Chainsaw Coil Testing (5 Pro Tips For Accurate Diagnosis)

Would you rather spend hours troubleshooting a chainsaw that won’t start, or diagnose the problem in minutes with the right knowledge? I’ve been there, wrestling with unresponsive chainsaws in the biting cold of winter. Trust me, knowing how to test your chainsaw coil is a game-changer. It can save you time, money, and a whole lot of frustration. In this guide, I’m sharing my top five pro tips for accurate chainsaw coil testing, based on years of experience in the field. Let’s dive in!

Chainsaw Coil Testing: 5 Pro Tips for Accurate Diagnosis

The ignition coil, sometimes called a magneto, is the heart of your chainsaw’s ignition system. It generates the high-voltage spark needed to ignite the fuel-air mixture in the cylinder. A faulty coil is a common culprit when your chainsaw refuses to start or runs erratically. But before you rush out to buy a new one, let’s make sure it’s actually the problem.

1. Understanding the Chainsaw Ignition System Basics

Before we get into testing, let’s quickly review how a chainsaw ignition system works. The flywheel, connected to the engine crankshaft, has magnets embedded in it. As the flywheel spins, these magnets pass by the ignition coil, inducing a voltage. This voltage is stepped up significantly by the coil and sent to the spark plug, creating a spark that ignites the fuel.

A typical chainsaw ignition system consists of:

Flywheel: Contains magnets to generate the initial voltage.
Ignition Coil: Steps up the voltage to create a spark.
Spark Plug: Provides the gap where the spark jumps to ignite the fuel.
Kill Switch: Grounds the ignition system to stop the engine.

Understanding these components is crucial for accurate diagnosis. A problem in any of these areas can mimic a faulty coil.

Personal Story: I once spent an entire afternoon convinced my chainsaw coil was bad. I even ordered a replacement! Turns out, the spark plug wire was loose. A simple fix, but a costly mistake due to a lack of understanding.

2. Visual Inspection: The First Line of Defense

Never underestimate the power of a good visual inspection. Before you grab your multimeter, take a close look at the ignition coil and its surroundings.

What to look for:

Cracks or Damage: Inspect the coil for any visible cracks, chips, or signs of physical damage. These can indicate internal failure.
Loose Connections: Check the wiring connections to the coil, spark plug, and kill switch. Make sure they are secure and free of corrosion.
Corrosion: Look for any signs of rust or corrosion on the coil terminals or wiring. Corrosion can impede electrical flow.
Air Gap: Verify that the air gap between the coil and the flywheel is correct. This gap is critical for proper voltage induction.

Air Gap Specification: The air gap is usually between 0.010 and 0.020 inches (0.25 to 0.50 mm). Consult your chainsaw’s service manual for the exact specification. I use a business card, which is approximately 0.012 inches thick, as a handy gauge for setting the gap.

How to check the air gap:

Loosen the coil mounting bolts.
Insert a business card (or a feeler gauge of the correct thickness) between the coil and the flywheel magnet.
Rotate the flywheel until the magnet is aligned with the coil.
Push the coil against the flywheel, ensuring the business card is snug.
Tighten the coil mounting bolts.
Remove the business card.

Case Study: I was helping a friend troubleshoot his old Stihl chainsaw. He was convinced the coil was dead. A quick visual inspection revealed a heavily corroded connection at the kill switch. Cleaning the connection restored the chainsaw to perfect working order.

Takeaway: A thorough visual inspection can often reveal obvious problems, saving you time and unnecessary parts replacements.

3. Resistance Testing: Measuring Coil Health

Resistance testing is a key step in diagnosing a chainsaw coil. It involves using a multimeter to measure the resistance of the coil’s primary and secondary windings. These measurements provide valuable insights into the coil’s internal condition.

Tools you’ll need:

Multimeter: A digital multimeter is recommended for accurate readings.
Chainsaw Service Manual: Consult your chainsaw’s service manual for the correct resistance specifications.

Testing Procedure:

Disconnect the Coil: Disconnect the coil from the spark plug and any other wiring.
Primary Winding Test: Set your multimeter to the ohms (Ω) setting. Connect the multimeter probes to the two terminals on the coil’s primary winding. This is typically the smaller winding with the lower resistance.
Secondary Winding Test: Connect one multimeter probe to the spark plug wire terminal and the other probe to one of the primary winding terminals. This measures the resistance of the secondary winding, which has a much higher resistance than the primary.
Compare Readings: Compare your multimeter readings to the specifications in your chainsaw’s service manual.

Interpreting the Results:

Zero Resistance (Short Circuit): If the multimeter reads zero or very close to zero, it indicates a short circuit within the coil. This means the coil is faulty and needs to be replaced.
Infinite Resistance (Open Circuit): If the multimeter reads infinite resistance (OL or a very high number), it indicates an open circuit. This means there is a break in the winding, and the coil is faulty.
Out-of-Spec Resistance: If the resistance is significantly higher or lower than the specified range, it suggests a problem with the coil. The coil may still produce a spark, but it may be weak or intermittent.

Example Resistance Specifications (These are approximate and vary by model):

Primary Winding: 0.5 – 3 ohms
Secondary Winding: 2,000 – 10,000 ohms

Caution: Always consult your chainsaw’s service manual for the exact resistance specifications for your specific model.

Personal Story: I once had a chainsaw that would start intermittently. Resistance testing revealed that the secondary winding resistance was slightly out of spec. The coil was producing a weak spark that was only strong enough to ignite the fuel-air mixture under ideal conditions. Replacing the coil solved the problem.

Takeaway: Resistance testing is a reliable method for assessing the internal health of your chainsaw coil. Comparing your measurements to the manufacturer’s specifications is crucial for accurate diagnosis.

4. Spark Testing: Confirming Spark Delivery

Spark testing is a direct way to confirm whether the ignition coil is producing a spark. This test verifies that the coil is not only generating voltage but also delivering it to the spark plug.

Tools you’ll need:

Spark Plug Tester: A spark plug tester is the safest and most reliable way to perform this test. These testers are inexpensive and widely available.
Insulated Pliers: Use insulated pliers to handle the spark plug wire to avoid electric shock.

Testing Procedure:

Remove the Spark Plug: Disconnect the spark plug wire and remove the spark plug from the cylinder.
Connect the Spark Plug Tester: Attach the spark plug to the spark plug tester. Connect the tester to the spark plug wire.
Ground the Tester: Ground the tester to the engine block. This completes the circuit.
Pull the Starter Rope: Pull the starter rope briskly. Observe the spark plug tester for a spark.

Alternative Method (If you don’t have a spark plug tester):

Reattach the Spark Plug: Reconnect the spark plug to the spark plug wire.
Ground the Spark Plug: Hold the spark plug against the engine block, ensuring the metal part of the plug is in contact with the metal of the engine. Use insulated pliers to hold the spark plug wire.
Pull the Starter Rope: Pull the starter rope briskly. Observe the spark plug gap for a spark.

Interpreting the Results:

Strong, Blue Spark: A strong, blue spark indicates that the ignition coil is working correctly and delivering sufficient voltage to the spark plug.
Weak, Yellow Spark: A weak, yellow spark suggests that the coil is producing voltage, but it may not be strong enough to reliably ignite the fuel-air mixture.
No Spark: No spark indicates that the coil is not producing any voltage, or that there is a break in the circuit.

Important Safety Note: Always use insulated pliers when handling the spark plug wire to avoid electric shock. Never touch the spark plug wire or spark plug while the engine is being cranked.

Personal Story: I was working on a Husqvarna chainsaw that had a weak, intermittent spark. The engine would start sometimes, but it would often stall. Spark testing revealed that the coil was producing a weak, yellow spark. Replacing the coil with a new one restored the chainsaw to reliable operation.

Takeaway: Spark testing provides direct confirmation of spark delivery. A strong, blue spark is essential for reliable engine starting and performance.

5. Kill Switch Testing: Ruling Out a Simple Problem

Sometimes, the problem isn’t the coil at all, but rather the kill switch. A faulty kill switch can ground the ignition system, preventing the coil from producing a spark. Testing the kill switch is a quick and easy way to rule out this possibility.

Tools you’ll need:

Multimeter: A digital multimeter is needed to test the continuity of the kill switch.

Testing Procedure:

Disconnect the Kill Switch: Disconnect the kill switch wire from the ignition coil. This isolates the kill switch from the rest of the ignition system.
Set the Multimeter: Set your multimeter to the continuity setting (usually indicated by a diode symbol or a sound wave symbol).
Test the Switch in the “On” Position: Connect the multimeter probes to the two terminals of the kill switch. With the switch in the “on” (run) position, the multimeter should not show continuity (OL or no reading). This means the switch is not grounding the ignition system.
Test the Switch in the “Off” Position: Move the kill switch to the “off” (stop) position. The multimeter should now show continuity (a beep or a reading close to zero). This means the switch is grounding the ignition system, as it should when the engine is turned off.

Interpreting the Results:

Continuity in Both Positions: If the multimeter shows continuity in both the “on” and “off” positions, it indicates that the kill switch is faulty and is constantly grounding the ignition system.
No Continuity in Either Position: If the multimeter shows no continuity in either position, it indicates that the kill switch is not grounding the ignition system, even when it is in the “off” position. This could also be a problem, as the engine may not be able to be turned off.
Correct Operation: If the kill switch shows no continuity in the “on” position and continuity in the “off” position, it is working correctly.

Personal Story: I was troubleshooting a chainsaw that would not start. I had already tested the coil and spark plug, and they seemed to be fine. As a last resort, I tested the kill switch. It turned out that the kill switch was stuck in the “off” position, constantly grounding the ignition system. Replacing the kill switch solved the problem.

Takeaway: Testing the kill switch is a simple but important step in diagnosing chainsaw ignition problems. A faulty kill switch can prevent the coil from producing a spark, even if the coil itself is working correctly.

Additional Tips for Chainsaw Coil Testing

Here are a few extra tips that I’ve learned over the years that can help you with chainsaw coil testing:

Check the Flywheel Key: A sheared flywheel key can cause the flywheel to be misaligned, affecting the timing of the ignition system. Inspect the flywheel key for any signs of damage.
Clean the Flywheel and Coil Surfaces: Dirt, debris, and rust can interfere with the magnetic field between the flywheel and the coil. Clean the surfaces of both components with a wire brush or sandpaper.
Use a Known Good Spark Plug: A faulty spark plug can mimic the symptoms of a bad coil. Before you start testing the coil, make sure you are using a known good spark plug.
Test the Coil Under Load: In some cases, a coil may test good when it is cold but fail under load when it heats up. If you suspect this is the case, try testing the coil after the engine has been running for a few minutes.
Consult a Professional: If you are not comfortable performing these tests yourself, or if you are still unable to diagnose the problem, consult a qualified chainsaw mechanic.

Final Thoughts

Diagnosing chainsaw problems can be frustrating, but with the right knowledge and tools, you can often pinpoint the issue and save yourself time and money. By following these five pro tips for chainsaw coil testing, you’ll be well-equipped to troubleshoot ignition problems and get your chainsaw back up and running. Remember to always consult your chainsaw’s service manual for specific instructions and specifications. Happy sawing!

Chainsaw Coil Testing (5 Pro Tips for Accurate Diagnosis)