However strange it may seem, having problems shutting off the engine on an ATV is more common than you may think. The bike may run as usual, but when you turn off the key or hit the kill switch, the engine keeps running. What may be causing this strange and potentially dangerous behavior?

When faced with an issue like this, the problem is more often than not electrical. To support this presumption, let’s have a look at what the ignition key or kill switch does to turn off the engine.

How does the ignition key or kill switch turn off the engine?

Any gasoline engine needs three components to run; fuel, air, and spark. When you take away any of these elements, the engine will shut off. 

When you turn off the ignition key or hit the killswitch, the ATV should take away the spark. The spark plug will no longer ignite the fuel, effectively shutting off the engine.

The Ignition key and killswitch are both parts of an electric circuit that, when active, sends an electric signal to the CDI or ECU, telling them to power the coil that then sends a high voltage charge to the spark plug. 

When you turn off the key or press the kill switch, it is no longer supposed to send a signal to the CDI or ECU. In a case where the engine won’t shut off, there is usually an issue somewhere in this circuit that causes an electric signal to be sent regardless of the switches’ position. 

Here are the basic principles of how the ignition key stops the ATV and what components are involved in the circuit. 

Please note that these are general guidelines. Your ATV may be wired differently. Your user manual’s wiring diagram will always be your best source of learning how your bike is built. 

  • The ignition key gets power from the battery and from the stator through a voltage regulator. The positive side of the ignition key is always hot. 
  • The ignition key is wired to the ECU or CDI. The wire you are looking for is often black with white or yellow tracing but refer to the wiring diagram in your service manual as your ATV may be wired differently. 
  • When the ignition key is on, the ECU or CDI gets an electrical signal, telling it to power the ignition coil, which is powering the spark plugs.
  • When the ignition switch is off, the electric signal is directed to ground, telling the ECU or CDI to stop powering the ignition coil. On modern ATVs, with electronic fuel injectors, the ECU will also tell the fuel pump and fuel injectors to stop sending fuel into the combustion chamber.
  • The kill switch is connected in parallel with the wires from the ignition switch. When the kill switch is off, it will send the signal to ground, regardless of the ignition switch position.

Focus your troubleshooting on these components, and you have a good chance of finding the cause of your issue. 

Look up an exploded parts diagram on a site like partzillla.com. This will help you get an overview of what the different components look like and where they are located on your machine,

Recommended troubleshooting tools and resources

In some cases, you will be able to identify the problem without any tools. Still, to make troubleshooting a bit easier, it is recommended that you have the following tools and resources at hand:

  • Multimeter
  • Service manual for your ATV model with a wiring diagram.

Now, let’s begin troubleshooting by having a closer look at typical issues that may prevent your ATV from shutting off the engine.

Make sure the ground wires are correctly connected

It’s a good idea to start your troubleshooting by inspecting the ground wires and connectors. They are relatively easily accessible, easy to repair, and do not require expensive spare parts.

When the grounding in the ignition system is bad, the ignition switch or killswitch aren’t able to short the signal to the CDI to ground as it should. 

Start by your ignition switch and trace the ground wires via the kill switch, the CDI, and the ignition coil. 

Make sure all connectors are correctly fastened to their grounding spots and have proper contact with bare metal. If you notice any corrosion sign, disconnect the wire and clean both the connector and connection point with some sandpaper to ensure a solid connection. 

Please make sure no pins in the connector plugs are loose, causing them to back out when you connect the plugs. It can be a bit hard to see whether the pins are in place or not. By gently pulling on each pin from the back of the plug, you can verify that they are correctly seated.

Also, inspect the main engine ground connection as it’s a commonplace for corrosion, causing a bad ground.

You can use a multimeter to verify that there is ground present at both the ignition, kill switch, and the CDI box.

Another way of testing if the grounding is good is by connecting a piece of wire from a bare spot on the frame and directly to the ignition or kill switch Caution! This tip should only be performed with extreme caution as there is a chance of getting electrocuted if you are not careful. Never touch bare metal parts and use a pair of insulated pliers to hold the wire as a precaution.

Make sure the ignition wires are not damaged, causing a ground fault

If the ignition wires become pinched or otherwise get damaged so that a hot wire comes in contact with either a ground cable or a grounded portion of the ATV, you will have a so-called ground fault. 

This may prevent the ground cables from shorting to ground as they should. 

A previous owner may have tapped into the wiring to install accessories but didn’t do a proper job either when installing or when removing the accessory before selling the ATV. It’s quite a common mistake just to wrap a couple of rounds of electrical tape around a hot wire or connector. While this may work as a short term fix, the tape may eventually come loose due to dirt, water, and vibrations. 

Trace the wires coming from the ignition switch and kill switch to see if there is any visible sign of damage or tinkering, such as damaged insulation, DIY electrical tape patch jobs, or burnt wire.

Repair any damaged wiring or replace the whole switch and cable assembly if you are not comfortable with your wiring skills. 

Make sure the ignition switch and kill switch is working properly

If your wiring seems to be in good condition, your next best bet will be to look at the ignition switch / key cylinder or killswitch.

An ignition switch or kill switch can fail due to internal wear or corrosion. If there is an internal short inside the switch, the CDI-wire may be hot at all times, continuously telling the CDI to run the engine.

Typical signs of a bad switch are when:

  • The ATV turns off only when you turn the key very slowly
  • The ATV turns off when the key is turned approximately 3/4th of a normal turn. 

Disassemble the switch to look for corrosion. Note that not all ignition switches can be opened without ruining it.

If you notice only a slight corrosion sign, it may be worth a shot to clean the contact points and spray the whole assembly with a proper dose of electrical cleaner. Pack the unit with di-electric grease to protect the internals from further corrosion.

If you have access to a multimeter, you can test the switch for continuity. Depending on your ATV’s wiring design, you should get a positive reading when the key is off, and no reading when it is off or vice versa. 

If all else fails, you can replace the whole switch assembly. A complete ignition switch assembly will set you back around $20 to $50. 

Make sure the ECU or CDI is not damaged 

When you are positive that the ignition wiring and switches are ok, it’s time to address the CDI or ECU/ECM module. 

The ECM on my Polaris Sportsman Xp1000 is hidden in front of the ai filter box. Some disassembly is required to reach it.

The CDI stands for Capacitor Discharge Ignition and controls the spark. 

An ECU (Engine Control Unit) or ECM (Engine Control Module) does a lot more than just controlling the spark such as controlling fans, power steering, lights, transmission, and more. 

These boxes may fail due to water exposure, heat, or old age, but they are not easy to troubleshoot. They are not something you replace unless you have to, as they can be quite expensive. 

Before you spend your hard-earned cash on a new unit, there are a couple of things you can do to look for signs of damage. 

Disconnect the connectors from the box to see if there is any sign of water or corrosion. Make sure the seals are undamaged and seated properly. If you see any sign of water getting into the unit, it’s a good chance it is failing. 

When the engine keep running a few seconds after turning off the ATV

If you shut off the ATV, but the engine keeps running for a few seconds after turning the key, you are likely experiencing a phenomenon known as “overrun” or “dieseling.” 

What happens is that the cylinder and spark plug is still hot enough to ignite any fuel that is still present, even after the ignition is turned off. 

This may happen on carbureted ATVs as there is no effective way to turn off the mechanical fuel pump and carb.

This problem may be caused by the timing being off, the bike running too lean, or not using the right spark plug. 

  • Replace the spark plug to the correct spec. 
  • Make sure the carburetor is set according to manufacturer specs.
  • Lookup a timing diagram for your specific model, pull the magneto and make sure all the timing marks line up properly. Setting the timing is a job best left to a professional if you are below average mechanically inclined.