A kill switch is important kit. It’s not just a convenient way to kill the motor. It could be a lifesaver too. I’m a mechanic and shortly you’ll understand how most dirt bike kill switches work.
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 to fix it.
What’s a Dirt Bike Kill Switch For?
Fumbling for an ignition switch with a stuck wide open throttle wearing riding gloves, while exciting, isn’t very practical. Of course, most dirt bikes won’t have an ignition switch fitted, however, some road-legal models may. If your bike doesn’t have a key and ignition, just ignore all references to it, either way, the ignition wiring circuit works the same.
The kill switch is designed for ease of use in case of an emergency. The rider can access the kill switch without removing their hand from the handlebars.
Emergency services are trained to locate the Bright red button when attending an accident.
Can You Use 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 (If fitted).
However, manufacturers don’t recommend using the kill switch to shut down your bike 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.
Dirt Bike Kill Switch CDI Or ECU?
It must be said not all dirt bike kill switches are wired the same or operate in the same fashion. Some kill switches initiate a kill by providing a ground and completing a circuit, some remove ground and render the circuit open and other more sophisticated systems may use a power supply to signal kill.
It depends on the bike and what system is employed. Modern bikes may have an ECU (Engine Control Unit) which means the engine fueling and ignition system is controlled by a sophisticated computer, not unlike a car.
Older bikes will likely be fitted with the less sophisticated CDI box, they are also referred to as the black box, module, brain, etc. Each system handles the kill switch wiring differently, indeed each manufacturer handles their wiring differently no matter if they are CDI or ECU controlled.
In this post, we’ll cover the kill switch on a basic CDI circuit and also an ECU circuit. The circuit diagrams here are for illustration purposes only. To troubleshoot a faulty kill circuit you’ll need a model-specific wiring diagram to nail it successfully.
Older CDI bikes work like this:
Many basic CDI systems use a kill switch that when activated, shorts out the CDI power feed and others may short out the ignition coil feed itself. Either way, activating the kill switch provides a shorter ground path for the CDIs, effectively shorting it out and shutting down the engine. It’s a very simple setup.
The ignition coil, as you know, supplies the high voltage needed to fire the spark plug. No spark equals no fun.
And so to start a dirt bike with a basic CDI system we must reverse the process, ie remove the short to ground. To start a dirt bike, the ignition must be set to”On” (if fitted) 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. (your bike may not have an ignition switch)
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 (opens the circuit) 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 (if equiped with starter motor)
The switch in the diagram below is set to run, closing the circuit, kills the engine.
- Stator – this is where the stroy begins, because it creates the voltage needed to make it all happen. It powers the CDI box as the engine cranks (also charges battery via rectifier/regulator if fitted)
- Crank sensor also known as the Pickup – creates a small voltage, used by CDI box to identify the correct time to fire the plug (timing)
- CDI box – processes the crankshaft signals and employing a Capacitor, Diodes, and an SCR, the timed voltage is sent to the coil
- Coil – produces high voltage by sending power through primary and secondary winding which is then sent down the plug wire where it arrives at the spark plug
- Spark plug – the final destination, the business end if you like. The spark plug provides an almost perfect path for the coil’s power.
It’s the imperfect path that makes the spark possible. As you know the coil’s voltage is always searching for a path to ground, recognizing an opportunity – it jumps the plug electrode gap and a spark is born. It’s a beautiful story.
Modern ECU bikes work like this:
Modern bikes work a little differently from CDI systems. The general idea is the same, but in many cases, the kill switch will be signaling the ECU to shut down the engine as opposed to a CDI system, where we make the physical change to the circuit directly – open or close a circuit.
ECUs allow for this more sophisticated type of system because they are programmed using computer code. And it allows for easy programming of fail-safe systems like kill the engine if the ECU detects the bike has fallen, don’t allow a start if the kickstand is down (if fitted), don’t allow start if Neutral not selected, etc.
As with the CDI dirt bike, bikes equipped with the starter motor, battery, and road-going dirt bikes with ignition switches will add complexity to the starting circuit. And as before a wiring diagram is essential to troubleshoot successfully.
Here’s the circuitry for a modern ECU bike with a brief explanation below of what’s going on. Remember not all ECU bikes will handle it this way.
In the above diagram, the kill switch is set to run which provides the ECU with a ground supply. Power, therefore, flows from the stator via the rectifier which provides power for both the ECU and the coil. The ECU receives an input signal from the pickup which as you know is triggered by the rotating crankshaft. The ECU uses the pickup input signals to control coil activation.
Additional inputs such as neutral switch, kickstand switch, ECU lean angle sensor (if applicable) may affect coil activation, depending on ECU programming and equipment installed.
Moving the switch to “kill” removes the ground and shuts down the flow of power.
Common Dirt Bike Kill Switch Problems
Generally, kill switches are pretty robust and don’t cause too many problems. Diagnoses of the kill switch are 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 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 column common)
- Wiring short to chassis ground
The symptoms of some of the above will vary, for some bikes, they may fail to shut off as opposed to failing to start, it depends on how the switch is wired.
Testing Dirt Bike Switch
This is a simple switch and checking it simply too. You can use a DVOM or a test light. I prefer the test light method, but only possible for battery-equipped bikes. I’ve listed both here on the ATV tools page.
We already know when the switch is set to run, the path across the switch for most bikes should be open (broken). We also know that when the switch is set to kill, the ground path across the switch is closed (connected). So with that knowledge, let’s go ahead and check the switch.
Note your switch may operate in reverse, meaning closed circuit for “run” and open circuit for “kill”. With that in mind, makes sense to focus on a change in meter readings when the switch is operated.
Using a Voltmeter
Let’s go ahead and test them now using a voltmeter first and then a simple test light. In this test, we’ll be checking resistance in the switch.
- Using a DVOM set to ohms, gain access to the wiring loom feeding the switch. If there’s a block connector, and there will be, disconnect it. Never check resistance on a live circuit, the circuit must be isolated first.
- Check the connector for loose or corroded pins
- Back probe either side of the pins and check for continuity when the kill switch is set to “Kill”, the value for most bikes should be 00.8 (continuity). (Some may read “1”, that could mean it’s faulty but could also mean your switch operates in reverse. What’s important here is that the resistance value actually changes when the switch is moved between run and kill. That’s what we’ll do next.
- Flick the switch to RUN, for most bikes the meter should read OL (Open Loop) or 1. Both readings indicate an open circuit which is correct for most basic CDI type bikes.
Using a Test Light
A test light is in many ways a better test. First off we can test the circuit live and that allows us to load the circuit. Checking resistance is Ok, but it doesn’t always show a fault. Loading a circuit with a test light will help show up partial resistance caused by broken wires, dirty or loose connectors.
A dim test might then let us know we have a problem. Of course, this test will only suit bikes with a battery. Here’s how I’d go about testing with a test light:
- 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).
- Locate the kill switch block connector and disconnect it.
- Slide the switch to “Kill”, if your bikes kill switch is a basic ground side then the test light will light up.
- Sliding the switch to “Run” should turn the light out.
If you find your kill switch operates in reverse, that’s OK. As said, what’s important is that the two tests “Run” and “Kill” give a different result.
That’s it, you are a pro!
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