How to Check a Chainsaw Coil (5 Pro Tips for Accurate Testing)

Have you ever yanked the starter cord on your chainsaw, only to be met with frustrating silence? It’s a feeling I know all too well. More often than not, the culprit is a faulty ignition coil. The coil is a critical component, responsible for generating the high-voltage spark that ignites the fuel-air mixture in your chainsaw’s engine. Without a properly functioning coil, your chainsaw is essentially a very heavy, very expensive paperweight.

Over the years, I’ve spent countless hours troubleshooting chainsaw ignition systems. From my early days working on the family farm to my current role as a consultant for small-scale logging operations, I’ve learned a thing or two about diagnosing and fixing coil issues. Testing a chainsaw coil might seem daunting, but with the right knowledge and a few simple tools, you can quickly determine if it’s the source of your problems.

In this guide, I’ll share five professional tips for accurately testing your chainsaw coil. I’ll walk you through the process step-by-step, explaining the tools you’ll need, the tests you’ll perform, and the interpretations of your results. Whether you’re a seasoned logger or a weekend warrior, this guide will empower you to diagnose and potentially fix your chainsaw’s ignition problems, saving you time, money, and frustration.

How to Check a Chainsaw Coil: 5 Pro Tips for Accurate Testing

Understanding the Chainsaw Ignition System

Before diving into the testing process, it’s crucial to understand the basics of a chainsaw ignition system. This knowledge will not only help you troubleshoot coil issues but also give you a better understanding of how your chainsaw operates.

The ignition system’s primary function is to generate a spark that ignites the fuel-air mixture in the cylinder, initiating the combustion process that drives the engine. The key components of the system include:

• Flywheel: A rotating component connected to the crankshaft. It contains magnets that induce a current in the ignition coil.
• Ignition Coil: A transformer that converts the low-voltage current from the flywheel into a high-voltage pulse.
• Spark Plug: A device that receives the high-voltage pulse from the coil and creates a spark gap in the cylinder.
• Ignition Module (in some models): An electronic component that controls the timing of the spark.
• Kill Switch: A switch that grounds the ignition coil, preventing the spark from occurring and stopping the engine.

When the flywheel rotates, its magnets pass near the ignition coil, inducing a small electrical current. The coil amplifies this current into a high-voltage pulse, typically in the range of 15,000 to 30,000 volts. This high-voltage pulse is then sent to the spark plug, creating a spark across the gap between the electrode and the ground. This spark ignites the fuel-air mixture, causing an explosion that pushes the piston and drives the crankshaft.

A faulty coil can manifest in several ways, including:

• No spark: The most obvious symptom. The engine won’t start because there’s no spark to ignite the fuel-air mixture.
• Weak spark: The engine may start but run poorly, with misfires or lack of power.
• Intermittent spark: The engine may start and run for a while, then suddenly stall due to a loss of spark.
• Difficult starting: The engine may be hard to start, requiring multiple pulls on the starter cord.

Takeaway: Understanding the basic components and function of the ignition system is the first step towards effectively diagnosing coil problems. Familiarize yourself with the role of each component, and you’ll be better equipped to pinpoint the source of your chainsaw’s starting issues.

Tip 1: Visual Inspection – The First Line of Defense

Before you even think about grabbing a multimeter, the first thing I always do is a thorough visual inspection of the coil and its surrounding components. This simple step can often reveal obvious problems that would otherwise be missed.

Here’s what I look for:

• Cracks or Damage: Carefully examine the coil for any visible cracks, chips, or other damage to the plastic housing. Even small cracks can allow moisture to penetrate, leading to short circuits and coil failure. I’ve seen coils that looked perfectly fine at first glance, but upon closer inspection, revealed hairline cracks that were the source of the problem.
• Corrosion: Check the coil terminals and wiring connections for signs of corrosion. Corrosion can impede the flow of electricity, resulting in a weak or intermittent spark. I often use a wire brush or sandpaper to clean corroded terminals and ensure a good connection.
• Loose Wiring: Inspect the wiring connected to the coil for any loose or frayed wires. Loose connections can cause intermittent spark and engine stalling. Gently tug on each wire to ensure it’s securely attached to the terminal. I’ve encountered situations where a wire had simply come loose from the terminal, causing the engine to die unexpectedly.
• Air Gap: The air gap between the coil and the flywheel magnets is crucial for proper operation. If the gap is too wide or too narrow, it can affect the coil’s ability to generate a strong spark. The correct air gap is typically between 0.010 and 0.014 inches (0.25 to 0.35 mm). I use a feeler gauge to accurately measure the air gap and adjust it if necessary. I once worked on a chainsaw where the coil had shifted slightly, resulting in an excessively wide air gap. Adjusting the gap back to the correct specification restored the engine’s performance.
• Spark Plug Wire: Inspect the spark plug wire for any cracks, cuts, or damage. A damaged spark plug wire can leak voltage to ground, resulting in a weak or no spark. I often use an ohmmeter to test the spark plug wire for continuity, ensuring that it’s not broken or shorted.

Personal Story: I remember one time, a logger friend of mine was having trouble with his chainsaw. He had already replaced the spark plug and fuel filter, but the engine still wouldn’t start. After a quick visual inspection, I noticed that the insulation on the spark plug wire was cracked and brittle. Replacing the wire solved the problem instantly.

Takeaway: A thorough visual inspection is a quick and easy way to identify obvious coil problems. Don’t underestimate the power of your eyes. Taking the time to carefully examine the coil and its surrounding components can save you a lot of time and effort in the long run.

Tip 2: The Spark Test – The Most Basic Indicator

The spark test is a simple yet effective way to determine if your chainsaw coil is producing a spark. It’s a quick and easy way to narrow down the possible causes of your engine problems.

Here’s how to perform a spark test:

1. Remove the Spark Plug: Carefully remove the spark plug from the cylinder head using a spark plug wrench.
2. Reattach the Spark Plug Wire: Reattach the spark plug wire to the spark plug.
3. Ground the Spark Plug: Hold the spark plug against a metal part of the engine, such as the cylinder head or engine block. This provides a ground path for the spark.
4. Pull the Starter Cord: Briskly pull the starter cord and observe the spark plug gap. You should see a bright blue spark jumping across the gap.

Interpreting the Results:

• Strong Blue Spark: This indicates that the coil is likely functioning properly. The problem may lie elsewhere, such as in the fuel system or compression.
• Weak Yellow Spark: This suggests that the coil may be weak or that there is a problem with the ignition system. Check the coil connections, air gap, and spark plug wire.
• No Spark: This indicates that the coil is likely faulty and needs to be replaced. However, before replacing the coil, double-check the kill switch and wiring connections to ensure that they are not grounding the coil.

Safety Precautions:

• Wear Gloves: I always wear gloves when performing a spark test to protect myself from electrical shock.
• Avoid Flammable Materials: Ensure that there are no flammable materials nearby, as the spark can ignite fuel vapors.
• Work in a Well-Ventilated Area: Perform the test in a well-ventilated area to avoid inhaling fuel fumes.

Personal Story: I once helped a friend troubleshoot his chainsaw that wouldn’t start. We performed the spark test and found that there was no spark at all. We initially assumed that the coil was bad, but after checking the kill switch, we discovered that it was stuck in the “off” position. A simple adjustment to the kill switch restored the spark and got the chainsaw running again.

Takeaway: The spark test is a simple and effective way to determine if your chainsaw coil is producing a spark. If you don’t see a strong blue spark, it’s a good indication that the coil may be faulty. However, before replacing the coil, be sure to check the kill switch and wiring connections.

Tip 3: Resistance Testing with a Multimeter – The Definitive Diagnostic

While the spark test provides a basic indication of coil function, a multimeter provides a more precise way to assess the coil’s health. By measuring the resistance of the primary and secondary windings, you can identify internal shorts or open circuits that could be causing problems.

Here’s how to perform a resistance test:

1. Disconnect the Coil: Disconnect the coil from the spark plug wire and any other wiring connections.
2. Set the Multimeter: Set your multimeter to the ohms (Ω) setting. You may need to select the appropriate resistance range depending on your multimeter. Typically, a range of 200 ohms for the primary winding and 20,000 ohms for the secondary winding is appropriate.
3. Test the Primary Winding: Touch one probe of the multimeter to the primary terminal of the coil (typically the terminal connected to the ignition module or wiring harness) and the other probe to the coil’s ground terminal (usually the metal mounting bracket). Record the resistance reading.
4. Test the Secondary Winding: Touch one probe of the multimeter to the spark plug terminal of the coil and the other probe to the coil’s ground terminal. Record the resistance reading.

Interpreting the Results:

• Primary Winding Resistance: The primary winding resistance is typically very low, often less than 1 ohm. A reading of zero or infinity indicates a short circuit or open circuit, respectively. Consult your chainsaw’s service manual for the specific resistance range for your coil model.
• Secondary Winding Resistance: The secondary winding resistance is typically much higher than the primary winding resistance, often in the range of 2,000 to 10,000 ohms. Again, consult your chainsaw’s service manual for the specific resistance range for your coil model. A reading of zero or infinity indicates a short circuit or open circuit, respectively.

Example Data:

Let’s say you’re testing the coil on a Stihl MS 271 chainsaw. According to the service manual, the primary winding resistance should be between 0.5 and 1.5 ohms, and the secondary winding resistance should be between 4,000 and 6,000 ohms. If your multimeter readings fall outside of these ranges, it’s a strong indication that the coil is faulty.

Personal Story: I once worked on a chainsaw that was producing a weak spark. The spark test indicated that the coil was functioning, but the engine was still running poorly. After performing a resistance test, I discovered that the secondary winding resistance was significantly lower than the specified range. This indicated an internal short in the coil, which was causing the weak spark. Replacing the coil resolved the problem.

Takeaway: Resistance testing with a multimeter is a definitive way to diagnose coil problems. By measuring the resistance of the primary and secondary windings, you can identify internal shorts or open circuits that could be causing a weak spark or no spark at all. Always consult your chainsaw’s service manual for the specific resistance ranges for your coil model.

Tip 4: Checking the Kill Switch and Wiring – Don’t Overlook the Simple Things

Before you declare your coil dead and order a replacement, it’s crucial to rule out any issues with the kill switch and its associated wiring. A faulty kill switch or a shorted wire can ground the coil, preventing it from producing a spark, even if the coil itself is perfectly fine.

Here’s how to check the kill switch and wiring:

1. Locate the Kill Switch: The kill switch is typically a small switch located on the handle of the chainsaw. It’s usually marked with an “O” (for off) and an “I” (for on).
2. Inspect the Wiring: Carefully inspect the wiring connected to the kill switch for any damage, such as cuts, frays, or loose connections. Pay close attention to the area where the wires pass through the engine housing, as this is a common area for damage to occur.
3. Disconnect the Kill Switch Wire: Disconnect the wire that runs from the kill switch to the coil. This will isolate the kill switch from the ignition system.
4. Perform a Spark Test: With the kill switch wire disconnected, perform a spark test as described in Tip 2. If the engine now produces a strong blue spark, it indicates that the kill switch or its wiring is the source of the problem.
5. Test the Kill Switch Continuity: Use a multimeter to test the continuity of the kill switch. Set the multimeter to the continuity setting (often indicated by a diode symbol). Touch one probe of the multimeter to each terminal of the kill switch. With the switch in the “on” position, the multimeter should show no continuity (an open circuit). With the switch in the “off” position, the multimeter should show continuity (a closed circuit). If the kill switch does not behave as expected, it needs to be replaced.

Personal Story: I once spent hours troubleshooting a chainsaw that wouldn’t start. I had already replaced the spark plug, fuel filter, and even the coil, but the engine still wouldn’t fire. Finally, in desperation, I decided to check the kill switch wiring. To my surprise, I found that the wire had been pinched between the engine housing and the handle, causing a short circuit. Repairing the wire solved the problem instantly.

Takeaway: Don’t overlook the simple things. A faulty kill switch or shorted wiring can ground the coil and prevent it from producing a spark. Always check the kill switch and its wiring before replacing the coil. Disconnecting the kill switch wire and performing a spark test is a quick and easy way to determine if the kill switch is the source of the problem.

Tip 5: The Air Gap Adjustment – Fine-Tuning for Optimal Performance

As I mentioned earlier, the air gap between the coil and the flywheel magnets is critical for proper ignition. If the gap is too wide, the coil won’t be able to generate a strong enough spark. If the gap is too narrow, the coil can rub against the flywheel, causing damage and premature failure.

Here’s how to adjust the air gap:

1. Locate the Coil and Flywheel: Locate the ignition coil and the flywheel on your chainsaw. The coil is typically mounted near the flywheel, with a small gap between the two components.
2. Loosen the Coil Mounting Screws: Loosen the screws that hold the coil in place. You should be able to move the coil slightly.
3. Insert a Feeler Gauge: Insert a feeler gauge of the correct thickness between the coil and the flywheel magnets. The correct air gap is typically between 0.010 and 0.014 inches (0.25 to 0.35 mm). Consult your chainsaw’s service manual for the specific air gap specification for your model.
4. Tighten the Coil Mounting Screws: With the feeler gauge in place, tighten the coil mounting screws. Be careful not to overtighten the screws, as this could damage the coil or the engine housing.
5. Remove the Feeler Gauge: Remove the feeler gauge.
6. Verify the Air Gap: Double-check the air gap to ensure that it’s still within the specified range.

Using a Business Card:

If you don’t have a feeler gauge, you can use a business card as a makeshift spacer. The thickness of a standard business card is typically around 0.012 inches (0.3 mm), which is within the acceptable range for most chainsaws.

Personal Story: I once worked on a chainsaw that was running but lacked power. The engine would start and idle, but it would bog down under load. I checked the fuel system, the spark plug, and even the compression, but everything seemed to be in order. Finally, I decided to check the air gap between the coil and the flywheel. To my surprise, I found that the gap was significantly wider than the specified range. Adjusting the air gap back to the correct specification restored the engine’s power and performance.

Takeaway: The air gap between the coil and the flywheel magnets is critical for proper ignition. Adjusting the air gap to the correct specification can improve engine performance and prevent premature coil failure. Always consult your chainsaw’s service manual for the specific air gap specification for your model.

Beyond the 5 Tips: Additional Considerations and Best Practices

While these five tips will cover the vast majority of chainsaw coil troubleshooting scenarios, there are a few additional considerations and best practices I’d like to share:

• Check the Flywheel Key: The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If the flywheel key is sheared or damaged, it can cause the flywheel to rotate out of time, resulting in a weak or no spark. Inspect the flywheel key for damage and replace it if necessary.
• Consider the Age of the Coil: Even if a coil passes the resistance tests, it may still be weak or unreliable due to age. Over time, the insulation on the coil windings can break down, leading to internal shorts and reduced spark output. If your chainsaw is several years old and the coil has never been replaced, it may be worth considering a replacement, even if it seems to be functioning properly.
• Use Quality Replacement Parts: When replacing a faulty coil, always use a high-quality replacement part from a reputable manufacturer. Cheap aftermarket coils may not meet the original specifications and can fail prematurely. I’ve learned this lesson the hard way by using cheaper parts.
• Protect Your Chainsaw from the Elements: Moisture and extreme temperatures can damage the coil and other ignition system components. Store your chainsaw in a dry, protected area when not in use.
• Regular Maintenance: Regular maintenance, such as cleaning the spark plug, checking the air filter, and using fresh fuel, can help prevent ignition system problems.

Addressing Common Questions:

• Can I test a chainsaw coil without a multimeter? While a multimeter provides the most accurate assessment, you can perform a basic spark test as described in Tip 2. However, the spark test will only tell you if the coil is producing a spark, not if it’s producing a strong enough spark.
• How often should I replace my chainsaw coil? There’s no set replacement interval for chainsaw coils. However, if your chainsaw is several years old and the coil has never been replaced, it may be worth considering a replacement as a preventative measure.
• Can a bad coil damage my chainsaw engine? A faulty coil can cause the engine to run poorly, leading to increased wear and tear on other components. In extreme cases, a bad coil can cause the engine to overheat or even seize.

Final Thoughts:

Troubleshooting chainsaw ignition problems can be frustrating, but with the right knowledge and tools, you can diagnose and potentially fix the issue yourself. By following these five pro tips, you can accurately test your chainsaw coil and determine if it’s the source of your problems. Remember to start with a visual inspection, perform a spark test, test the resistance with a multimeter, check the kill switch and wiring, and adjust the air gap. And don’t forget to consider the age of the coil and use quality replacement parts.

The Next Steps:

Now that you have the knowledge and tools to test your chainsaw coil, it’s time to put them to use. Grab your multimeter, your feeler gauge, and your chainsaw, and start troubleshooting. And remember, if you’re not comfortable working on your chainsaw yourself, don’t hesitate to take it to a qualified repair shop.

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