Ever wondered what’s the point of a dirt bike kill switch and an ignition switch, do we really need both.
A dirt bike will start if the kill switch is disconnected. Your dirt bike kill switch serves an important function, it’s an emergency off button, and it must be fitted by law.
In this post, you’ll learn why a dirt bike will start when the kill switch is disconnected, and you’ll also learn the important function they serve.
What’s The Point Of A Kill Switch
The kill switch is a safety feature that’s fitted to all bikes. It serves as an emergency shut-off button and that’s why they are always easily accessible and colored red.
In the event of an accident, emergency services are trained to use the kill switch (aka e-Stop) or another road user could easily identify and operate the red kill switch button on the handlebars.
The button is designed to be used by the rider without removing a hand from the handlebars. Turning the ignition key off (most bikes) would do the same job, but the key is fiddly and can’t be as easily located or operated.
Consider experiencing a stuck wide-open throttle, while wearing riding gloves and fiddling with an ignition key, isn’t practical. I know many dirt bikes won’t be fitted with an ignition switch, battery, or starter motor and if that applies to your bike, just ignore the references to it.
Kill Switch Function
Kill switch wiring differs from bike to bike, but the net result is the same – hitting the kill switch turns off the engine. If your bike has an ignition switch also then the ignition will still be on and if your bike has a battery fitted the key will need to be turned off also, otherwise, the battery will drain.
Using the kill switch to shut your bike down as part of your normal shut-off procedure is perfectly OK. It’s a misconception by many riders that doing so long term will damage the battery or wiring. It’s not so, as in most cases, the kill switch and ignition switch are wired in parallel, meaning they are doing the same job.
Many bike handbooks state you should only use the kill switch in an emergency or electrical damage may occur. They are being diplomatic here, they are alluding to a couple of common human error situations:
- Forgetting to turn ignition off after hitting the kill switch causes the battery to run down.
- Forgetting to turn the kill switch back on when restarting, causes the rider to crank and crank, potentially running down the battery, cooking the starter motor and or flooding the engine with gas.
How Does Kill Switch Work
Kill switch’s wiring differs between all manufacturers and how they are wired also depends upon how your bike’s ignition switch is controlled. Most older bikes are the traditional black CDI box, but some newer models are computer-controlled.
The kill switch for either type is handled a little differently. I’ve outlined the two common ways for each type below, but to fault find a kill switch correctly a wiring diagram is the best place to start.
We’ll for now identify the two different types as CDI type and ECU (Engine Control Unit) type.
How the Basic CDI Box Type works
The kill switch for most bikes is a very simple on/off switch. When the kill switch is activated (switch in Kill position), it shorts out the voltage coming from the stator to the CDI box. The short (shorter path to ground) prevents power flow from the stator to the CDI and consequently to the coil.
The coil as you know makes the voltage for the spark plug, no spark equals no fun.
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 (if battery and starter fitted)
- Kill switch “RUN” – removes the ground path (and secondary ground path, if ignition switch fitted) 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 directly to the starter motor and cranks the engine (if fitted)
- Stator – creates voltage and powers the CDI box as the engine cranks.
- Pickup aka Trigger or crank sensor – creates a voltage, used by CDI box to identify the correct time to fire the plug (timing).
- CDI box – processes the trigger signals and using Capacitor, Diodes, and an SCR, it controls the coil voltage
- Coil – produces high voltage through primary and secondary winding’s which is converted to a spark inside the combustion chamber by the spark plug.
How the modern ECU controlled type works
The more sophisticated ECU-type bike use sensors and pre-programmed code to control ignition timing and fuelling. Their kill switches typically work like this but as said earlier, different manufacturers will handle it differently.
- Kill switch “Run” – Initiates system by providing a ground path for the ECU. Unlike the CDI type, the grounding of the kill switch occurs after the load (ECU).
- Clutch/Neutral switch activated – Signals the gear position to the ECU
- Stator – Creates voltage box as the engine cranks (kicked over) which powers the ECU and coil via the rectifier/regulator.
- Pickup aka Trigger or Crank sensor – Creates a voltage, used by ECU to identify the correct time to fire the plug (timing).
- Coil – Produces high voltage through primary and secondary winding’s which is converted to a spark inside the combustion chamber by the spark plug
Can You Delete a Kill Switch?
You can remove a kill switch, but doing so isn’t advised. For a basic type as per the diagram above, removing or disconnecting the ground allows the bike to start, that’s assuming all else is operating correctly.
That said some basic types may be wired in reverse, meaning they require a ground to start. A wiring diagram is required to identify a kill switch circuit correctly.
But yes it is a pretty simple procedure to remove a kill switch, either removing a ground or adding a ground usually does the job.
Points to note:
- It’s is mandatory to have a kill switch fitted to a motorcycle, for road use, and parks and recreation areas may not permit motorcycles without an operating kill switch.
- Insurance companies may void insurance cover and reject claims if a kill switch is deleted.
- In the event of an accident, injury caused by a deleted could result in a prosecution.
You may find the following posts useful: