Stihl 045 AV Kill Switch (Testing & Wiring)
The 045 AV stops when its ignition stop lead is grounded to the saw’s metal ground. In RUN, that connection must be open; in STOP, it must have continuity. Test the switch with the engine off and its lead disconnected from the ignition unit. Because 045 AV ignition parts and wire routes can vary, identify your installed system before changing connections.
If the saw will not start, a pinched or damaged stop lead can hold the ignition grounded. If the saw starts but ignores STOP, the switch or its ground path may be open. A meter can separate those faults from other ignition problems without guessing at parts. Work through the checks below, and verify the switch again before cutting.
Equipment Setup & Safety Prep
The stop circuit is a grounding circuit, not a fuel shutoff. The switch connects the ignition stop lead to the saw’s metal ground in STOP. Safe setup matters because the saw can start during testing if you turn the switch or pull the starter.
Work outdoors with the engine cool and the saw stable. Engage the chain brake, set the switch to STOP, and remove the spark-plug cap. Keep the cap and plug lead away from the plug and metal parts where they could arc. Do not test near spilled fuel.
You will need a multimeter with continuity or resistance mode and basic hand tools suited to your saw. Do not apply battery voltage or any other external power to the stop lead. It is not a powered input; added voltage can damage ignition parts.
Before opening the saw, check its model identification and inspect the installed ignition unit. Use the matching Stihl wiring diagram if available. Do not identify a wire by color alone or copy a terminal location from another 045 version.
Next step: Find the stop lead and the switch connection without pulling on the wire or disturbing other ignition parts.
Test the Kill Switch with a Multimeter
A multimeter checks whether the switch changes the stop lead’s connection to ground. Disconnect the stop lead from the ignition unit before testing the switch itself. This prevents the ignition unit from affecting the reading and helps you pinpoint a switch or wiring fault.
- Confirm the engine is off, the spark-plug cap is removed, and the plug lead is safely clear.
- Locate the stop lead where it connects to the ignition unit. Use the correct wiring diagram to confirm the lead; do not guess based on appearance.
- Disconnect that lead from the ignition unit. Keep the loose connector clear of metal.
- Set the meter to continuity or resistance mode. Touch the probes together first. Note how your meter indicates a closed connection; some meters beep, while others show a low resistance.
- Place one probe on the isolated stop lead or switch terminal and the other on clean, bare engine metal. Check the reading with the switch in RUN, then in STOP.
- Gently move the lead and connector while watching the meter. A reading that changes as you move them can point to a loose or damaged connection.
The expected pattern is open in RUN and continuity to ground in STOP. “Open” means the meter does not detect a connection; the display may show OL or a similar indication. Do not rely on one exact resistance number: meter leads, contact quality, and meter design can affect the display.
| Switch position | Expected result from stop lead to clean engine ground | What it means |
|---|---|---|
| RUN | Open circuit | The switch is not grounding the ignition stop lead |
| STOP | Continuity or low resistance | The switch is grounding the lead to stop the engine |
| Either position changes when wire moves | Unstable reading | Inspect the lead, connector, switch, and routing |
If the switch does not show both expected states, inspect its terminal and ground path before replacing parts. A dirty probe contact can also mislead you, so make sure the probe touches bare, clean metal.
Inspect Wiring and Isolate the Fault
The stop lead must stay clear of hot or moving parts and must not touch ground unintentionally in RUN. A switch that passes a bench-style meter check can still fail if its wire is pinched, its connector is loose, or its ground contact is poor.
Follow the lead from the switch toward the ignition unit. Look for rubbed insulation, pinching under a cover, corrosion, loose connectors, and contact with the cylinder or flywheel area. Check that any switch or ground connection sits as shown in the diagram for the installed ignition system. Avoid forcing wires into a route that looks familiar from another saw.
If the switch fails either test state, repair the damaged connection or replace the faulty switch or lead with parts that match the installed version. Secure the wire in its correct route. Then repeat the meter test before reconnecting it to the ignition unit.
If the switch passes, reconnect the stop lead and inspect the ignition-unit terminal and ground connections. A no-spark complaint may still have another cause. A qualified technician can briefly isolate the stop lead at the ignition unit and use an appropriate inline spark tester to check whether an unwanted ground is suppressing spark. Keep hands and test leads clear of the flywheel and high-voltage lead.
That test is for diagnosis only. If spark returns with the stop lead isolated, the stop circuit is likely grounding when it should not. Reconnect the stop lead before operating the saw. With it disconnected, STOP may not stop the engine.
Common Fault Patterns: Diagnostic Examples
These examples show how meter results guide the next check; they are not proof that every 045 AV has the same fault. I start with the switch test because it separates a grounding problem from a fault elsewhere in the ignition system.
Example: no start, switch reads continuity in RUN. With the stop lead isolated from the ignition unit, continuity in RUN is not the expected result. I would inspect the switch and the full lead route for a short to ground, then retest. Replacing the ignition unit before checking that route risks spending money without evidence.
Example: engine runs, but STOP does not shut it down. If the meter shows open in STOP, the switch may not be closing the ground path. Check the switch connection and ground contact against the correct diagram, then retest. Do not keep using the saw while its stop function is unreliable.
Next step: Repair the specific failed connection and confirm both RUN and STOP readings before starting the saw.
Verify Shutdown Before Cutting
A meter test confirms electrical behavior while the engine is off. A start-and-stop check confirms that the assembled saw responds to the switch. Refit covers and the spark-plug cap before starting, and use the model-appropriate starting procedure outdoors.
With the saw in RUN, start it and confirm it operates normally. Select STOP and confirm the engine stops promptly. If it does not, shut it down by a safe method, do not resume cutting, and repair the circuit. Never treat pulling off the spark-plug cap as a routine shutdown method.
Do not bypass the switch or leave the stop lead disconnected. An open or bypassed stop circuit can leave the engine running when STOP is selected. The stop switch is a safety control, not an optional convenience.
Wiring Care and Seasonal Checks
A brief inspection can catch wear before it becomes a no-start or failed-stop problem. Check the stop lead, connector, and switch during normal saw maintenance, especially if covers have been removed or wiring has been disturbed.
Keep the lead in its intended route and away from the flywheel, hot engine parts, and sharp edges. Replace damaged insulation or connectors with parts suited to the installed ignition system. Do not splice in a way that changes the circuit or leaves an exposed conductor able to touch ground.
For storage, keep the saw clean and dry and avoid bending or trapping the stop lead under a cover. If the saw has been stored or repaired, repeat the RUN-open and STOP-continuity test before relying on it.
Conclusion
The decisive test is simple: with the stop lead disconnected from the ignition unit, check for an open circuit in RUN and continuity to ground in STOP. If either result is wrong, inspect the switch, ground, connector, and wire route. Use the correct diagram for the installed 045 AV ignition system, then verify shutdown before cutting.
FAQ
What does the kill switch do on a Stihl 045 AV?
It grounds the ignition stop lead to the saw’s metal ground in STOP, which stops ignition. It does not shut off fuel.
Should the stop lead show continuity in RUN?
No. With the lead isolated from the ignition unit, RUN should be open from the stop lead to engine ground.
Should the stop lead show continuity in STOP?
Yes. In STOP, the switch should provide continuity from the stop lead to clean engine ground.
Do I test the switch with the stop lead connected to the ignition unit?
No. Disconnect the lead from the ignition unit before testing the switch so you can assess the switch and wiring on their own.
What does continuity in both switch positions mean?
It can point to a short to ground, damaged insulation, a faulty switch, or a wiring connection that is not being tested as intended. Check the route and diagram, then test again.
What if the meter shows open in both positions?
The switch may not be closing in STOP, or a connector or ground path may be open. Inspect those parts and confirm you have identified the correct lead.
Can wire color identify the stop lead?
Do not rely on color alone. The installed ignition components and routing can vary, so confirm the lead with the matching wiring diagram.
Can I start the saw with the stop lead disconnected?
Do not operate it that way. The switch may not stop the engine, creating a serious safety risk.
Can I apply battery voltage to test the stop circuit?
No. The circuit works by grounding the stop lead. External voltage can damage ignition components.
What if the switch passes the meter test but the saw still has no spark?
Reconnect the stop lead and check other ignition-system causes using the correct service information. A qualified technician can isolate the stop circuit briefly with a proper spark tester, then reconnect it before operation.
(This article was written by one of our staff writers, Robert Callahan. Visit our Meet the Team page.)
