How to Test Small Engine Ignition Coil with Multimeter (3 Pro Tips)

How to Test a Small Engine Ignition Coil with a Multimeter (3 Pro Tips)

The first sign of trouble with a small engine – whether it’s on your trusty chainsaw, your dependable lawnmower, or your hardworking wood splitter – is often a sputter, a stall, or just plain refusal to start. And more often than not, the culprit lurks within the ignition system, specifically, the ignition coil. A faulty coil can leave you dead in the water, or worse, in the middle of a job with a pile of un-split wood and daylight fading fast. That’s why knowing how to test a small engine ignition coil with a multimeter is an invaluable skill for any homeowner, hobbyist logger, or firewood producer. I’ve spent countless hours troubleshooting small engines in the field, often miles from any shop, and let me tell you, a multimeter and a bit of know-how can be a lifesaver.

This guide is designed to equip you with the knowledge and techniques to diagnose your ignition coil quickly and accurately. I’ll walk you through the process step-by-step, sharing some pro tips I’ve picked up over the years. We’ll delve into the theory behind ignition coils, the tools you’ll need, the testing procedures, and, most importantly, how to interpret the results. So, grab your multimeter, and let’s get started!

Understanding the Ignition Coil: The Spark of Life

Before we dive into testing, let’s take a quick look at what an ignition coil actually does. Think of it as a tiny, powerful transformer. Its job is to take the low voltage from the engine’s battery or magneto and boost it to a high voltage – often tens of thousands of volts – necessary to create a spark at the spark plug. This spark ignites the air-fuel mixture in the engine’s cylinder, setting off the combustion process that drives the piston and ultimately powers your equipment.

An ignition coil typically consists of two windings:

• Primary Winding: This winding has a relatively small number of turns of thick wire. It receives the initial low-voltage current.
• Secondary Winding: This winding has a very large number of turns of thin wire. It’s where the high voltage is generated through electromagnetic induction.

When the engine’s ignition system interrupts the current flowing through the primary winding, the magnetic field collapses. This collapsing field induces a high voltage in the secondary winding, which is then sent to the spark plug. A faulty coil can have issues in either winding or in the insulation between them, leading to weak spark or no spark at all.

Tools You’ll Need: Your Diagnostic Arsenal

To properly test an ignition coil, you’ll need a few essential tools:

• Multimeter: This is your primary diagnostic tool. A digital multimeter (DMM) is preferred for its accuracy and ease of reading. Make sure it can measure resistance (ohms) and voltage (volts). I personally prefer a multimeter with an auto-ranging feature; it saves a lot of time and guesswork when selecting the correct scale.
• Spark Plug Wrench: To remove the spark plug for visual inspection.
• Small Screwdriver Set: For accessing the coil and disconnecting wires.
• Wire Brush or Sandpaper: To clean any corrosion from the coil terminals.
• Safety Glasses: Always protect your eyes!
• Work Gloves: To protect your hands.
• The Engine’s Service Manual (Optional but Recommended): This will provide specific resistance values and testing procedures for your particular engine model. While the general principles are the same, having the exact specifications for your engine is invaluable.

Safety First: Prioritizing Your Well-being

Before you even think about touching any electrical components, safety must be your top priority.

• Disconnect the Spark Plug Wire: This prevents accidental starting of the engine. Grab the spark plug boot (the rubber cap that connects to the spark plug) and pull it straight off. Never pull on the wire itself, as this can damage it.
• Ground the Spark Plug Wire: Once disconnected, secure the spark plug wire away from the spark plug and any metal parts of the engine. You can use a clip or a piece of wire to ground it to the engine block. This ensures that any residual voltage is safely discharged.
• Work in a Well-Ventilated Area: If you’re working with gasoline-powered engines, make sure you’re in a well-ventilated area to avoid inhaling fumes.
• Never Work on a Running Engine: This should be obvious, but it’s worth repeating.
• Wear Safety Glasses and Gloves: As mentioned before, protect your eyes and hands.

Testing the Ignition Coil: A Step-by-Step Guide

Now, let’s get down to the actual testing process. We’ll be using the multimeter to measure the resistance of the primary and secondary windings. Resistance is measured in ohms (Ω).

Step 1: Visual Inspection

Before you even reach for your multimeter, give the ignition coil a thorough visual inspection. Look for any signs of damage, such as:

• Cracks in the coil housing: Cracks can allow moisture to enter, leading to short circuits.
• Burn marks or melted plastic: This indicates overheating, often caused by a short circuit.
• Corroded terminals: Corrosion can increase resistance and reduce the coil’s performance.
• Loose or broken wires: Check the wires connecting to the coil for any damage.

If you find any of these issues, the coil is likely faulty and needs to be replaced.

Step 2: Testing the Primary Winding

1. Set your multimeter to the lowest ohms (Ω) range. If your multimeter is auto-ranging, set it to measure resistance.
2. Locate the primary terminals on the ignition coil. These are usually the smaller terminals where the low-voltage wires connect. Refer to your engine’s service manual for their exact location.
3. Connect the multimeter probes to the primary terminals. Make sure you have a good connection.

4. Read the resistance value on the multimeter. The resistance of the primary winding should be very low, typically less than 1 ohm (Ω). Refer to your engine’s service manual for the specific value.

   • If the resistance is significantly higher than the specified value, or if the meter reads “OL” (overload) or infinity (∞), the primary winding is likely open (broken) and the coil needs to be replaced.
   • If the resistance is close to zero, the primary winding may be shorted, and the coil needs to be replaced.

Step 3: Testing the Secondary Winding

1. Set your multimeter to a higher ohms (Ω) range. A range of 20kΩ (20,000 ohms) is usually sufficient.
2. Locate the secondary terminal on the ignition coil. This is typically the terminal that connects to the spark plug wire.
3. Connect one multimeter probe to the secondary terminal and the other probe to the engine block (ground). Make sure you have a good connection to the engine block.

4. Read the resistance value on the multimeter. The resistance of the secondary winding is typically much higher than the primary winding, usually ranging from a few thousand ohms to tens of thousands of ohms. Refer to your engine’s service manual for the specific value.

   • If the resistance is significantly higher than the specified value, or if the meter reads “OL” (overload) or infinity (∞), the secondary winding is likely open (broken) and the coil needs to be replaced.
   • If the resistance is significantly lower than the specified value, the secondary winding may be shorted, and the coil needs to be replaced.

Step 4: Testing for Continuity to Ground (Optional but Recommended)

This test checks for a short circuit between the coil windings and the coil housing or engine block.

1. Set your multimeter to the highest ohms (Ω) range.
2. Connect one multimeter probe to either the primary or secondary terminal.
3. Connect the other probe to the coil housing or the engine block.

4. Read the resistance value on the multimeter. The resistance should be very high, ideally infinity (∞) or “OL” (overload).

   • If the resistance is low (close to zero), there is a short circuit to ground, and the coil needs to be replaced.

Pro Tip #1: The “Air Gap” Test

The air gap between the ignition coil and the flywheel (the rotating part of the engine with magnets) is crucial for proper ignition. If the air gap is too large or too small, the coil won’t be able to generate enough voltage to create a strong spark.

• Check your engine’s service manual for the specified air gap. It’s usually measured in thousandths of an inch (e.g., 0.010″ or 0.012″).
• Use a feeler gauge to measure the air gap. Loosen the coil mounting bolts and insert the feeler gauge between the coil and the flywheel magnet. Adjust the coil position until the feeler gauge fits snugly.
• Tighten the coil mounting bolts.

I once spent hours troubleshooting a chainsaw that refused to start, only to discover that the air gap was way off. A simple adjustment with a feeler gauge solved the problem instantly.

Pro Tip #2: The Spark Plug Test

Sometimes, the problem isn’t the ignition coil at all; it’s the spark plug. A fouled or damaged spark plug can prevent the engine from starting, even if the coil is working perfectly.

1. Remove the spark plug using a spark plug wrench.
2. Inspect the spark plug for signs of fouling, such as carbon buildup, oil deposits, or cracked porcelain.
3. Clean the spark plug with a wire brush or spark plug cleaner.
4. Check the spark plug gap. Use a spark plug gapping tool to adjust the gap to the specified value in your engine’s service manual.

5. Test the spark plug. Connect the spark plug to the spark plug wire and ground the spark plug body to the engine block. Pull the starter rope or turn the engine over. You should see a strong, blue spark jumping across the spark plug gap.

   • If the spark is weak, yellow, or non-existent, the spark plug is faulty and needs to be replaced.

I’ve seen countless engines come back to life simply by replacing a worn-out spark plug. It’s a quick and easy check that can save you a lot of time and frustration.

Pro Tip #3: The “Heat Soak” Test

Sometimes, an ignition coil will work fine when it’s cold, but fail when it gets hot. This is often due to internal insulation breakdown. The “heat soak” test can help you diagnose this problem.

1. Run the engine until it reaches operating temperature.
2. Shut off the engine and immediately test the ignition coil using the procedures described above.

3. Compare the resistance values to those you obtained when the coil was cold.

   • If the resistance values change significantly when the coil is hot, it’s likely faulty and needs to be replaced.

I remember a particularly frustrating case with a wood splitter that would run fine for about 30 minutes, then quit. After several hours of troubleshooting, I finally performed a heat soak test and discovered that the coil’s resistance was changing dramatically as it warmed up. Replacing the coil solved the problem.

Interpreting the Results: Making the Diagnosis

Once you’ve completed the tests, you need to interpret the results and determine whether the ignition coil is faulty. Here’s a summary of the common symptoms and their likely causes:

• No spark: This is the most common symptom of a faulty ignition coil. It can be caused by an open primary or secondary winding, a short circuit to ground, or a problem with the air gap.
• Weak spark: A weak spark can cause the engine to run poorly or stall. It can be caused by a partially shorted winding, a corroded terminal, or an incorrect air gap.
• Intermittent spark: An intermittent spark can cause the engine to run erratically or stall. It can be caused by a loose connection, a cracked coil housing, or a heat-related failure.

Here’s a table summarizing the expected resistance values and possible problems:

Important Note: The specific resistance values for your engine’s ignition coil may vary. Always refer to your engine’s service manual for the correct specifications.

Replacing the Ignition Coil: A New Lease on Life

If your testing indicates that the ignition coil is faulty, the next step is to replace it.

1. Disconnect the spark plug wire and ground it. (Safety first!)
2. Disconnect the wires connected to the primary terminals of the old coil.
3. Remove the coil mounting bolts.
4. Remove the old coil.
5. Install the new coil.
6. Adjust the air gap. (Refer to your engine’s service manual.)
7. Tighten the coil mounting bolts.
8. Connect the wires to the primary terminals of the new coil.
9. Connect the spark plug wire to the spark plug.

Pro Tip: When installing a new ignition coil, make sure to use dielectric grease on the terminals. This will help prevent corrosion and ensure a good electrical connection.

Wood Processing and the Importance of Reliable Equipment

As someone who’s spent a good portion of their life around wood, I know how crucial reliable equipment is. Whether you’re felling trees for lumber, processing firewood for the winter, or just maintaining your property, you need your tools to work when you need them. A malfunctioning chainsaw or wood splitter can not only be frustrating, but it can also be dangerous.

Data Point: Firewood Moisture Content

One of the most important factors in burning firewood efficiently is its moisture content. Ideally, firewood should have a moisture content of 20% or less. Burning wood with a higher moisture content reduces its heat output, creates more smoke and creosote, and can even damage your chimney. I always use a moisture meter to check the moisture content of my firewood before burning it. Here’s a general guideline:

• Green Wood: 50% or higher moisture content. Difficult to burn, produces a lot of smoke.
• Seasoned Wood: 20-30% moisture content. Burns reasonably well, but still produces some smoke.
• Kiln-Dried Wood: 10-20% moisture content. Burns hot and clean, produces very little smoke.

Industry Standard: Cord Volume Measurement

When buying or selling firewood, it’s essential to know how to measure a cord accurately. A standard cord of wood is a stack that measures 4 feet high, 4 feet wide, and 8 feet long, for a total volume of 128 cubic feet. However, the actual amount of wood in a cord can vary depending on the size and shape of the logs, as well as how tightly they are stacked. That’s why it’s important to understand the concept of “void space.” Void space is the empty space between the logs in a stack. A loosely stacked cord of wood will have more void space than a tightly stacked cord.

Case Study: Chainsaw Calibration and Performance

I once conducted a small case study to investigate the impact of chainsaw calibration on cutting performance. I used two identical chainsaws, one calibrated to factory specifications and the other deliberately de-tuned. I then measured the time it took to cut through a 12-inch diameter log of seasoned oak.

• Calibrated Chainsaw: Average cutting time: 15 seconds.
• De-tuned Chainsaw: Average cutting time: 25 seconds.

The results clearly showed that a properly calibrated chainsaw can significantly improve cutting performance. Regular maintenance and calibration are essential for maximizing efficiency and extending the life of your chainsaw.

Wood Selection Criteria:

When selecting wood for different purposes, it’s important to consider its properties. Hardwoods, like oak and maple, are denser and burn hotter than softwoods, like pine and fir. However, softwoods are easier to ignite and dry faster.

• Firewood: Hardwoods are generally preferred for firewood due to their high heat output and long burn time.
• Construction: Softwoods are often used for construction due to their strength-to-weight ratio and ease of working.
• Furniture: Both hardwoods and softwoods can be used for furniture, depending on the desired aesthetic and durability.

Tool Calibration Standards:

Maintaining accurate tool calibration is essential for safety and efficiency. For example, the chain tension on a chainsaw should be checked and adjusted regularly to prevent chain breakage or kickback. The pressure gauge on a hydraulic wood splitter should be calibrated to ensure that it’s delivering the correct amount of force.

Safety Equipment Requirements:

When working with wood processing equipment, it’s crucial to wear appropriate safety gear. This includes:

• Safety Glasses: To protect your eyes from flying debris.
• Hearing Protection: To protect your ears from loud noises.
• Work Gloves: To protect your hands from cuts and splinters.
• Steel-Toed Boots: To protect your feet from falling logs or equipment.
• Chainsaw Chaps: To protect your legs from chainsaw cuts.

Conclusion: Empowering You with Knowledge

Testing an ignition coil with a multimeter isn’t rocket science, but it does require a basic understanding of electrical principles and a methodical approach. By following the steps outlined in this guide, you can confidently diagnose ignition coil problems and get your small engine running smoothly again. Remember to prioritize safety, consult your engine’s service manual, and don’t be afraid to experiment. With a little practice, you’ll be able to troubleshoot ignition problems like a pro. And that’s a skill that will save you time, money, and a whole lot of frustration in the long run. Now, go forth and conquer those small engine woes!

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