What Is a String Trimmer Ignition Coil? (Resistance Test)
A string-trimmer ignition coil is the electrical part that helps create the spark needed to start and run a gasoline engine. A multimeter can check its winding resistance, but test points and acceptable readings vary by model. Isolate the coil, follow the service manual, and use a spark tester too: resistance alone cannot prove the coil works in use.
Is your trimmer not starting, or does it stop after warming up? The coil may be at fault, but a bad plug, loose wire, or kill-switch problem can cause similar symptoms. A careful resistance test can help narrow the cause without guessing. I’ll explain what the coil does, how to test it, and what the results can and cannot tell you.
Coil Identification, Equipment Setup & Safety Preparation
An ignition coil is part of a gasoline trimmer’s magneto ignition system. It turns electrical energy created as the flywheel spins into the high voltage needed for a spark at the plug. Its wiring and test points differ by model, so find the correct service information before judging a reading.
What the ignition coil does
The flywheel has magnets that pass the coil as the engine turns. This changing magnetic field helps the coil build and release electrical energy. The high-tension lead, also called the HT lead, carries the resulting voltage to the spark-plug cap and plug.
Inside the coil are two wire windings: the primary winding and the secondary winding. The primary winding is the lower-voltage side; the secondary winding produces the much higher voltage used for ignition. A kill-switch wire lets the stop switch shut off the engine by grounding the ignition circuit.
What you need before testing
Use a digital multimeter with resistance (Ω) and continuity settings. You may also need a small screwdriver or wrench to remove a cover, and an approved inline or adjustable spark tester for the later spark check. The service manual is essential for the coil’s correct test points and pass/fail values.
Before touching ignition parts:
- Stop the engine, let it cool, and work outdoors in a well-ventilated area.
- Remove the spark-plug boot from the plug. Keep the boot away from the plug while you work.
- Prevent accidental starting. Follow the manufacturer’s instructions for disabling the ignition.
- Keep fuel and fuel vapors away from sparks, open flames, and other ignition sources.
- Do not hold the bare high-tension lead or plug while cranking the engine.
Inspect the spark plug, plug cap, HT lead, coil mounting, flywheel magnets, and wires. Look for cracks, corrosion, loose connections, dirt, or damage. A simple wiring fault can look like a failed coil.
Field Diagnosis: Resistance Testing and Spark Verification
Resistance testing checks whether the coil windings and related connections have an open circuit or an unexpected reading. It does not recreate the heat, voltage, and vibration of a running engine. Use the correct manual values, then confirm the ignition with an approved spark tester.
Isolate the coil and set the meter
First disconnect the coil’s kill-switch wire. If that wire remains connected, a fault in the stop switch or harness may ground the ignition and create a misleading test result. Leave the coil’s other connections as specified by the manual.
Set the meter to resistance (Ω). Touch the meter probes together and note their resistance. On a very low-resistance primary test, lead resistance can distort the result. Use the meter’s zero or relative function if available, or subtract the measured lead resistance when appropriate.
Measure primary and secondary resistance
The primary test is commonly made between the coil’s specified primary or kill-terminal connection and the coil core or ground. The secondary test is commonly made between the HT lead and the specified coil ground or core. Follow the manual’s exact method, especially if the plug cap must be removed.
Some small-engine coils have primary readings in a broad range of about 0.2–2 Ω and secondary readings around 2–10 kΩ. These are examples, not universal pass/fail limits. Coil design and test points vary, so compare your result with the specification for the exact trimmer model.
A resistor plug cap can add about 5 kΩ to a secondary reading. Test the cap separately if the manual allows it, or account for it when comparing the coil reading with the specified value. A reading that seems high may come from the cap, not the coil.
| Test | Typical test points* | Example range on some coils | What to keep in mind |
|---|---|---|---|
| Primary winding | Specified primary or kill terminal to coil core/ground | About 0.2–2 Ω | Meter-lead resistance matters at low values. |
| Secondary winding | HT lead to specified coil ground/core | About 2–10 kΩ | The manual may require removing the cap. |
| Resistor cap | Across the disconnected cap | About 5 kΩ | Treat this as a common example, not a universal value. |
*Use the model’s service manual for exact test points and limits.
Read the result without overcalling it
An open-circuit reading, shown as “OL” or an equivalent symbol, can point to a break in the winding or a poor probe connection. A reading far outside the manual’s range also deserves investigation. Check that the probes touch clean metal and that the kill wire is disconnected before deciding the coil has failed.
| Result or symptom | What it may suggest | Next step |
|---|---|---|
| Reading matches the manual | Winding resistance is within the stated range | Continue to a spark test. |
| Reading is open or far out of range | Possible winding fault, poor contact, or incorrect test setup | Recheck connections and test points; consult the manual. |
| No spark, resistance appears normal | Possible plug, cap, kill circuit, ground, air-gap, or heat-related fault | Check the ignition system as a whole. |
| Runs cold, stops warm | A temperature-dependent coil or connection fault is possible | Repeat checks when the symptom occurs, safely and as directed. |
Verify spark safely
Reconnect the coil as directed and use an approved inline or adjustable spark tester at the engine maker’s specified gap. Do not test by holding the plug or bare HT lead near the engine. Keep the test away from spilled fuel, fuel vapors, and other ignition sources.
A visible spark in open air does not prove the ignition will work under compression inside the cylinder. If there is no spark, check the plug, cap and lead, kill circuit, coil ground, flywheel magnets and key, and the prescribed coil-to-flywheel air gap before replacing the coil.
Maintenance, Air-Gap Checks & Ignition Troubleshooting
The air gap is the small space between the ignition coil and flywheel magnets. It affects how the magnets interact with the coil, but its correct size depends on the engine. Check it against the service manual rather than using a general figure as a final setting.
Some small engines use a coil-to-flywheel gap near 0.20–0.30 mm (0.008–0.012 in). That range is not universal. Use the model’s specified measurement and adjustment method; a feeler gauge may be called for, but the manual controls.
Also inspect the flywheel magnets for visible damage or contamination, and check that the coil is secure. Do not assume that a new coil will fix a no-spark problem if the mounting, wiring, or flywheel parts are at fault.
Diagnostic scenarios: what the pattern can tell you
These are illustrative troubleshooting scenarios, not proof of a particular failure. They show how to combine resistance checks with symptoms and other inspections. A single reading rarely identifies the faulty part on its own.
- No spark, but winding readings match the manual: Check the plug and cap, disconnect and inspect the kill-switch circuit, and verify the coil ground and air gap. A normal resistance result does not rule out a fault that appears only under operating conditions.
- Trimmer starts cold, then stops after warming up: A coil winding or connection may behave differently as it heats. Repeat the specified test when the fault occurs, if the manual’s procedure allows it, and check the plug, cap, and wiring as well.
- Secondary reading seems too high: If the plug cap is still attached, its resistor may affect the result. Test or account for it as directed before comparing the measurement with the coil-only specification.
Building on this, resistance is best treated as one part of diagnosis. It can reveal some winding faults, but spark testing and inspection help identify problems in the rest of the ignition path.
Shutdown, Cleaning & Seasonal Storage
After testing or repair, put the ignition system back into its normal configuration before starting the trimmer. Secure the plug and boot, restore wiring and guards, and make sure the cutting attachment is clear of people and objects. Follow the manual for any storage or cleaning steps.
Return the trimmer to service
Reconnect the kill-switch wire and any other wiring you disconnected. Refit covers and guards, and make sure the spark-plug boot is firmly seated. Test-start outdoors, with the cutting attachment pointed away from people, pets, and objects.
If the trimmer still has no spark or runs poorly, stop and consult the model’s service information or a qualified repair provider. Do not keep running it with loose wiring, missing guards, or an unresolved ignition fault.
Store it according to the manual
Shut the engine off and let it cool before cleaning or storing the trimmer. Keep the coil, plug lead, and wiring free of debris and moisture, and follow the manufacturer’s instructions for fuel and seasonal storage. Do not use solvents or cleaning methods unless the manual approves them.
Conclusion and FAQ
A multimeter can help check an ignition coil’s primary and secondary winding resistance, but the correct values and test points depend on the trimmer. Disconnect the kill wire, account for the plug cap, and compare measurements with the exact service manual. Then verify spark safely and check related parts before replacing the coil.
What does a string-trimmer ignition coil do?
It helps create the high voltage that produces a spark at the spark plug. Magnets on the flywheel pass the coil as the engine turns, and the coil sends ignition energy through the HT lead and plug cap to the spark plug.
How do I test a trimmer ignition coil with a multimeter?
Disconnect the kill-switch wire, then measure the primary and secondary windings at the test points specified in the service manual. Check the plug cap separately if required. Compare the readings with the model’s specifications; generic resistance ranges are only rough examples.
What resistance should a string-trimmer ignition coil have?
There is no single correct value for every trimmer coil. Some small-engine primary readings are roughly 0.2–2 Ω, and some secondary readings are about 2–10 kΩ. Use the exact model’s service data to decide whether a reading passes.
Should I disconnect the kill wire before testing?
Yes, when the service procedure calls for it. A connected kill circuit can ground the ignition and affect the result. Disconnect the specified wire before measuring, then reconnect it correctly before operating the trimmer.
Does the spark-plug cap affect the resistance reading?
It can. A resistor cap commonly adds about 5 kΩ to a secondary-side measurement. The amount and test method vary, so check whether the manual expects the cap to be attached, removed, or tested separately.
Can a coil pass a resistance test and still be bad?
Yes. A coil can show an acceptable cold reading but fail when hot or under operating voltage. Resistance testing also cannot prove that the ignition will spark reliably under compression. Use the prescribed spark test and check the rest of the ignition circuit.
What air gap should I use between the coil and flywheel?
Use the exact air-gap setting in the trimmer’s service manual. Some small engines use about 0.20–0.30 mm, or 0.008–0.012 in, but this is not a universal specification. An incorrect gap can affect ignition.
Is it safe to test spark by holding the plug wire?
No. Do not hold a bare HT lead or spark plug while cranking the engine. Use an approved inline or adjustable spark tester, follow the manufacturer’s procedure, and keep the test away from fuel vapors and other ignition sources.
What should I check if there is no spark?
Check the spark plug, plug cap and HT lead, kill-switch circuit, coil ground, flywheel magnets and key, and the specified air gap. A failed coil is one possible cause, but other faults can produce the same no-spark symptom.
Why does my trimmer lose spark when it gets hot?
Heat can reveal a winding or connection fault that is not present when the engine is cold. A coil is one possibility, but inspect the plug, cap, wiring, and kill circuit too. Follow the service manual for any warm-condition test.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
