Does A Motorcycle Coil Have To Be Grounded?


Motorcycles have a lot of parts that help them run efficiently. Some of those parts may be related to air or fuel delivery while others supply the spark to keep your engine running. The motorcycle coil maintains performance and reliability. 

Does a motorcycle coil have to be grounded? The coil itself does not have to be electrically grounded to the frame of your motorcycle. The current generated in the coil passes through the spark plug wires and is supplied a ground to the engine through the spark plugs. So the spark plug is grounded, but not the coil itself.

It can be incredibly overwhelming when it comes to electrical components on a motorcycle.  Trust me, I’ve been there and understand the mess it can leave you in.  This article will help you understand why grounding a motorcycle coil is important and what you can do if you find yourself with coil grounding issues.

Why A Motorcycle Ignition System Has To Be Grounded

The motorcycle coil is what’s used to supply enough power to your spark plugs and/or wires to ignite the fuel inside of the motorcycle engine. This means it amplifies the power sent to it to give enough power to keep the motorcycle running. 

To understand why a motorcycle coil needs to be grounded through the spark plugs, we need to understand how electricity works. Your ground is a part of a circuit. A circuit is a complete pathway that electricity can follow. If the circuit is incomplete or missing an important part, it can stop the electricity from arriving at its desired destination or even send it somewhere else. 

The coil on your motorcycle is a part of an electrical circuit and it receives power from your battery/ignition system. In order for it to use this power given to it, it needs to have a complete circuit. If the motorcycle coil is grounded through the spark plug it should work, as long as the windings inside the coil have not failed internally. 

Once this circuit is complete, the electricity flows from the battery to the motorcycle coil to send power to ignite the spark plugs. This can be very helpful to know if you do not have spark coming from your spark plugs. It doesn’t always mean the coil is bad though; it could mean that your spark plug is not grounded properly. 

If the motorcycle coil is not grounded through the spark plug properly this means you will not have spark coming from your spark plugs. You may notice that your motorcycle has a hard time starting or won’t start at all if it has a faulty ground. 

If you suspect your motorcycle ignition system is not grounded correctly and you’re not getting spark to one or multiple cylinders there are tests I perform to figure out which part is at fault.

  • Use a multimeter and perform a resistance test on both ends of your spark plug wire. Check the manufacturer’s recommended resistance levels. If your wire has a much higher resistance than it should then replace it.
  • Unscrew one spark plug at a time and then connect it back to the spark plug wire. Hold the sparking end of the spark plug against the engine block then crank the engine. If you see a spark that’s a good sign. If there is no spark then you have an electrical problem upstream of the spark plug. If it’s a very weak spark then buy a spark testing tool (they’re very cheap) and see if the spark is jumping the appropriate gap. There’s lots of videos on Youtube showing how to do this.
  • Look for any wires around your coil that could be disconnected, burnt, cut and rubbing against the frame, or seem really hot to the touch.
  • Check your fuse box and make sure the ignition fuse is not blown (this one has stumped me for days before).

Electrical problems can be hard to track down on the side of the road and could take hours of testing if you don’t know what to look for. 

A motorcycle coil that is not grounded through the spark plug correctly could also cause an intermittent crank no-start issue. This means it only turns on sometimes and other times it may only crank. This means there is a ground that could be incomplete only part of the time. This type of condition can cause the motorcycle coil to prematurely fail (we will revisit this later in the article as well). The usual cause of these symptoms could be a bad coil, a bad spark plug wire, or the spark plug wire is just loose.

While motorcycle coils can be different from one motorcycle to the next, they all function in a very similar way. No matter the type of motorcycle, issues will arise if the coil system isn’t grounded properly. 

Where A Motorcycle Coil Needs To Be Grounded 

Motorcycles need to look great, or they are a pretty hard sell. Could you imagine if the wires on your motorcycle were always exposed? They wouldn’t be very attractive let alone incredibly dangerous. Though maybe there is some kind of wiring Picasso that can make it look really awesome. Most motorcycle manufacturers will tuck wires in places where they cannot be seen by the rider to ensure a sleek look while maintaining safety for the rider.

The motorcycle coil that is on every bike has a few wires that connect to it. One is a power wire and the other is a signal wire. The power side comes from the battery. Think of the battery as a big city with a bunch of electric particles living in it that really need to leave to go to work. The ground side of the coil is completed at the end of the spark plug, this path is what allows them to go to work as it completes the path for the electricity to follow. The signal wire is what tells the electricity when to leave the coil.

The ground on a motorcycle is a spot that is not interfered with by other power sources. This means that you cannot connect power to power. You have to have something separate like the frame of your motorcycle to complete the circuit. The frame and engine block is where most motorcycles will have their grounds. 

The motorcycle coil is grounded where the threaded portion of the spark plug is connected to the engine block. The engine block and the frame are both grounded together, and connected to the negative terminal of the battery, so all grounding points should be grounded together. If you’re ever wondering if a certain wire is grounded then either get a wiring diagram and follow the lines or visually follow the line on the bike to see if it attaches to the frame or the engine block. If it does then it’s a ground wire. 

If you can find your motorcycle coil (almost allows located under the tank) then it should be easy to find which wire is the positive inlet wire, the signal wire, and the spark plug wire. Hint, the spark plug wire is the big fat one. Most leading manufacturers will have their positive wires yellow or orange or red.  Be sure to check in your owner’s manual to confirm what color the positive and signal wires are on your motorcycle. If you don’t have your owner’s manual, you can find yours here on eManualonline.com. You can then follow that wire to wherever it connects on the motorcycle. 

If you do have bolts that hold your ignition coil, or coils, tight to the frame this is generally not a grounding bolt, but just a way to hold the coils secure. Sometimes manufacturers also use the coil-to-frame connection as a heat sink mechanism. Coils get hot as they operate, and if they are connected to the frame it can help dissipate that heat faster. Your owner’s manual will also be able to tell you where the coil is (or at least was) from the manufacturer.

While a motorcycle coil problem can be annoying and seemingly difficult to fix, there are some key things to look for if you are having this issue. 

Symptoms Of A Coil Not Grounded Correctly

A motorcycle coil system that is not grounded correctly can cause multiple things to happen. One of the first ways an improperly grounded ignition system will manifest itself is by your motorcycle cranking but does not start. This is a sign the coil circuit is not complete, because you may be getting fuel and air but if there is no spark then the engine will not run.

Another symptom could be that your motorcycle engine turns off at random. This could be a sign of a bad coil, bad signal wire, or a problem with your stator. Again, without spark your engine will not run, even if it loses the spark while it was running. 

Something else that may signal a bad motorcycle coil is a misfire. This is when you may have a motorcycle coil for each cylinder. Some older bikes might only have one motorcycle coil total while newer ones may have one on each cylinder. If only one of the motorcycle coils has a problem then it will cause a no-spark condition on one cylinder. In this case the motorcycle will still run but it will run poorly.

Black smoke coming out of the exhaust could also be a sign of a bad motorcycle coil ground. If the fuel and air are mixed in the cylinder and do not ignite with the spark, that unburned fuel will go out of the exhaust. This will be burned in the exhaust causing black smoke to come out. 

Can It Cause Permanent Damage If A Coil System Is Not Grounded Correctly?

Yes, if the spark plug on a motorcycle is not threaded in or the spark plug wire is not connected it can cause damage to important parts of your motorcycle. If that current can’t dissipate through the spark plug it’s going to try and find somewhere else to dissipate to, like your nearby hands. Heat will also continue to build inside the coil. This is not good for your coils or your spark plug wires. 

The wires can get hot and melt if the ground through the spark plug is intermittent as this could possibly cause more resistance in the circuit. Electricity will always take the path of least resistance, but if the only path it has is high resistance it can burn up components on your motorcycle.

If you aren’t igniting the fuel in your motorcycle on one cylinder it can create carbon deposits on your valves, which could affect your motorcycle’s engine compression. If those valves cannot seal because of carbon build-up it can cause a misfire as well. 

In some cases, if the ground is bad on your motorcycle coil system it can be taxing to find the problem and mitigate it.

It is important to know where your motorcycle coil is beforehand so that you can rectify any potential issues quickly if you break down on the side of the road while riding. Be on the watch for the symptoms of a bad motorcycle coil. Ride safe, and have fun!

This article has been reviewed in accordance with our editorial policy.

Kyle Cannon

Kyle currently works as a mechanical engineer and graduated with a minor in automotive engineering. He loves restoring motorcycles, has a vast knowledge of how they work, and has sold his restoration projects to customers from all over the United States.

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