plug: Regularly check the spark plug for signs of wear or fouling. Replace the spark plug as needed.
Inspect the Wiring: Regularly inspect the wiring connected to the ignition coil for signs of damage. Repair or replace any damaged wiring.
Avoid Overheating the Engine: Avoid overheating the engine by running it at full throttle for extended periods.
Use the Chainsaw Properly: Use the chainsaw properly and avoid abusing it.
Follow the Manufacturer’s Recommendations: Follow the manufacturer’s recommendations for maintenance and operation.

Seasonal Storage Tips

If you plan to store your chainsaw for an extended period, such as during the off-season, take these additional steps:

Drain the Fuel Tank: Drain the fuel tank to prevent the fuel from going stale and damaging the engine.
Remove the Spark Plug: Remove the spark plug and spray a small amount of engine oil into the cylinder. This will help prevent corrosion.
Store the Chainsaw in a Dry Place: Store the chainsaw in a dry, well-ventilated place.
Cover the Chainsaw: Cover the chainsaw to protect it from dust and debris.

Case Studies: Real-World Ignition Coil Troubleshooting

Let’s look at a couple of real-world examples where I’ve encountered ignition coil issues and how I approached the troubleshooting process. These case studies will illustrate the practical application of the techniques we’ve discussed.

Case Study 1: The “Stubborn Stihl”

A friend of mine, a seasoned logger, called me in a panic. His Stihl MS 261, his go-to saw for felling smaller trees, refused to start. He’d already checked the fuel, air filter, and spark plug, but nothing seemed to work.

Symptoms: Chainsaw would not start.
Initial Checks: Fuel, air filter, and spark plug were all in good condition.
Troubleshooting Steps:
1. Spark Test: Performed a spark test and found a weak, intermittent spark.
2. Coil Resistance Test: Checked the resistance of the ignition coil and found that the secondary winding resistance was significantly higher than the specified range.
Diagnosis: Faulty ignition coil.
Solution: Replaced the ignition coil with a new one.
Outcome: The chainsaw started immediately and ran smoothly.

In this case, the spark test was the key to identifying the problem. The weak spark indicated that the ignition system was not functioning properly, and the coil resistance test confirmed that the ignition coil was the culprit.

Case Study 2: The “Finicky Farm Boss”

Another time, a neighbor asked me to take a look at his Husqvarna Farm Boss. The chainsaw would start and run for a few minutes, but then it would suddenly die. He suspected a fuel issue, but he couldn’t find anything wrong with the fuel system.

Symptoms: Chainsaw would start and run for a few minutes, then die.
Initial Checks: Fuel system appeared to be in good condition.
Troubleshooting Steps:
1. Spark Test: Performed a spark test after the chainsaw died and found no spark.
2. Coil Resistance Test: Checked the resistance of the ignition coil and found that the coil was grounded.
Diagnosis: Grounded ignition coil.
Solution: Replaced the ignition coil with a new one.
Outcome: The chainsaw started and ran without any issues.

In this case, the key was to perform the spark test after the chainsaw died. The fact that there was no spark indicated that the ignition system was failing when the engine got hot. The coil resistance test confirmed that the coil was grounded, which was likely caused by heat-related breakdown of the insulation.

These case studies highlight the importance of a systematic approach to troubleshooting chainsaw problems. By carefully observing the symptoms, performing the appropriate tests, and interpreting the results correctly, you can diagnose and fix a wide range of ignition coil issues.

The Future of Chainsaw Technology: Ignition Systems and Beyond

As technology advances, so does the design and functionality of chainsaws. While the fundamental principles of the ignition system remain the same, manufacturers are incorporating new technologies to improve performance, efficiency, and reliability.

Advancements in Ignition Systems

Electronic Ignition Modules: Many modern chainsaws now use electronic ignition modules instead of traditional magneto systems. Electronic ignition modules offer several advantages, including more precise timing, improved starting performance, and increased reliability.
Digital Ignition Systems: Some high-end chainsaws feature digital ignition systems that use microprocessors to control the timing and duration of the spark. Digital ignition systems offer even greater precision and control than electronic ignition modules.
Coil-on-Plug (COP) Systems: Some chainsaws are now using coil-on-plug (COP) ignition systems, where the ignition coil is mounted directly on the spark plug. COP systems eliminate the need for spark plug wires, reducing the risk of voltage loss and improving spark quality.

Other Technological Advancements in Chainsaws

Fuel Injection: Fuel injection systems are becoming increasingly common in chainsaws. Fuel injection offers several advantages over carburetors, including improved fuel efficiency, reduced emissions, and better performance at high altitudes.
Automatic Chain Oilers: Automatic chain oilers are now standard on most chainsaws. Automatic chain oilers ensure that the chain is properly lubricated, reducing wear and tear and extending the life of the chain.
Chain Brakes: Chain brakes are a safety feature that stops the chain immediately if kickback occurs. Chain brakes are now required on all new chainsaws.
Anti-Vibration Systems: Anti-vibration systems reduce the amount of vibration that is transmitted to the operator, reducing fatigue and improving comfort.
Lightweight Materials: Manufacturers are using lightweight materials, such as magnesium and carbon fiber, to reduce the weight of chainsaws. Lightweight chainsaws are easier to handle and reduce fatigue.

The Impact on Wood Processing

These technological advancements are having a significant impact on wood processing, making it more efficient, safer, and more environmentally friendly. Fuel injection systems reduce fuel consumption and emissions. Automatic chain oilers reduce wear and tear on the chain. Chain brakes improve safety. Anti-vibration systems reduce fatigue. And lightweight materials make chainsaws easier to handle.

As technology continues to advance, we can expect to see even more innovations in chainsaw design and functionality. These innovations will further improve the efficiency, safety, and environmental friendliness of wood processing.

Remember, a well-maintained chainsaw is the cornerstone of any successful wood processing endeavor. By understanding how the ignition system works and by practicing preventative maintenance, you can keep your chainsaw running smoothly for years to come.

Checking the coil resistance is a crucial step in diagnosing ignition problems, but it’s just one piece of the puzzle. Don’t be afraid to explore other potential causes, such as a faulty spark plug, a clogged fuel filter, or a problem with the carburetor.

And finally, don’t hesitate to seek professional help when needed. Complex electrical problems may require specialized diagnostic equipment and expertise.

So, the next time your chainsaw refuses to roar to life, don’t panic. Armed with the knowledge you’ve gained from this article, you’ll be well-equipped to tackle the problem and get your chainsaw back in action. Happy cutting!