Stihl FS 55R Parts Guide (Troubleshooting Coil Issues Explained)

As someone who’s spent countless hours in the field, I know that the durability of your equipment is paramount. A reliable tool isn’t just a convenience; it’s a necessity when you’re working in remote locations or facing demanding timelines. That’s why understanding how to troubleshoot and maintain your equipment is crucial. Let’s dive into the Stihl FS 55R and specifically address those pesky coil issues that can bring your work to a screeching halt.

Stihl FS 55R Parts Guide: Troubleshooting Coil Issues Explained

The Stihl FS 55R is a workhorse, a dependable tool for many landscapers, homeowners, and forestry workers. I’ve used mine for everything from trimming delicate flowerbeds to clearing thick brush along logging trails. But like any machine, it has its quirks. One of the most common issues I’ve encountered, and one that many users report, is related to the ignition coil. This guide will walk you through troubleshooting and resolving these coil-related problems, ensuring your FS 55R keeps running smoothly.

Understanding the Ignition Coil

The ignition coil is the heart of your FS 55R’s ignition system. It’s responsible for generating the high-voltage spark that ignites the fuel-air mixture in the engine. Without a properly functioning coil, your trimmer won’t start or will run erratically.

Function: Transforms low-voltage electrical current from the magneto into high-voltage (typically 15,000-30,000 volts).
Location: Usually found mounted near the flywheel, under the engine shroud.
Key Components: Primary winding, secondary winding, and a laminated iron core.

Common Symptoms of a Faulty Ignition Coil

Before you start tearing apart your FS 55R, it’s essential to accurately diagnose the problem. Here are the most common symptoms that indicate a potential issue with the ignition coil:

No Spark: This is the most obvious sign. The engine won’t start because there’s no spark to ignite the fuel.
Weak Spark: The engine starts but runs poorly, misfires, or lacks power.
Intermittent Starting: The trimmer starts sometimes, but not others. This can be caused by a coil that’s failing intermittently.
Engine Cuts Out: The engine starts and runs for a short period, then suddenly dies. This can happen when the coil overheats and fails.
Difficult Starting When Hot: The engine starts fine when cold but is difficult to start after it’s been running and is hot.

Required Tools and Materials

Before you begin troubleshooting, gather the necessary tools and materials. Having everything on hand will save you time and frustration.

1. Safety First: Disconnect the Spark Plug

Before you begin any work on your FS 55R, disconnect the spark plug wire from the spark plug. This prevents accidental starting and potential injury.

Procedure: Locate the spark plug wire and firmly pull it off the spark plug.
Why: This eliminates the risk of the engine starting while you’re working on it.

2. Inspect the Spark Plug

The spark plug is a critical component of the ignition system. A fouled or damaged spark plug can mimic the symptoms of a faulty coil.

Procedure: Remove the spark plug using the spark plug wrench. Inspect the electrode for wear, carbon buildup, or damage. Clean the spark plug with a wire brush or replace it with a new one if necessary.
Specifications: The correct spark plug gap for the Stihl FS 55R is typically 0.5 mm (0.020 inches). Use a feeler gauge to ensure the gap is correct.
Why: A faulty spark plug can prevent the engine from starting or cause it to run poorly. I’ve seen many instances where simply replacing the spark plug resolves the issue.
Personal Experience: I once spent hours troubleshooting what I thought was a coil problem, only to discover that the spark plug was heavily fouled with carbon. A quick cleaning and re-gapping solved the problem.

3. Test for Spark

To determine if the ignition system is producing a spark, perform a spark test.

Procedure:
1. Reconnect the spark plug wire to the spark plug.
2. Hold the spark plug against the engine block, ensuring good metal-to-metal contact.
3. Pull the starter cord.
4. Observe the spark plug electrode. You should see a bright blue spark jumping across the gap.
Troubleshooting:
- No Spark: If there’s no spark, the problem could be the ignition coil, the spark plug wire, or the kill switch.
- Weak Spark: A weak or yellow spark indicates a potential problem with the ignition coil or the flywheel magnet.
Safety Note: Be careful not to touch the spark plug or the engine block while pulling the starter cord, as you could receive an electrical shock.
Pro Tip: Perform this test in a dimly lit area to make it easier to see the spark.

4. Inspect the Spark Plug Wire and Boot

The spark plug wire and boot can become damaged over time, leading to poor electrical conductivity.

Procedure: Inspect the spark plug wire for cracks, cuts, or other damage. Ensure the boot is securely attached to the spark plug and the ignition coil.
Troubleshooting: If you find any damage, replace the spark plug wire and boot.
Personal Experience: I’ve seen cases where the spark plug wire had corroded internally, preventing a strong spark from reaching the spark plug. Replacing the wire solved the problem.

5. 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 engine from starting even if the ignition coil is functioning correctly.

Procedure:
1. Disconnect the wire leading from the kill switch to the ignition coil.
2. Perform the spark test again.
3. If the engine now produces a spark, the kill switch is faulty and needs to be replaced.
Why: A stuck or shorted kill switch can ground the ignition system, preventing the coil from firing.

6. Inspect the Flywheel Magnet

The flywheel magnet is responsible for generating the electrical current that powers the ignition coil. A weak or damaged magnet can cause a weak or no spark.

Procedure:
1. Remove the engine shroud to access the flywheel.
2. Inspect the flywheel magnet for cracks, chips, or other damage.
3. Clean the magnet with a wire brush to remove any rust or debris.
Testing Magnet Strength: You can test the strength of the magnet by holding a screwdriver near it. The magnet should strongly attract the screwdriver.
Troubleshooting: If the magnet is weak or damaged, the flywheel may need to be replaced.
Important Note: Be careful when removing the flywheel, as it is often held in place with a strong retaining nut. Use the appropriate tools and follow the manufacturer’s instructions.

7. Check the Air Gap Between the Coil and Flywheel

The air gap between the ignition coil and the flywheel magnet is critical for proper ignition. If the gap is too large or too small, the coil won’t be able to generate a strong spark.

Procedure:
1. Loosen the screws holding the ignition coil in place.
2. Use a feeler gauge to set the air gap to the manufacturer’s specifications. The correct air gap for the Stihl FS 55R is typically 0.3 mm (0.012 inches).
3. Tighten the screws holding the ignition coil in place.
Why: An incorrect air gap can prevent the coil from generating a strong enough magnetic field to produce a spark.
Pro Tip: Use a business card or a piece of cardboard as a makeshift feeler gauge if you don’t have a dedicated feeler gauge. The thickness of a business card is usually close to the required air gap.
Torque Specifications: Tighten the coil mounting screws to the torque specified in the Stihl FS 55R service manual (typically around 5-7 Nm).

8. Test the Ignition Coil with a Multimeter

A multimeter can be used to test the primary and secondary windings of the ignition coil for continuity and resistance.

Procedure:
1. Set the multimeter to the resistance (Ohms) setting.
2. Disconnect the ignition coil from the wiring harness.
3. Test the resistance between the primary coil terminals (typically the two small terminals).
4. Test the resistance between the secondary coil terminals (typically the spark plug wire and one of the small terminals).
5. Compare your readings to the manufacturer’s specifications.
Specifications: The primary coil resistance should typically be between 0.5 and 2 Ohms, while the secondary coil resistance should be between 3,000 and 10,000 Ohms. Refer to the Stihl FS 55R service manual for the exact specifications.
Troubleshooting:
- Open Circuit (Infinite Resistance): Indicates a broken winding.
- Short Circuit (Zero Resistance): Indicates a shorted winding.
- Resistance Outside of Specifications: Indicates a faulty coil.
Important Note: Multimeter testing can only provide an indication of the coil’s condition. It’s not always definitive, as a coil can fail under load even if it tests okay with a multimeter.

9. Visual Inspection of the Ignition Coil

Carefully inspect the ignition coil for any signs of physical damage.

Procedure:
1. Remove the ignition coil from the engine.
2. Inspect the coil for cracks, burns, or melted plastic.
3. Look for any signs of corrosion or damage to the terminals.
Troubleshooting: If you find any physical damage, the coil needs to be replaced.
Personal Experience: I’ve seen coils that looked perfectly fine on the outside but were internally damaged due to overheating. These coils would often fail intermittently, making them difficult to diagnose.

10. Replace the Ignition Coil

If all other troubleshooting steps have failed to identify the problem, and the ignition coil is suspected to be faulty, replace it with a new one.

Procedure:
1. Disconnect the old ignition coil from the wiring harness.
2. Remove the screws holding the old coil in place.
3. Install the new ignition coil, ensuring the air gap is set correctly.
4. Connect the new coil to the wiring harness.
Important Note: Use only a genuine Stihl replacement ignition coil or a high-quality aftermarket coil. Avoid using cheap, generic coils, as they may not perform reliably.
Torque Specifications: Tighten the coil mounting screws to the torque specified in the Stihl FS 55R service manual (typically around 5-7 Nm).

11. Test After Replacement

After replacing the ignition coil, perform a spark test to ensure the new coil is functioning correctly.

Procedure:
1. Reconnect the spark plug wire to the spark plug.
2. Hold the spark plug against the engine block, ensuring good metal-to-metal contact.
3. Pull the starter cord.
4. Observe the spark plug electrode. You should see a bright blue spark jumping across the gap.
If the engine still doesn’t start: Recheck all the previous steps and consider other potential issues, such as a fuel problem or a compression problem.

Advanced Troubleshooting Techniques

If you’ve followed all the above steps and are still experiencing problems, here are some more advanced troubleshooting techniques to consider.

Checking the Wiring Harness

The wiring harness connects the ignition coil to the other components of the ignition system. A damaged or corroded wiring harness can prevent the coil from functioning correctly.

Procedure:
1. Inspect the wiring harness for cracks, cuts, or other damage.
2. Check the connectors for corrosion or loose connections.
3. Use a multimeter to test the continuity of the wires in the harness.
Troubleshooting: If you find any damage or corrosion, repair or replace the wiring harness.

Testing the Magneto

The magneto is a device that generates the electrical current that powers the ignition coil. A faulty magneto can cause a weak or no spark.

Procedure:
1. Disconnect the ignition coil from the magneto.
2. Use a multimeter to test the output voltage of the magneto while pulling the starter cord.
3. Compare your readings to the manufacturer’s specifications.
Specifications: The output voltage of the magneto should typically be several hundred volts AC. Refer to the Stihl FS 55R service manual for the exact specifications.
Troubleshooting: If the output voltage of the magneto is low or nonexistent, the magneto may need to be replaced.

Checking for a Ground Fault

A ground fault occurs when the ignition system is accidentally grounded, preventing the coil from firing.

Procedure:
1. Disconnect the ignition coil from the wiring harness.
2. Use a multimeter to test the resistance between the ignition coil terminals and the engine block.
3. The resistance should be infinite (open circuit).
Troubleshooting: If the resistance is low or zero, there is a ground fault. Trace the wiring to find the source of the fault and repair it.

Preventing Future Ignition Coil Problems

Prevention is always better than cure. Here are some tips to help prevent future ignition coil problems on your Stihl FS 55R:

Use Fresh Fuel: Old or stale fuel can damage the engine and ignition system. Always use fresh, high-quality fuel mixed with the correct amount of oil. I recommend using fuel that is no more than 30 days old.
Maintain the Air Filter: A dirty air filter can restrict airflow to the engine, causing it to overheat and potentially damage the ignition coil. Clean or replace the air filter regularly.
Keep the Engine Clean: Dirt and debris can accumulate around the engine, trapping heat and potentially damaging the ignition coil. Clean the engine regularly with a brush and compressed air.
Store the Trimmer Properly: When storing the trimmer for extended periods, drain the fuel tank and run the engine until it stalls. This will prevent fuel from gumming up the carburetor and potentially damaging the ignition system. Store the trimmer in a dry, well-ventilated area.
Regular Maintenance: Follow the manufacturer’s recommended maintenance schedule. This includes checking the spark plug, air filter, fuel filter, and other components. Regular maintenance can help prevent problems before they occur.
Avoid Overheating: Overheating can damage the ignition coil and other engine components. Avoid running the trimmer at full throttle for extended periods, especially in hot weather.
Use the Correct Spark Plug: Using the wrong spark plug can damage the ignition system. Always use the spark plug specified in the Stihl FS 55R owner’s manual.
Check the Spark Arrestor: A clogged spark arrestor can restrict exhaust flow, causing the engine to overheat. Clean the spark arrestor regularly.

Case Studies: Real-World Ignition Coil Problems

Here are a few real-world case studies that illustrate common ignition coil problems on the Stihl FS 55R:

Case Study 1: The Intermittent Starter

A homeowner was experiencing intermittent starting problems with his FS 55R. The trimmer would start sometimes, but not others. He replaced the spark plug, but the problem persisted. After testing the ignition coil with a multimeter, he found that the primary coil resistance was outside of the manufacturer’s specifications. He replaced the ignition coil, and the trimmer started reliably.

Diagnosis: Faulty ignition coil with intermittent primary winding failure.
Solution: Replaced the ignition coil.

Case Study 2: The Sudden Shutdown

A landscaper reported that his FS 55R would start and run for a short period, then suddenly shut down. He suspected a fuel problem, but after cleaning the carburetor, the problem persisted. After inspecting the ignition coil, he found a small crack in the plastic housing. He replaced the ignition coil, and the trimmer ran without any further problems.

Diagnosis: Cracked ignition coil housing leading to internal short circuit when heated.
Solution: Replaced the ignition coil.

Case Study 3: The Weak Spark

A forestry worker was experiencing a lack of power with his FS 55R. The trimmer would start, but it would bog down under load. He checked the spark plug and found that it was heavily fouled with carbon. After cleaning the spark plug, the problem persisted. He then tested the spark and found that it was weak and yellow. After checking the air gap between the coil and flywheel and ensuring it was within specifications, he replaced the ignition coil, and the trimmer regained its power.

Diagnosis: Weak spark due to a failing ignition coil.
Solution: Replaced the ignition coil.

Choosing a Replacement Ignition Coil

When replacing the ignition coil on your Stihl FS 55R, it’s important to choose a high-quality replacement. There are many aftermarket coils available, but not all of them are created equal. Here are some factors to consider when choosing a replacement coil:

Brand: Stick with reputable brands that are known for producing high-quality parts. Genuine Stihl parts are always a good choice, but there are also several aftermarket brands that offer reliable coils.
Quality: Look for coils that are made with high-quality materials and construction. Avoid coils that look cheap or poorly made.
Warranty: Choose a coil that comes with a warranty. This will protect you in case the coil fails prematurely.
Reviews: Read online reviews to see what other users have to say about the coil. This can give you valuable insights into the coil’s performance and reliability.

Conclusion: Keeping Your Stihl FS 55R Running Smoothly

Troubleshooting ignition coil problems on your Stihl FS 55R can seem daunting, but with a systematic approach and the right tools, it’s a manageable task. By following the steps outlined in this guide, you can accurately diagnose the problem and get your trimmer back up and running quickly. Remember to prioritize safety, use high-quality replacement parts, and perform regular maintenance to prevent future problems. With a little care and attention, your Stihl FS 55R will continue to be a reliable workhorse for years to come.

Takeaway: Understanding the ignition system and knowing how to troubleshoot coil issues is essential for keeping your Stihl FS 55R in top condition. Don’t hesitate to consult the service manual or seek professional help if you’re unsure about any of the steps.