Stihl FS 45 Ignition Coil Test (Resistance)
A resistance reading alone cannot confirm whether an FS 45 ignition module is good or bad. STIHL does not publish one universal coil-resistance pass/fail value for every version. Inspect the plug and wiring, isolate the stop-switch circuit for checks, then confirm spark with an approved tester. Use the serial-number-matched service information before adjusting or replacing parts.
There is a practical luxury in a trimmer that starts when you need it, rather than after an hour of guessing. If your FS 45 has no spark, a multimeter can help you investigate, but a single resistance number is not a reliable verdict on its ignition module.
The key is to separate a module fault from simpler causes, such as a damaged plug cap or a stop-switch wire shorting to ground. The steps below use resistance as a screening check, then confirm the fault with a spark test.
What resistance testing can and cannot tell you
Resistance testing measures how strongly a component opposes electrical current. On an FS 45, it can help reveal an open circuit or an unexpected connection, but it cannot give a universal pass/fail answer for the ignition module. Readings vary with meter design, probe placement, temperature, and connected parts.
STIHL does not provide one resistance threshold that applies to every FS 45 ignition module. The module contains electronic components, so an ohmmeter may show different results depending on probe polarity and the circuit being measured. A reading that looks unusual, or an open reading, does not by itself prove the module has failed.
This is why resistance should be one part of the diagnosis, not the replacement decision. First inspect the parts you can see. Then isolate the stop circuit, check resistance according to the applicable service procedure, and verify spark with a proper tester.
Prepare the FS 45 safely
Safe preparation prevents accidental starting and misleading readings. Work outdoors with the engine cool, the ignition switched off, and the cutting attachment clear of people and objects. Secure the trimmer so it cannot move. Before measuring resistance, disconnect the spark-plug lead and the module’s stop-switch wire.
The FS 45 is a handheld grass trimmer or brushcutter, not a chainsaw. Chain, bar, and wood-cutting specifications do not apply. Confirm the exact model and version from the machine’s label, then use its serial-number-matched manual or service information for plug, gap, and installation details.
Before you start:
- Wear eye protection and gloves.
- Keep fuel vapors away from sparks, flames, and hot surfaces.
- Do not pull-start the engine while a meter is connected.
- Do not probe a live ignition system with a multimeter.
- Keep the stop-switch wire disconnected only while performing the stationary test. Reconnect it before operation.
A shorted stop-switch lead can suppress spark and imitate a failed module. Leaving that wire disconnected removes the normal way to stop the engine, so never run the trimmer in that condition.
Inspect and test in a sensible order
A staged check helps identify common faults before you consider the ignition module. Start with the plug and visible wiring, then use resistance as a comparison check, and finish by confirming spark with an approved tester. Record what you find; changes between tests can be more useful than one isolated number.
1. Inspect the plug, lead, and wiring
Look for a cracked plug insulator, a damaged plug cap, a loose high-tension lead, or wiring that is pinched, rubbed through, or corroded. Check the module’s mounting and ground points for looseness or debris. Inspect the flywheel area for visible damage or dirt that could interfere with the magnetic pickup.
Verify that the spark plug is the correct type and that its gap matches the manual for your exact version. Do not assume a plug is good because it looks clean; a known-good, correct plug can help rule it out. Avoid touching the plug terminal while testing spark.
2. Use the multimeter as a screening tool
Set a digital multimeter to resistance, following its instructions. With the ignition off, the plug lead disconnected, and the stop-switch wire detached from the module, check the plug cap and lead, then the coil circuit only as directed by the correct STIHL service procedure.
A resistor-type plug cap can add resistance to a measurement. Disconnect it only if the service procedure allows that, and note whether it was connected. Do not apply a generic online resistance threshold: the meter may be measuring the cap, lead, and module together, rather than one isolated part.
For a useful record, write down:
- Meter make or model and selected range
- Probe locations and polarity
- Whether the plug cap was connected
- Approximate component temperature
- The displayed reading, including an open or unstable result
If the meter reading changes when you reverse the probes, that does not automatically mean the module is defective. Electronic parts can respond differently to meter polarity. Compare readings only when the setup and probe positions are the same.
3. Confirm spark with an approved tester
Reconnect the spark-plug lead and follow the tester maker’s instructions for an approved inline or grounded spark tester. Keep the plug or tester securely grounded as directed, and keep fuel vapors away. Do not hold the plug by hand during a spark test.
If spark is absent, switch the ignition off again before disconnecting the module’s stop-switch wire. Repeat the spark test with that wire isolated, following the tester instructions. If spark returns, the result points toward a stop-switch or wiring fault; it does not, by itself, prove the module is perfect. Repair the stop circuit and reconnect it before starting the machine.
4. Check mounting and air gap if the module remains suspect
Inspect the flywheel magnets and module mounting for damage, looseness, or debris. If the module has been moved or removed, set the air gap using the specification for the exact FS 45 version and the prescribed service method. A commonly cited family reference is about 0.3 mm, but confirm it in the machine’s applicable service information before adjustment.
Reassemble the covers and guards, reconnect every wire, and verify the stop switch shuts the engine off. Only then test-start the trimmer, with the cutting attachment safely clear. If spark is still absent after the stop circuit, plug, cap and lead, wiring, flywheel, and gap have been checked, use the STIHL service test procedure or ask a dealer to test the module.
Read the results without guessing
The table shows what each finding may suggest. It does not replace the service procedure for your exact machine. In particular, neither an unusual resistance reading nor a spark returning with the stop wire isolated is enough on its own to justify replacing the ignition module.
| Finding | What it may indicate | Next check |
|---|---|---|
| Damaged plug, cap, or lead | A fault outside the module | Replace or test the affected part as specified |
| No spark with stop wire connected | Module, plug, wiring, or stop-circuit fault | Inspect wiring, then repeat the approved spark test with the stop wire isolated |
| Spark returns with stop wire isolated | Stop switch or its wiring may be grounding the ignition | Repair the stop circuit; reconnect and verify shutdown |
| Open or unexpected resistance reading | Open connection, connected resistor, meter effects, or module behavior | Confirm probe points and service procedure; do not condemn the module from this alone |
| No spark after other checks | Module may be faulty, but diagnosis is not complete by resistance alone | Follow STIHL’s service test or have a dealer assess it |
| Spark present, but engine still will not start | The ignition module may not be the cause | Check other starting or fuel-related causes using the manual |
A useful field scenario is a trimmer that shows no spark with its stop wire connected, then sparks when the wire is isolated. That result makes the stop circuit the next place to investigate; it is not a reason to leave the wire disconnected or to buy a module immediately.
In another common diagnostic situation, a meter displays an open reading across a connected cap-and-module path. Because the cap may include a resistor and the module contains electronic parts, that result is inconclusive unless the service procedure specifies those probe points and expected behavior. These are examples of how to interpret tests, not reports from a particular repair.
Common reference data and limits
Some FS 45 family data can help identify the machine, but version-specific service information takes priority. These figures are reference points, not substitutes for a serial-number-matched manual or a diagnosis. Do not use carburetor adjustments to compensate for a suspected ignition fault.
| Item | Common family reference | Use it carefully |
|---|---|---|
| Engine displacement | About 27.2 cm³ | Confirm the model/version |
| Fuel mix | 50:1 with STIHL two-stroke oil or an approved equivalent | Follow the manual and fuel-oil instructions |
| Nominal idle speed | About 2,800 rpm | Do not adjust idle to mask an ignition fault |
| Module air gap | Commonly about 0.3 mm | Verify the correct specification and setting method |
Fuel that is stale or mixed incorrectly can cause starting or running trouble that is not an ignition-coil failure. If spark is present, avoid replacing the module simply because the trimmer still will not start; continue diagnosis using the machine’s manual.
Reconnect, verify, and return the trimmer to service
Finish by restoring the machine to its normal safety configuration. Refit covers and guards, reconnect the stop-switch wire and spark-plug lead, and check that all wiring is secure and clear of moving parts. Verify that the stop switch shuts the engine off before returning the trimmer to service.
If the stop switch does not work, do not operate the machine. Follow the manual for fuel storage and seasonal care; old fuel can create symptoms that are easily confused with ignition trouble. The key takeaway is simple: use resistance readings to guide the next check, not to make the replacement decision.
Frequently asked questions
What resistance should an FS 45 ignition coil show?
There is no single STIHL-published resistance pass/fail value that applies to every FS 45 module. Use the applicable service procedure and treat the reading as a screening check.
Can an ohmmeter prove the ignition module is bad?
No. The module contains electronic parts, and readings can vary with the meter, probe polarity, temperature, and connected cap or lead. Confirm the fault with the prescribed service test.
Should I disconnect the stop-switch wire before a resistance check?
Yes, when following the specified resistance-check procedure, isolate the module’s stop-switch wire so the switch circuit cannot create a misleading ground path. Switch the ignition off first.
Can I run the trimmer with the stop wire disconnected?
No. Disconnecting it can disable the normal engine stop control. Reconnect the circuit and verify that the stop switch works before operating the trimmer.
What if spark returns when the stop wire is isolated?
That points toward a stop-switch or wiring fault. It does not prove the module is fault-free, and the trimmer must not be operated with the circuit left disconnected.
What air gap should I set on the module?
About 0.3 mm is a commonly cited FS 45 family reference, but the correct specification and procedure depend on the exact version. Confirm them before adjustment.
Should I replace the module if the meter reads open?
Not on that result alone. Check the probe points, cap and lead setup, and applicable service procedure before deciding whether the module needs further testing.
Is a correct-looking spark plug enough to rule out the plug?
No. Confirm that the plug type and gap match the manual. If needed, test with a known-good plug of the correct type.
What should I do if there is still no spark after these checks?
Use the STIHL service test procedure or have a dealer test the module. Do not substitute an improvised resistance threshold for the specified test.
(This article was written by one of our staff writers, Harrison Croft. Visit our Meet the Team page.)
