I’ve seen and dealt with a fair share of kill switches in the 19 years I’ve been riding motorcycles and driving trains (I became a train driver in 2014).
This includes testing kill switches, activating kill switches in emergencies, and deactivating already activated kill switches too.
>> If you’re trying to install a kill switch, then jump straight to our motorcycle kill switch installation guide.
Precisely, What is A Motorcycle kill switch?
A kill switch (also known as emergency stop (E-stop), emergency off (EMO) or emergency power off (EPO), is a safety mechanism used to immediately shut off a motorcycle’s engine in an emergency.
Because of the purpose a kill switch serves, motorcycle manufacturers install them in places where they can be easily accessed (i.e., handlebars, under the seat with a yellow or red button/switch) and activated (usually via an on/off switch) by riders or bystanders to quickly shut off the engine.
Generally, there are four types of kill switches commonly used for motorbikes, which include:
- Ignition wire kill switches (most common type)
- Fuel pump cut-off switches
- Motorcycle battery disconnect switches
- And, Fuse box kill switches
But, How exactly Do Motorcycle Kill Switches Work?
Kill switches (specifically in motorcycles) work by short-circuiting (or disrupting) the current passing through an engine’s ignition system (i.e., ignition coil, CDI, Spark plug, battery) to prevent the spark plug from creating the spark that is needed (for the power stroke) to ignite the fuel/air mixture contained within the combustion chamber.
It’s important to note that kill switches are not just limited to ignition systems, as they can also be wired into fuel systems (to starve the engine of fuel and cause the engine to shut off), or any other electrical system that’s essential (i.e., needed) for the engine to run.
What Engine Components Are Affected When The Kill Switch Is Activated?
Motorcycle ignition systems commonly comprise a battery, coil pack, spark plugs (including its associated wires), pickup coils or crank position sensors, and capacitor discharge ignition boxes (i.e., CDI units).
The specific engine components that are affected by an activated kill switch include:
- Ignition Coil: Activated kill switches will stop power to the ignition coil
- HT leads/cables: This cable is what runs from the ignition coil to the spark plug cap
- Spark plug cap + Spark plug: Attached to the end of the HT cable and will stop working when no power is being supplied from the ignition coil
- CDI box: Power to or from the CDI (capacitor discharge ignition box) will be either disrupted or short-circuited
But it doesn’t stop there, as the engine’s cycle (i.e., strokes) is also affected during this process too.
Let me explain…
How does A Motorcycle’s Engine cycle Relate To An Activated kill switch?
The four-stroke cycle in a combustion engine is made up of an intake stroke(1), compression stroke(2), power stroke(3), and exhaust stroke(4).
Conversely, a two-stroke engine’s cycle comprises just a compression stroke (1), and a power stroke (2).
The design of a two-stroke engine allows for air and fuel to be simultaneously taken in (via the transfer port) and pushed out (via the exhaust port) using the compression and suction (i.e., the vacuum) created (within the combustion chamber) from the piston is moving up to ‘top dead centre, and then down to ‘bottom dead centre.
Doing so replaces the need for ‘intake valves’, ‘exhaust valves’ and their associated strokes.
Note: The image above explains how all of this works.
>> Learn more about TDC (top dead centre) and BDC (bottom dead centre) here.
But How Is A Power Stroke Related to A Kill Switch being Activated?
In normal conditions (of the combustion engine’s cycle), a spark (from the spark plug) is required to ignite the compressed fuel and air mix contained in the combustion chamber (also understood as the ‘power stroke’).
Once the spark plug… well… sparks… combustion takes place!
The combusting hydrocarbons (i.e, burning fuel and air) create pressure (i.e., expansion within the cylinder) that forces down the piston (including the piston arm), resulting in an output of power (from the engine).
Here’s an image that will give you an insight into how this works!
Conversely, with no air and fuel being ignited, there is no combustion. And with no combustion, there is no power… or better said, no engine operation (i.e., causing the engine to shut off).
so, Based On the Above, Would It be Bad To Crank an Engine With An Activated Kill switch?
Cranking an engine (i.e., trying to fire up the bike) with the kill switch activated is essentially forcing the engine to draw in additional air and fuel (via the combustion chamber) that cannot be ignited (as an activated kill switch will prevent the spark plugs ignition).
The problem with this is that even when the kill switch has been de-activated, an excess (i.e., rich mixture) of air and fuel will likely remain within the chamber, and result in a breach (i.e., “excess”) of the fuel’s upper explosive limit.
This will result in an imbalance of the ratio needed for fuel and air to be combusted within the engine, and in turn, raise the threshold on the amount of spark needed for an explosion.
The result… You can crank the engine until your heart’s content, but to no real success, as the bike won’t start… or at the very least will take a much longer time to start until the excess fuel has been removed from the chamber.
Is There A Difference In Using The Key Vs Using The Kill Switch (To Shut Off An Engine)?
Two words, “damage control”.
Using the key ignition to shut down a motorcycle ensures that all mechanical and electrical components are turned off in a specially timed order (set by manufacturers) to prevent damage.
A kill switch, however, in most cases will only shut off current to the CDI, ignition coil and spark plug. And may result in unassociated electrical equipment such as lights, fuel pumps and beepers being still left on (as these components will still be fed current from the ’12-volt’ battery).
This design constraint (i.e., disadvantage) allows for the engine to be shut off in an emergency, but will not guarantee the prevention of damage to its associated components.
Nevertheless, there’s no need to shy away from using the kill switch, as many modern bikes come with this safety feature fitted (by manufacturers) as standard.
And, like with anything, the more you use it (i.e., the kill switch), the higher the chances of it becoming defective and needing replacement, which is likely a problem that you do not want to have in an emergency.
When Is Activating The kill switch more Appropriate than Using The key?
Emergency and theft prevention are the two most common reasons why you would operate the kill switch instead of using the key.
However, with that being said, there are also scenarios where having a kill switch at your disposal could be life-changing… This includes:
A Stuck Open (Or jammed-in) Throttle
I’ve had my fair share of throttles being stuck open… and they are NOT fun. This problem can occur while popping out for a ride, or even worse when popping a wheelie, and there’s no real timing to when this problem pops up (“no popping pun intended”).
This could be the result of a snapped throttle cable, jammed-in or stuck open carburettor throttle valve (common on older bikes), debris stuck under the throttle grip (causing it to stick, preventing rollback), or even… a broken gear selector after a crash.
So how do you reduce the possibility of this happening to you?
Well, I’m a strong believer that the best type of cure is “prevention”, which is why proper motorcycle maintenance such as regularly cleaning (i.e., removing debris from) grippers, lubricating handlebar shafts (including the throttle slide), and inspecting throttle cables (to ensure there is no fraying) are all vital processes for the upkeep of your bike.
My advice would be to set up a regular notification (with something like Google calendar) to remind you of when (i.e., every 3 months or so) to give the bike a once over.
Pro Tip: Installing handguards (link to Amazon) is a great way to reduce the amount of debris getting under your grippers.
However, when a part is damaged beyond repair it should be immediately replaced.
In the Event Of a Fire
Ever had your battery catch on fire?… I have! And the unfortunate reality is that this happens far more frequently than manufacturers like to advertise.
And if you’ve ever experienced this problem, then you will likely remember the smell (of a skunk’s backside) and the sound (somewhat like sizzling bacon) which both ring alarm bells that you’re in extreme levels of danger.
But, while batteries are the common cause, there are also many other ways that a bike can catch fire, which include:
- After having a crash
- Faulty wiring
- Decomposed fuel lines
- Broken seals
- Engine overheating
- Design flaws, you name it.
And that’s just scraping the surface!
Regardless, a kill switch could be your shining knight in armour when trying to shut off a fuel pump, battery or ignition coil in flames.
Fuel leaks are another common cause of motorcycle fires, and are usually the result of leaky carburettors, damaged fuel lines (i.e., fuel pipes), or damaged and broken fuel tanks (including nozzles).
Even the simple drop of a bike (which is likely common for new riders) could be all it takes to damage a component in the fuel system. This leads me to conclude that if you ever get a strong smell of fuel, then immediately disconnect the battery, locate the source of the leak, and fix it! Pronto!
And of course, if you ride a two-stroke, then you’ll have to read between the lines. As two-stroke engines run a much richer mix that causes them to spit out an excess of unburnt fuel from the exhaust.
With close to forty thousand motorcycles being stolen in the UK each year, it’s probably a wise idea to increase the security of your motorcycle. But the unfortunate truth is that even with a kill switch installed, it’s likely that thieves will simply lift the bike and take it away (as I’ve had multiple motorbikes stolen just like this in the past).
Regardless, installing a kill switch or two is still a great way to reduce the chances of becoming a victim, and will also increase the chances of you getting the bike back in decent condition. Because with a GPS tracker and kill switch installed, you may well get to the bike before the thieves can start it up.
Discover if your bike is commonly stolen!
I found some really cool data that the Met Police have shared which reveals the total amount of motorcycles (and their specific makes and models) that were stolen in 2021.
You can use this data to find out if the bike you own is a common stone motorcycle and take the right steps in protecting yourself earlier.
>> Met Police’s recorded data for the number of stolen motorcycles in 2021
A Quick Guide For Installing A Motorcycle Kill Switch
Installing a kill switch involves splicing into the wires associated with the engine’s ignition coil and CDI unit (although I’ve seen people do this in varying ways, as the goal is to simply ground, i.e., earth, or break any circuit that will stop the engine).
The perfect place to enter would be (via the writing loom) between the ignition coil and the CDI unit/box. But, if your bike has a different type of circuit, then you can simply splice into the black wire that provides power to the ignition coil (note, this is NOT the thick HT cable between the spark plug cap and ignition coil, as that would be the wrong wire).
Next, splice the black wire from the kill switch’s loom with the black wire in the ignition loom. Finally, ground the other wire from the ‘kill switch’ on any large section of the motorcycle’s frame using a ring terminal (link to Amazon).
To finish up, put everything back together, test out your newly installed circuit to ensure it works and Bobs your uncle! Job done!
How do You Shut Off A Motorcycles Engine when the Kill Switch Or Key Ignition Is Not Available?
Important disclaimer (Please Read)
The methods I share in the following sections are not recommended by manufacturers (including regulators) and are not official (by any standards).
Instead, these are methods that I’ve discovered from messing with my bikes in the back garden (and may be suitable to use in particular situations).
Essentially, I’m arming you with the knowledge, but what you choose to do with it is none of my beeswax.
Sidestand/Kicktsand Kill switch
Sidenote: Harley Davidsons are not fitted with this mechanism
I’m laughing as I write this because there are countless times I have come across riders left scratching their heads when trying to figure out why their bike will not start or move when stationary.
Only to realise that the kickstand is still down and preventing the engine’s ignition.
Kickstand switches are designed to cut the engine (by shorting the ignition coil) when the kickstand is down. The way they work is by opening (i.e., breaking) the circuit when the kickstand is down, and closing (i.e., completing) the circuit when the kickstand is up.
Keeping the above in mind could be your get-out-of-jail-free card in the event you experience a bike not starting up or moving in gear, not starting when stationary, or on fire with no kill-switch or key operation available.
Just kick out the stand to cut it out!
Note: There are ways to bypass this circuit in the event you suspect the kickstand killswitch is causing a problem with starting or riding your motorcycle.
Unplug the HT cables and spark plug cap
This is a method you should not apply unless absolutely necessary. However, unplugging the spark plug cap will shut off the engine.
Here’s what it looks like:
Just be sure to use an electrically insulated tool (i.e., an insulated screwdriver) to pop it off!
Blocking Up The Exhaust
As we know, the operation of a combustion engine requires a successful combination of air, fuel, ignition and exhaust. This means, that in theory, if we remove one of the four operations from the combustion cycle, the engine should be forced into shut down.
I’ve personally tried this! and it works!
Blocking the exhaust up is NO EASY job, it’s scary (as you’re unsure of what’s going to happen next), smelly, dirty, and difficult, as you’re trying to contain the exhaust back pressure.
This method takes a good 5 to 10 seconds (and a firm amount of pressure) to shut off the engine and can be achieved by using a jumper, dirty rag or any type of material that’s heatproof with a surface area large enough to cover up the hole on the exhaust.
This method should only be used this method when you have exhausted all other options (no pun intended).
Blocking air supply to the Carb (for older Designs with carb & Choke)
Just like blocking the exhaust, restricting air supply to the carburettor (and combustion chamber) will cause the engine to shut off.
Fortunately, older bikes that are manufactured with carburettors are also likely to be manufactured with chokes.
Wait, What the heck is A choke?
A choke is a small two-step mechanism fitted to an engine that allows you to control the amount of air (via a valve) entering the carburettors fuel and air mixture (being mixed in the venturi).
Less air equates to running a rich mix (i.e. an increased amount of fuel within the mixture) while more air is commonly referred to as running a lean mix (i.e. a higher amount of air but less fuel within the mixture).
Manipulating airflow intake is particularly useful to use in scenarios such as engine flooding, cold starts, riding in unusual climates and much more.
If your bike has been fitted with a choke! Happy days…Use it! But if it hasn’t, then you can pull off the air filter (usually mounted to the carburettor) and block off the air supply with a piece of material large enough to cover the hole.
That being said, I would NOT recommend the latter in an emergency, but if you have no other option, then this could be the best bet.
What happens If you Accidentally hit the kill switch while riding?
Hitting the switch while riding will either cause a lack of throttle response (on manual gearboxes) or sudden engine shut-off (on automatic gearboxes).
Huh! Wait… why does the kill switch not immediately shut off a manual engine?
The answer is simple. Even when you “kill power” to the ignition, continuous power (i.e., the speed at which you are travelling) continues to pass from the rear wheel into the gearbox (via the chain), and then into the engine (via the clutch).
For the purpose of this example, I would like you to picture the clutch as a bridge between the gearbox and the engine.
This means when in gear (i.e., bridged from the gearbox to the engine), an increase of speed in the gearbox will reflect as an increase of speed in the engine.
>> Learn more about how engine RPMs and gearboxes work here.
Conversely, when the clutch is disengaged, the engine no longer receives power from the gearbox (to run collectively), and cannot run independently (because there is no ignition) resulting in an engine shut-off.
To explain this further, I have labelled the image below with notes to comprehend this process.
Can you restart the engine While riding After the Kill Switch Has been Activated?
Starting an engine up after activating the kill switch is easy, regardless of if you are stationary or moving at speed.
Simply deactivate the kill switch (i.e., switch it off), operate the ignition (turn the key or push the button) to start the bike and continue riding.
Does using a Kill switch impact the health of An engine?
I’m in agreement with the motorcycle manufacturers on this one.
I would recommend that you do not use the kill switch frequently because of the problems that come as a byproduct of the kill switch being activated.
A kill switch left activated can drain the 12v battery (and not properly shut down the entire bike) of a motorcycle and can cause engine flooding in the event that you attempt to crank the engine with no ignition.
Over time, this will most certainly reduce the health of your engine.
Frequently Asked Questions
Got questions about kill switches? Great! We’ve got answers.
Here are a few common questions that I picked up when researching this post.
Kill switches can be traced back to as far as the 1880s (formally referred to as ‘the deadman handle’ and used to operate vehicles or identify if an operator becomes incapacitated). They are great for stopping theft and shutting off engines in emergencies.
A majority of modern motorcycles come with kill switches manufactured as standard, but there is truly no limit to the number of kill switches that can be fitted to a motorcycle.
The operation of a kill switch is simple, they either break or disrupt or complete the circuit they’re wired to (whether that be the ignition coil, 12v battery supply, fuel pump, or fuse box).