Stihl MS250 Ignition Coil Troubleshooting (3 Pro Fixes)

Ever wondered what makes your perfectly seasoned oak taste so amazing in your wood-burning stove? A reliable spark, courtesy of a functioning ignition system, is the unsung hero. When your Stihl MS250, a workhorse of a chainsaw for many homeowners and small-scale loggers, starts acting up, the ignition coil is often the culprit. I’ve spent countless hours in the woods, relying on my MS250 for everything from felling small trees to bucking firewood, and I know the frustration of a chainsaw that won’t start. Let’s dive into some troubleshooting.

Stihl MS250 Ignition Coil Troubleshooting: 3 Pro Fixes

Understanding the Stihl MS250 Ignition System

Before we jump into the fixes, let’s cover some basics. The ignition system is responsible for creating the spark that ignites the fuel-air mixture in the engine. It’s a relatively simple system, but a failure in any component can prevent the saw from starting.

Ignition Coil: This is the heart of the system. It transforms the low voltage from the flywheel into a high-voltage spark.
Flywheel: The flywheel contains magnets that, as it rotates, induce a current in the ignition coil.
Spark Plug: The spark plug provides the gap across which the high-voltage spark jumps, igniting the fuel-air mixture.
Kill Switch: This switch grounds the ignition coil, preventing it from producing a spark and stopping the engine.
Wiring: Wires connect all these components, ensuring a complete circuit.

Key Terms:

Green Wood: Freshly cut wood with a high moisture content (often above 30%). It’s heavier and harder to split than seasoned wood.
Seasoned Wood: Wood that has been dried to a moisture content of 20% or less. It burns more efficiently and produces less smoke. I typically aim for 15-18% moisture content for my firewood.
Air Gap: The small space between the ignition coil and the flywheel magnets. This gap is crucial for proper ignition.

Personal Story: I remember one time, I was deep in the woods, felling some ash trees for a friend. My MS250 suddenly died. After checking the fuel and air filter, I suspected the ignition. It turned out a tiny twig had wedged itself between the flywheel and the ignition coil, disrupting the air gap. A simple fix, but it taught me the importance of keeping that area clean.

Fix 1: Checking and Adjusting the Air Gap

The air gap between the ignition coil and the flywheel is critical. If the gap is too large, the coil won’t generate enough voltage. If it’s too small, the coil can rub against the flywheel, causing damage.

Tools Needed:

Feeler gauge (specifically, a 0.010″ or 0.25mm feeler gauge is ideal)
Screwdriver (usually a Torx T27)
Wrench (if the coil is held on with bolts)

Step-by-Step Instructions:

Safety First: Disconnect the spark plug wire from the spark plug. This prevents accidental starting.
Access the Ignition Coil: Remove the chainsaw’s top cover. This usually involves unscrewing a few screws or bolts. Refer to your owner’s manual for specific instructions.
Locate the Ignition Coil: The ignition coil is typically located next to the flywheel. It’s a small, black or gray component with a wire leading to the spark plug.
Loosen the Ignition Coil: Loosen the screws or bolts that hold the ignition coil in place. Don’t remove them completely, just enough so you can move the coil slightly.
Insert the Feeler Gauge: Place the 0.010″ (0.25mm) feeler gauge between the ignition coil and the flywheel magnets. The gauge should be positioned so it touches both the coil and the flywheel.
Adjust the Coil: Gently push the ignition coil towards the flywheel until it makes contact with the feeler gauge.
Tighten the Screws/Bolts: While holding the coil in place against the feeler gauge, tighten the screws or bolts. Ensure they are snug but not overtightened.
Remove the Feeler Gauge: Remove the feeler gauge. The air gap should now be correctly set.
Reassemble the Chainsaw: Replace the top cover and reconnect the spark plug wire.
Test: Try starting the chainsaw. If it starts, you’ve successfully adjusted the air gap.

Strategic Insights:

Importance of Correct Air Gap: A properly set air gap ensures optimal spark generation. Too large a gap results in a weak spark, while too small a gap can cause physical damage to the coil.
Feeler Gauge Alternatives: If you don’t have a feeler gauge, you can use a business card or a piece of thin cardboard as a substitute. However, a feeler gauge provides the most accurate measurement.
Troubleshooting: If the chainsaw still doesn’t start after adjusting the air gap, proceed to the next fix.

Case Study: I once helped a neighbor who was struggling with his MS250. He had replaced the ignition coil himself, but the saw still wouldn’t start. After checking the air gap, I found it was way too large – almost 0.030″. Adjusting it to 0.010″ immediately solved the problem.

Fix 2: Testing the Ignition Coil with a Multimeter

If adjusting the air gap doesn’t solve the problem, the ignition coil itself might be faulty. A multimeter can be used to test the coil’s resistance, indicating whether it’s functioning correctly.

Tools Needed:

Multimeter
Screwdriver (for accessing the coil)

Step-by-Step Instructions:

Safety First: Disconnect the spark plug wire from the spark plug.
Access the Ignition Coil: Remove the chainsaw’s top cover.
Disconnect the Coil Wires: Disconnect the wires connected to the ignition coil. Usually, there’s a primary wire (going to the kill switch) and a secondary wire (going to the spark plug).
Set the Multimeter: Set your multimeter to measure resistance (Ohms – Ω). Select a range appropriate for measuring relatively low resistance values (e.g., 200Ω or 2000Ω).
Test the Primary Resistance: Place one multimeter probe on the primary terminal of the ignition coil (the one that connects to the kill switch wire) and the other probe on the coil’s metal body (ground). Record the resistance reading. A typical reading for a good coil is between 2-6 Ohms.
Test the Secondary Resistance: Place one multimeter probe on the secondary terminal of the ignition coil (the one that connects to the spark plug wire) and the other probe on the coil’s metal body (ground). Record the resistance reading. A typical reading for a good coil is between 2,000-6,000 Ohms. Consult your saw’s service manual for specific resistance values.
Interpret the Results:
- Zero Resistance (Short Circuit): If the multimeter reads zero resistance, it indicates a short circuit within the coil. The coil is likely bad.
- Infinite Resistance (Open Circuit): If the multimeter reads infinite resistance, it indicates an open circuit within the coil. The coil is also likely bad.
- Out-of-Range Resistance: If the resistance is significantly outside the typical range, the coil may be faulty.
Reassemble the Chainsaw: If the coil tests good, reconnect the wires and reassemble the chainsaw. If the coil tests bad, replace it.

Technical Details:

Multimeter Accuracy: The accuracy of your multimeter is important. A digital multimeter generally provides more accurate readings than an analog one.
Coil Specifications: Always refer to your Stihl MS250 service manual for the exact resistance specifications for your ignition coil. These values can vary slightly depending on the coil’s part number.
Cost of Replacement: A replacement ignition coil for the Stihl MS250 typically costs between $20 and $50, depending on the brand and vendor.

Personal Story: I once spent hours troubleshooting a chainsaw that wouldn’t start, only to discover a hairline crack in the ignition coil housing. The crack wasn’t visible to the naked eye, but it allowed moisture to enter the coil, causing it to fail intermittently. The multimeter test quickly revealed the problem.

Benefits of Testing:

Accurate Diagnosis: Testing the ignition coil with a multimeter provides a definitive diagnosis, preventing unnecessary replacement of other components.
Cost Savings: Replacing a faulty ignition coil is often cheaper than taking the chainsaw to a repair shop.

Fix 3: Checking the Kill Switch and Wiring

Even if the ignition coil and air gap are good, a faulty kill switch or damaged wiring can prevent the chainsaw from starting. The kill switch grounds the ignition coil, preventing it from producing a spark. If the switch is stuck in the “off” position or the wiring is damaged, the coil will be grounded even when the switch is in the “on” position.

Tools Needed:

Multimeter
Screwdriver (for accessing the kill switch and wiring)
Wire stripper/crimper (if repairing wiring)

Step-by-Step Instructions:

Safety First: Disconnect the spark plug wire from the spark plug.
Locate the Kill Switch: The kill switch is usually located on the handle of the chainsaw.
Inspect the Wiring: Visually inspect the wiring connected to the kill switch. Look for any signs of damage, such as frayed wires, broken insulation, or loose connections.
Test the Kill Switch with a Multimeter:
- Disconnect the wires from the kill switch.
- Set your multimeter to measure continuity (usually indicated by a beep or a low resistance reading).
- Place one multimeter probe on each terminal of the kill switch.
- With the switch in the “on” position, the multimeter should not show continuity (open circuit).
- With the switch in the “off” position, the multimeter should show continuity (closed circuit).
Test the Wiring for Continuity:
- Disconnect the wiring from both the ignition coil and the kill switch.
- Use the multimeter to test the continuity of each wire. Place one probe on each end of the wire. The multimeter should show continuity (closed circuit).
- Also, check for shorts to ground. Place one probe on the wire and the other on the chainsaw’s metal body (ground). The multimeter should not show continuity (open circuit).
Repair or Replace Faulty Components:
- If the kill switch is faulty, replace it.
- If the wiring is damaged, repair it using wire strippers and crimpers. Replace the entire wire if necessary.
Reassemble the Chainsaw: Reconnect the wiring, replace the top cover, and reconnect the spark plug wire.

Original Insights from Projects:

Common Wiring Problems: I’ve found that the wiring near the kill switch is often exposed to vibration and moisture, leading to corrosion and breaks. Using dielectric grease on the connections can help prevent corrosion.
Kill Switch Failure Modes: Kill switches can fail in two ways: they can get stuck in the “off” position, or they can become internally shorted, constantly grounding the ignition coil.

Case Study: I once had a customer who complained that his MS250 would only run intermittently. After checking the ignition coil and air gap, I focused on the wiring. I discovered that the wire connecting the kill switch to the ignition coil was almost completely severed inside the insulation. The wire would occasionally make contact, allowing the saw to run, but the slightest movement would break the connection. Replacing the wire solved the problem.

Benefits of Checking Kill Switch and Wiring:

When preparing firewood, the drying process is crucial. Green wood is difficult to burn and produces a lot of smoke. Seasoned wood, with a moisture content of 20% or less, burns much more efficiently.

Air Drying: This is the most common method. Stack the firewood in a well-ventilated area, off the ground, and cover the top to protect it from rain. Air drying typically takes 6-12 months, depending on the climate and wood species.
Kiln Drying: This method uses a kiln to dry the wood quickly. Kiln-dried firewood typically has a moisture content of 10-15%.
Moisture Meter: Use a moisture meter to check the moisture content of the firewood. Insert the probes into the wood and read the percentage.

Strategic Advantages of Hydraulic Splitters:

Splitting firewood by hand can be physically demanding and time-consuming. A hydraulic splitter can significantly increase efficiency, especially when dealing with large or knotty logs.

Increased Productivity: A hydraulic splitter can split several cords of firewood per day, compared to a few hours of hand-splitting.
Reduced Physical Strain: Using a hydraulic splitter reduces the risk of back injuries and other physical strains.
Splitting Large Logs: Hydraulic splitters can handle logs that are too large or difficult to split by hand.
Cost Considerations: Hydraulic splitters range in price from a few hundred dollars to several thousand dollars, depending on the size and features.

Skill Levels Required:

Basic Troubleshooting: Adjusting the air gap and checking the kill switch and wiring are tasks that most homeowners can perform with basic tools and skills.
Advanced Troubleshooting: Testing the ignition coil with a multimeter requires some knowledge of electrical circuits and the use of a multimeter.
Carburetor Repair: Carburetor repair is best left to experienced mechanics.

Conclusion and Next Steps

Troubleshooting a Stihl MS250 ignition coil can seem daunting, but by following these three pro fixes, you can diagnose and resolve many common issues. Remember to always prioritize safety and refer to your owner’s manual for specific instructions.

Next Steps:

Gather Your Tools: Assemble the necessary tools, including a feeler gauge, multimeter, screwdrivers, and wire strippers/crimpers.
Start with the Simplest Fix: Begin by checking and adjusting the air gap. This is the easiest fix and often resolves the problem.
Test the Ignition Coil: If adjusting the air gap doesn’t work, test the ignition coil with a multimeter.
Check the Kill Switch and Wiring: If the coil tests good, check the kill switch and wiring for any signs of damage or malfunction.
Replace Faulty Components: Replace any faulty components, such as the ignition coil, kill switch, or wiring.
Test Your Chainsaw: After making the necessary repairs, test your chainsaw to ensure it’s running properly.

By following these steps, you can get your Stihl MS250 back in action and continue enjoying the satisfaction of processing your own firewood. Remember that regular maintenance, proper fuel storage, and careful handling are key to keeping your chainsaw running smoothly for years to come. And who knows, maybe you’ll even discover the perfect wood to give your next fire that unforgettable taste!

Stihl MS250 Ignition Coil Troubleshooting (3 Pro Fixes)