How To Tell If Ignition Coil Is Bad (Chainsaw Spark Diagnosis)

How to Tell If Ignition Coil Is Bad (Chainsaw Spark Diagnosis)

The ignition coil is the heart of a chainsaw’s electrical system, responsible for generating the high-voltage spark needed to ignite the air-fuel mixture in the cylinder. When it fails, your chainsaw will likely refuse to start, leaving you stranded and frustrated. This guide will walk you through the process of diagnosing a faulty ignition coil, using simple tools and techniques. I’ll share practical tips gleaned from years of working on chainsaws, helping you pinpoint the problem and get back to cutting.

Understanding the Ignition System

Before diving into the diagnostics, let’s understand how the ignition system works. The system comprises:

Magneto: A small generator driven by the engine’s flywheel. It produces a low-voltage current.
Ignition Coil: Steps up the low-voltage current from the magneto to a high-voltage (typically 15,000-30,000 volts) to create a spark.
Spark Plug: Receives the high-voltage current from the ignition coil and creates a spark across its gap, igniting the air-fuel mixture.
Primary Winding: The first coil of wire in the ignition coil. It receives the low voltage current.
Secondary Winding: The second coil of wire in the ignition coil. It has many more turns than the primary winding, allowing the coil to step up the voltage significantly.

Symptoms of a Bad Ignition Coil

Identifying the symptoms is the first step in diagnosing a faulty ignition coil. Here are the most common signs:

Tools You’ll Need

To diagnose your chainsaw’s ignition coil, you’ll need the following tools:

Spark Plug Wrench: To remove and inspect the spark plug.
Spark Tester: A device that connects between the spark plug and the spark plug wire to visually confirm spark.
Multimeter: To measure resistance and voltage.
Small Screwdriver: For adjusting carburetor settings (if necessary) and removing the ignition coil cover.
Feeler Gauge: To set the correct air gap between the ignition coil and flywheel.
Gloves: To protect your hands.
Safety Glasses: To protect your eyes.
Clean Rags: For cleaning components.
Compressed Air (Optional): To clean debris from the ignition coil area.

Safety First

Before you begin any diagnostics, take the following safety precautions:

Disconnect the Spark Plug Wire: Always disconnect the spark plug wire from the spark plug before working on the ignition system. This prevents accidental starting and electrical shock.
Wear Safety Glasses: Protect your eyes from flying debris.
Wear Gloves: Protect your hands from sharp edges and chemicals.
Work in a Well-Ventilated Area: Avoid inhaling fumes.
Allow the Engine to Cool: Never work on a hot engine.
Refer to the Chainsaw’s Manual: Consult your chainsaw’s manual for specific instructions and safety information.

Step-by-Step Diagnosis

Here’s a step-by-step guide to diagnosing a bad ignition coil:

1. Inspect the Spark Plug

The spark plug is the easiest component to check. Remove the spark plug using the spark plug wrench and inspect it for the following:

Condition: Look for cracks, chips, or excessive wear.
Color: The color of the spark plug can indicate engine problems. A light tan color is ideal. Black and sooty indicates a rich mixture, while white indicates a lean mixture.
Gap: Check the spark plug gap using a feeler gauge. The correct gap is typically between 0.020 and 0.030 inches (0.5-0.75mm), but refer to your chainsaw’s manual for the specific gap.

Data Point: A spark plug with a gap outside the recommended range can cause weak or no spark. I’ve seen cases where simply adjusting the gap restored proper ignition.

Action: Clean the spark plug with a wire brush or replace it if necessary. Adjust the gap to the specified setting.

2. Check for Spark

After inspecting the spark plug, check for spark. Reconnect the spark plug wire to the spark plug. Hold the spark plug against the engine’s metal case, ensuring a good ground. Pull the starter cord. You should see a strong, blue spark jumping across the spark plug gap.

If you don’t see a spark:

Try a new spark plug. Sometimes, a faulty spark plug can prevent spark even if the ignition coil is good.
Use a spark tester. Connect the spark tester between the spark plug wire and the spark plug. Pull the starter cord. The spark tester will visually indicate if there is spark.

Data Point: A weak or orange spark indicates a weak ignition coil or a problem with the magneto.

Action: If you still don’t see a spark after trying a new spark plug and a spark tester, proceed to the next step.

3. Inspect the Spark Plug Wire

The spark plug wire can also be a source of problems. Inspect the wire for:

Cracks or Damage: Look for any visible damage to the wire’s insulation.
Loose Connections: Ensure the wire is securely connected to the spark plug and the ignition coil.
Corrosion: Check for corrosion at the terminals.

Data Point: A damaged spark plug wire can leak voltage, reducing the spark’s strength. I once found a wire that had been chewed by a rodent, causing intermittent starting issues.

Action: Replace the spark plug wire if it’s damaged or corroded. Ensure the connections are clean and secure.

4. Check the Ignition Coil Air Gap

The air gap between the ignition coil and the flywheel is crucial for proper ignition. If the gap is too large or too small, the ignition coil won’t generate enough voltage.

Procedure:

Locate the ignition coil, typically mounted near the flywheel.
Loosen the ignition coil mounting screws.
Insert a feeler gauge of the correct thickness (usually 0.010-0.020 inches or 0.25-0.50mm, but consult your chainsaw’s manual) between the ignition coil and the flywheel magnets.
Push the ignition coil against the flywheel.
Tighten the mounting screws.
Remove the feeler gauge.

Data Point: An incorrect air gap is a common cause of weak or no spark. I’ve seen cases where adjusting the air gap alone resolved the problem.

Action: Adjust the air gap to the specified setting.

5. Test the Ignition Coil with a Multimeter

A multimeter can be used to test the resistance of the ignition coil’s primary and secondary windings. This will help determine if the coil is internally damaged.

Procedure:

Disconnect the Ignition Coil: Disconnect the ignition coil from the wiring harness.
Set the Multimeter: Set the multimeter to measure resistance (Ohms).
Measure Primary Winding Resistance: Place one probe on the ignition coil’s primary terminal (the one connected to the magneto) and the other probe on the ignition coil’s metal core or mounting bracket (ground).
Record the Reading: The resistance should be within the range specified in your chainsaw’s manual. A typical value is between 0.5 and 3 Ohms.
Measure Secondary Winding Resistance: Place one probe on the spark plug wire terminal and the other probe on the ignition coil’s metal core or mounting bracket (ground).
Record the Reading: The resistance should be within the range specified in your chainsaw’s manual. A typical value is between 2,000 and 15,000 Ohms.

Data Point: An open circuit (infinite resistance) or a short circuit (zero resistance) indicates a faulty ignition coil. Values outside the specified range also suggest a problem.

Action: Compare your readings to the specifications in your chainsaw’s manual. If the readings are outside the specified range, replace the ignition coil.

Example: In a project involving a Stihl MS 250 chainsaw, I measured the primary winding resistance at 0.8 Ohms and the secondary winding resistance at 8,500 Ohms. These values were within the manufacturer’s specifications, indicating that the ignition coil was likely good. However, in another case with a Husqvarna 455 Rancher, the primary winding showed infinite resistance, confirming a faulty ignition coil.

6. Check the Kill Switch

The kill switch is designed to ground the ignition system, preventing the engine from starting. A faulty kill switch can prevent the chainsaw from starting even if the ignition coil is good.

Procedure:

Disconnect the Kill Switch Wire: Disconnect the wire that runs from the kill switch to the ignition coil.
Test for Spark: Check for spark as described in step 2.

Data Point: If the chainsaw now has spark with the kill switch wire disconnected, the kill switch is faulty.

Action: Replace the kill switch.

7. Inspect the Flywheel

The flywheel contains magnets that generate the electrical current for the ignition system. A damaged or demagnetized flywheel can cause weak or no spark.

Procedure:

Inspect the Magnets: Look for cracks, chips, or corrosion on the magnets.
Check Magnet Strength: Use a screwdriver to test the strength of the magnets. The screwdriver should be strongly attracted to the magnets.

Data Point: Weak magnets can reduce the voltage produced by the magneto, resulting in a weak spark.

Action: If the magnets are damaged or weak, replace the flywheel.

8. Check the Magneto

The magneto is responsible for generating the initial electrical current. A faulty magneto can prevent the ignition coil from receiving the necessary voltage.

Procedure:

Disconnect the Magneto Wire: Disconnect the wire that runs from the magneto to the ignition coil.
Test for Voltage: Set the multimeter to measure AC voltage. Connect the multimeter probes to the magneto wire and ground.
Pull the Starter Cord: Pull the starter cord and observe the voltage reading.

Data Point: The magneto should produce a voltage of at least 20-30 volts AC. If the voltage is significantly lower, the magneto is likely faulty.

Action: Replace the magneto if it’s not producing enough voltage.

Case Studies and Real-World Examples

Here are a few case studies from my experience, illustrating common ignition coil problems and their solutions:

Case Study 1: Stihl MS 170 with Intermittent Spark: A homeowner complained that his Stihl MS 170 would start and run for a few minutes, then stall and refuse to restart. After checking the spark plug and fuel system, I suspected the ignition coil. I tested the coil with a multimeter and found that the secondary winding resistance was within the specified range when the coil was cold, but increased significantly when heated with a heat gun. This indicated a heat-related breakdown in the coil. Replacing the ignition coil resolved the problem.
Case Study 2: Husqvarna 450 with No Spark: A professional logger reported that his Husqvarna 450 suddenly stopped working in the field. I checked for spark and found none. The spark plug was new, and the spark plug wire was in good condition. I tested the ignition coil with a multimeter and found that the primary winding had infinite resistance, indicating an open circuit. Replacing the ignition coil restored spark and got the logger back to work.
Case Study 3: Echo CS-310 with Weak Spark: A firewood cutter complained that his Echo CS-310 was difficult to start and ran poorly. I checked for spark and found a weak, orange spark. The spark plug was fouled, and the air gap was too large. I cleaned the spark plug, adjusted the air gap, and the spark improved, but was still weak. I tested the ignition coil with a multimeter and found that the secondary winding resistance was slightly below the specified range. Replacing the ignition coil with an aftermarket part resulted in a strong, blue spark and improved engine performance.

Troubleshooting Tips and Tricks

Here are some additional tips and tricks for troubleshooting chainsaw ignition problems:

Check for Loose Connections: Ensure all electrical connections are clean and secure. Use dielectric grease to protect connections from corrosion.
Inspect the Wiring Harness: Look for damaged or frayed wires.
Test with a Known Good Ignition Coil: If possible, test the chainsaw with a known good ignition coil to confirm the diagnosis.
Use a Heat Gun: Use a heat gun to warm up the ignition coil and see if the problem worsens. This can help identify heat-related coil breakdown.
Check the Ground Connection: Ensure the ignition coil is properly grounded to the engine’s metal case.
Consider Aftermarket Parts: Aftermarket ignition coils can be a cost-effective alternative to OEM parts. However, be sure to choose a reputable brand.

Common Mistakes to Avoid

Here are some common mistakes to avoid when diagnosing chainsaw ignition problems:

Assuming the Ignition Coil is Bad: Don’t assume the ignition coil is bad without thoroughly checking the other components of the ignition system.
Ignoring the Spark Plug: The spark plug is the easiest component to check and is often the source of the problem.
Using the Wrong Air Gap: Using the wrong air gap can damage the ignition coil or prevent it from working properly.
Over-Tightening Mounting Screws: Over-tightening the ignition coil mounting screws can damage the coil or the engine.
Using a Damaged Multimeter: Ensure your multimeter is in good working condition and properly calibrated.

Preventative Maintenance

Preventive maintenance can help extend the life of your chainsaw’s ignition system and prevent future problems. Here are some tips:

Replace the Spark Plug Regularly: Replace the spark plug every 100 hours of use or annually, whichever comes first.
Keep the Air Gap Clean: Clean the air gap between the ignition coil and the flywheel regularly.
Store the Chainsaw Properly: Store the chainsaw in a dry, clean place.
Use Fresh Fuel: Use fresh, high-quality fuel and oil.
Avoid Overheating: Avoid overheating the engine by running it at full throttle for extended periods.

Specifications and Technical Requirements

Here’s a summary of key specifications and technical requirements:

Spark Plug Gap: 0.020-0.030 inches (0.5-0.75mm)
Ignition Coil Air Gap: 0.010-0.020 inches (0.25-0.50mm)
Primary Winding Resistance: 0.5-3 Ohms (typical)
Secondary Winding Resistance: 2,000-15,000 Ohms (typical)
Magneto Voltage: 20-30 volts AC (minimum)
Fuel-Oil Ratio: Refer to your chainsaw’s manual for the correct fuel-oil ratio (typically 50:1 or 40:1)
Fuel Type: Use unleaded gasoline with an octane rating of 87 or higher.

Conclusion

Diagnosing a bad ignition coil can seem daunting, but with the right tools and knowledge, you can pinpoint the problem and get your chainsaw back in action. Remember to follow the safety precautions, use the proper tools, and take your time. By understanding the ignition system and following the step-by-step diagnosis outlined in this guide, you can confidently troubleshoot ignition problems and keep your chainsaw running smoothly. Remember, a little preventative maintenance can go a long way in extending the life of your chainsaw and preventing future headaches. And if all else fails, don’t hesitate to consult a qualified chainsaw mechanic. Happy cutting!

How to Tell If Ignition Coil Is Bad (Chainsaw Spark Diagnosis)