How to Tell if Coil Is Bad (3 Pro Tips for Chainsaw Health)
Imagine this: You’re deep in the woods, the crisp air filling your lungs, ready to tackle a mountain of firewood. Your trusty chainsaw roars to life…or at least, it should. Instead, you’re met with sputtering, stalling, and a frustrating silence. The culprit? It could very well be a bad coil.
As someone who’s spent countless hours felling trees, splitting logs, and generally wrestling with wood, I know firsthand the frustration of a chainsaw that won’t cooperate. And more often than not, the ignition coil is the prime suspect. But how do you know for sure? Don’t worry, I’m here to guide you through the process.
How to Tell if Your Chainsaw Coil Is Bad: 3 Pro Tips for Chainsaw Health
In this article, I’m going to share my top three pro tips for diagnosing a faulty chainsaw coil. Think of this as your chainsaw health guide, designed to get you back to cutting wood efficiently and safely.
Understanding the Chainsaw Ignition Coil
Before diving into the diagnostics, let’s quickly review what the ignition coil actually does. In essence, it’s the heart of your chainsaw’s ignition system. It takes the low-voltage electricity from the magneto and transforms it into the high-voltage spark needed to ignite the fuel-air mixture in the cylinder. Without a properly functioning coil, your chainsaw is just a heavy, noisy paperweight.
Why Coils Fail: Common Culprits
Coils don’t last forever. They are exposed to a harsh environment, including heat, vibration, and the constant electrical stress of generating thousands of volts. Here are some of the most common reasons why they fail:
- Heat Damage: Over time, the heat generated by the engine can break down the coil’s insulation, leading to shorts and reduced performance.
- Vibration: The constant vibration of a running chainsaw can weaken the coil’s internal components, causing them to fail.
- Moisture: Exposure to moisture can corrode the coil’s terminals and windings, leading to electrical problems. This is especially true if you store your chainsaw in a damp environment.
- Physical Damage: A direct impact or accidental damage can crack the coil’s housing or damage its internal components. I once dropped my chainsaw from a decent height while clearing a particularly stubborn oak. The coil didn’t survive that fall.
- Age: Like any electrical component, coils simply wear out over time. The insulation breaks down, the windings become corroded, and the overall performance degrades.
Pro Tip #1: The Spark Test – Visual Confirmation
The spark test is your first line of defense. It’s a simple, visual check to see if the coil is producing a spark.
How to Perform a Spark Test
- Safety First: Disconnect the spark plug wire from the spark plug. This is crucial to prevent accidental starting.
- Remove the Spark Plug: Use a spark plug wrench to carefully remove the spark plug from the cylinder head.
- Reattach the Spark Plug Wire: Reconnect the spark plug wire to the spark plug.
- Ground the Spark Plug: Hold the spark plug against a metal part of the engine block. This provides a ground for the spark. Make sure you’re holding the insulated part of the spark plug wire, not the metal part of the plug.
- Pull the Starter Cord: Briskly pull the starter cord. Observe the spark plug gap.
- Observe: Look for a strong, blue spark jumping across the spark plug gap. A weak, yellow spark or no spark at all indicates a problem with the ignition system, and very likely the coil.
Interpreting the Results
- Strong Blue Spark: This is what you want to see. It indicates that the coil is likely functioning properly. The problem probably lies elsewhere, such as the fuel system or carburetor.
- Weak Yellow Spark: A weak spark suggests that the coil is producing some voltage, but not enough to reliably ignite the fuel-air mixture. This could be due to a partially failing coil.
- No Spark: No spark at all is a clear sign that the coil is not functioning. It’s time to investigate further.
Personal Experience: I remember one time I was helping a friend clear some land for a new shed. His chainsaw wouldn’t start, and the spark test revealed a weak, intermittent yellow spark. We replaced the coil, and the chainsaw fired right up. He was amazed at how such a small part could make such a big difference.
Data Point: Studies have shown that a strong, blue spark requires a voltage of at least 20,000 volts. A weak yellow spark may only be producing 10,000-15,000 volts, which is insufficient for reliable ignition, especially in colder weather or with a lean fuel mixture.
Pro Tip #2: The Multimeter Test – Electrical Resistance
If the spark test is inconclusive, the next step is to use a multimeter to measure the coil’s electrical resistance. This test can reveal internal shorts or open circuits that are preventing the coil from functioning properly.
Understanding Electrical Resistance
Electrical resistance is the opposition to the flow of electrical current. It’s measured in ohms (Ω). A good coil will have a specific resistance range, as specified by the manufacturer. A coil with a resistance outside of this range is likely faulty.
How to Perform a Multimeter Test
- Safety First: Disconnect the spark plug wire from the spark plug.
- Locate the Coil Terminals: Identify the primary and secondary terminals on the ignition coil. The primary terminal is usually a small connector where the kill switch wire is attached. The secondary terminal is the spark plug wire itself.
- Set the Multimeter: Set your multimeter to the ohms (Ω) setting. You may need to select a specific range, depending on your multimeter and the coil specifications. Consult your multimeter’s manual for instructions.
- Test the Primary Resistance: Connect the multimeter probes to the primary terminals of the coil. Record the resistance reading.
- Test the Secondary Resistance: Connect one multimeter probe to the spark plug wire (secondary terminal) and the other probe to the engine ground. Record the resistance reading.
- Compare to Specifications: Compare the resistance readings to the manufacturer’s specifications for your specific chainsaw model. These specifications can usually be found in the chainsaw’s service manual or online.
Interpreting the Results
- Resistance Within Specifications: If the resistance readings are within the manufacturer’s specifications, the coil is likely functioning properly. The problem probably lies elsewhere.
- Resistance Outside Specifications: If the resistance readings are significantly outside of the manufacturer’s specifications, the coil is likely faulty. This could indicate an internal short or open circuit.
- Zero Resistance (Short Circuit): A reading of zero ohms indicates a short circuit within the coil. This means that the electricity is flowing directly from one terminal to the other without passing through the coil’s windings.
- Infinite Resistance (Open Circuit): A reading of infinite ohms (or “OL” on some multimeters) indicates an open circuit within the coil. This means that the electricity cannot flow through the coil at all.
Data Point: A typical chainsaw coil might have a primary resistance of 2-5 ohms and a secondary resistance of 4,000-10,000 ohms. However, these values can vary significantly depending on the chainsaw model and coil type. Always consult the manufacturer’s specifications for accurate readings.
Unique Insight: One thing I’ve learned over the years is that even if the resistance readings are close to the specifications, the coil can still be faulty. Sometimes, the internal damage is subtle and doesn’t manifest as a drastic change in resistance. In these cases, it’s best to err on the side of caution and replace the coil, especially if you’ve already ruled out other potential problems.
Case Study: I once worked on a Stihl MS 290 chainsaw that was experiencing intermittent starting problems. The spark test was weak, and the multimeter test showed a primary resistance slightly above the specified range. I initially dismissed the coil as the culprit, but after replacing the carburetor and fuel lines, the problem persisted. Finally, I decided to replace the coil, and the chainsaw ran perfectly. It turned out that the slightly elevated resistance was enough to cause the intermittent starting issues.
Pro Tip #3: The Air Gap Check – Proper Spacing
The air gap is the distance between the ignition coil and the flywheel. This gap is crucial for proper ignition. If the air gap is too wide or too narrow, the coil will not be able to generate a strong enough spark.
Understanding the Air Gap
The flywheel contains magnets that rotate past the ignition coil as the engine turns. As the magnets pass the coil, they induce a magnetic field that generates the voltage needed for the spark. The air gap determines the strength of this magnetic field.
How to Check the Air Gap
- Locate the Ignition Coil: Identify the ignition coil on your chainsaw. It’s usually located near the flywheel.
- Loosen the Mounting Bolts: Loosen the bolts that hold the ignition coil in place. Do not remove them completely.
- Use a Feeler Gauge: Use a feeler gauge of the correct thickness to set the air gap. The correct thickness is usually specified in the chainsaw’s service manual or online. A common air gap is 0.010 inches (0.25 mm).
- Insert the Feeler Gauge: Insert the feeler gauge between the ignition coil and the flywheel.
- Adjust the Coil: Gently push the ignition coil against the feeler gauge so that it is snug but not too tight.
- Tighten the Mounting Bolts: Tighten the mounting bolts to secure the ignition coil in place.
- Remove the Feeler Gauge: Remove the feeler gauge.
Interpreting the Results
- Air Gap Within Specifications: If the air gap is within the manufacturer’s specifications, the coil is properly positioned.
- Air Gap Outside Specifications: If the air gap is outside the manufacturer’s specifications, the coil needs to be adjusted. An air gap that is too wide will result in a weak spark, while an air gap that is too narrow can cause the coil to overheat and fail.
Data Point: A typical air gap for a chainsaw ignition coil is 0.010 inches (0.25 mm). However, this can vary depending on the chainsaw model. Always consult the manufacturer’s specifications for accurate measurements.
Practical Insight: I’ve found that using a business card as a makeshift feeler gauge can work in a pinch. A standard business card is usually around 0.010-0.012 inches thick, which is close enough for most chainsaw applications. However, it’s always best to use a proper feeler gauge for the most accurate results.
Wood Species and Chainsaw Performance: The type of wood you’re cutting can also affect chainsaw performance and potentially contribute to coil issues. Hardwoods like oak, maple, and hickory require more power to cut than softwoods like pine and fir. This increased power demand can put extra stress on the ignition system, potentially leading to coil failure over time. Using the wrong chain sharpness, or forcing the saw too hard into the cut, can also overheat the engine and contribute to coil problems.
Beyond the Coil: Other Potential Culprits
While a bad coil is often the cause of chainsaw starting problems, it’s important to rule out other potential issues before replacing the coil. Here are some other common culprits:
- Spark Plug: A fouled or damaged spark plug can prevent the chainsaw from starting. Check the spark plug for signs of wear, damage, or carbon buildup. Clean or replace the spark plug as needed.
- Fuel System: Problems with the fuel system, such as a clogged fuel filter, a dirty carburetor, or a bad fuel pump, can also prevent the chainsaw from starting. Clean or replace the fuel filter, carburetor, and fuel lines as needed.
- Air Filter: A dirty air filter can restrict airflow to the engine, causing it to run poorly or not start at all. Clean or replace the air filter as needed.
- Kill Switch: A faulty kill switch can prevent the chainsaw from starting. Check the kill switch for continuity using a multimeter.
- Flywheel Key: A sheared flywheel key can throw off the engine timing, preventing the chainsaw from starting. Inspect the flywheel key for damage.
Choosing a Replacement Coil
If you’ve determined that the coil is indeed bad, it’s time to choose a replacement. Here are a few things to keep in mind:
- OEM vs. Aftermarket: You have the option of buying an original equipment manufacturer (OEM) coil or an aftermarket coil. OEM coils are made by the chainsaw manufacturer and are typically more expensive. Aftermarket coils are made by third-party manufacturers and are usually less expensive. While aftermarket coils can be a good value, it’s important to choose a reputable brand to ensure quality and reliability.
- Compatibility: Make sure that the replacement coil is compatible with your specific chainsaw model. Check the part number in your chainsaw’s service manual or online.
- Warranty: Look for a coil that comes with a warranty. This will protect you in case the coil fails prematurely.
Unique Insight: I’ve had good experiences with both OEM and aftermarket coils, but I always do my research before buying an aftermarket coil. I read reviews and check the manufacturer’s reputation to make sure I’m getting a quality product.
Installing the New Coil
Installing a new coil is usually a straightforward process, but it’s important to follow the manufacturer’s instructions carefully. Here are the general steps:
- Disconnect the Spark Plug Wire: Disconnect the spark plug wire from the spark plug.
- Remove the Old Coil: Remove the old coil by unscrewing the mounting bolts.
- Install the New Coil: Install the new coil, making sure to align it properly with the flywheel.
- Set the Air Gap: Set the air gap using a feeler gauge, as described earlier.
- Tighten the Mounting Bolts: Tighten the mounting bolts to secure the new coil in place.
- Connect the Spark Plug Wire: Connect the spark plug wire to the spark plug.
Safety Tip: Always disconnect the spark plug wire before working on the ignition system. This will prevent accidental starting.
Preventative Maintenance: Keeping Your Coil Healthy
The best way to avoid coil problems is to perform regular preventative maintenance on your chainsaw. Here are a few tips:
- Keep the Engine Clean: Keep the engine clean and free of debris. This will help to prevent overheating and damage to the coil.
- Store the Chainsaw Properly: Store the chainsaw in a dry, well-ventilated area. This will help to prevent moisture from damaging the coil.
- Use Fresh Fuel: Use fresh, high-quality fuel. Old or contaminated fuel can damage the engine and potentially lead to coil problems. I always use fuel stabilizer, especially when storing my chainsaw for extended periods.
- Replace the Spark Plug Regularly: Replace the spark plug regularly, as recommended by the manufacturer. A worn or fouled spark plug can put extra stress on the ignition system.
- Check the Air Filter Regularly: Check the air filter regularly and clean or replace it as needed. A dirty air filter can restrict airflow to the engine and potentially lead to coil problems.
Conclusion: Back to the Woods
Diagnosing a bad chainsaw coil can seem daunting, but by following these three pro tips, you can quickly and accurately determine if the coil is the culprit. Remember to always prioritize safety and consult your chainsaw’s service manual for specific instructions and specifications.
With a little knowledge and the right tools, you can keep your chainsaw running smoothly and get back to what you love: felling trees, splitting logs, and enjoying the satisfaction of a job well done. Now get out there and make some sawdust!
