A kill switch is important kit, it’s not just a convenient way to kill the motor, it could be a life saver too.
Dirt bike kill switches have two positions “RUN” and “KILL”.
- “KILL” – Operating a dirt bike kill switch provides a ground path for the CDI Capacitor Discharge Ignition) box, shorting it out. The shorted CDI unit stops sending voltage to the coil and the engine stalls (lack of spark).
- “RUN”- Setting the switch to “RUN”, removes the ground path to the CDI Box (Ignition “ON” also). When the engine is cranked over the CDI box is primed and sends voltage to the coil as per crank sensor signals.
By the end of this post you’ll understand how a dirt bike kill switch works, you’ll also learn how to diagnose a fault and how fix it.
What’s a kill switch for?
Fumbling for an ignition switch with a stuck wide open throttle wearing riding gloves, while exciting, isn’t very practical.
The kill switch is designed for ease of use in case of an emergency. The rider can access the kill switch without removing there hand from the handlebars.
Emergency services are trained to locate the Bright red button when attending an accident.
Can you use the kill switch to stop your bike?
Yes, that won’t hurt your bike in any way. As you’ll learn in this guide using the kill switch is identical to using the ignition switch.
Manufacturers don’t recommend it for a couple of reasons:
- Riders often forget to turn the ignition off after flicking the kill switch and that results in a flat battery.
- Riders often forget to set the switch to run, the bike won’t start and floods with gas and runs down the battery.
How they work
Setting the switch to kill provides a ground path to the CDI box, shorting it out and shutting down the coil.
The coil as you know makes the voltage for the spark plug, no spark equals no more fun.
To start the bike, the ignition must be on and the kill switch set to run. Both the ignition and kill switch provide a ground path to the CDI box which must be removed before the CDI box will operate.
Your ignition system consists of a few critical components, a typical start procedure looks like this:
- Ignition switch “On” – Initiates system, by removing the ground path to the CDI box, and sending power to the start button.
- Kill switch “RUN” – removes the secondary ground path to the CDI box
- Clutch/Neutral switch activated – Offers a ground path to the starter solenoid.
- Start button pressed – Power flows to the starter solenoid which connects battery power direct to starter motor and cranks the engine.
- Stator – creates voltage and powers the CDI box as the engine cranks.
- Crank sensor – creates a voltage, used by CDI box to identify the correct time to fire the plug (timing).
- CDI box – processes the crankshaft signals using Capacitor, Diodes and an SCR, the timed voltage is sent to the coil.
- Coil – produces high voltage through primary and secondary winding’s which is converted to a spark inside the combustion chamber by the spark plug.
Common kill switch problems
Generally the switches are pretty robust, and don’t cause too many problems. Diagnoses of the kill switch is mostly associated with troubleshooting a no start/spark and it’s a case of eliminating the switch as a possible root cause.
Common symptoms of a faulty kill switch or wiring include:
- No start – switch faulty or wire is grounding
- Switch not killing the engine – switch faulty or wire disconnected/broken
- Intermittent no start – faulty switch or wire grounding
- Engine stalls over rough terrain – faulty switch or wire grounding
- Backfiring – faulty switch or wire grounding
Switches are made from plastic and simply wear out, common problems include:
- Loose switch action
- Brittle plastic
- Loose contact points
- Worn contact points
- Corroded/dirty contact points
- Wire disconnects
- Loose or corroded block connector pins
- Broken wiring (around steering colum common)
- Wiring short to chassis ground
Testing the switch
This is a simple switch and checking it simple too. You can use a DVOM or a test light. I prefer the test light method.
We already know when the switch is set to run, the path across the switch should be open (broken).
We also know that when the switch is set to kill, the ground path across the switch is closed (connected).
Using a volt meter
Lets go ahead and test them now using a volt meter first and then a simple test light.
- Using a DVOM set to ohms, gain access to the wiring loom feeding the switch (2 wires). If there’s a block connector, and there should be, disconnect it. (If not we’ll pierce the wiring and make good afterwards using a wipe of silicone sealer)
- Check the connector for loose or corroded pins
- Back probe either side of the pins (switch side) and check for continuity when the kill switch is set to Kill, the value should be 00.8
- Flick the switch to RUN, the meter should read 1
Using a test light
A test light is a better test as it loads the circuit, a resistance reading can be misleading as when loaded the value can be wildly different.
- Hook up your test light by connecting the crocodile clip to battery positive (we connect to positive because we’re checking for ground with the probe).
- We won’t disconnect any kill switch wiring. Instead, find an access point, back probe at a block connector or pierce the wiring if you must.
- One of the two wires is a permanent ground feed and so the test light should light, the second wire should only light when the switch is set to “KILL”.