What Are the Symptoms of a Bad Coil on an Outboard Motor?

An outboard motor will provide you with years of service if you maintain it. Without the proper boat maintenance, you could face a bad coil on an outboard motor.

The truth is, the problems are rarely caused by the coil, but that doesn’t mean you should overlook this vital component.

When your outboard motor has a bad coil, you could face trouble starting the engine, no spark at the plug and misfires. However, many ignition problems are traced back to something else, such as the plugs.

What Does a Coil Do on a Boat Motor?

The marine coil works just like the component in your vehicle. This vital part is responsible for transforming the voltage from the battery into the volts needed to create a spark at the plugs.

This spark then ignites the fuel, which keeps your motor running.

Symptoms of a Bad Coil on an Outboard Motor

If your coil does go bad, there are several symptoms you might notice.

Engine Experiences a Hard Starting Condition

When the coil becomes weakened and damaged, it is unable to produce the energy that it once did.

What you are left with is a weaker spark at the plug gap, which makes it more difficult to start the engine. If you regularly foul the spark plugs, this is one indication that your boat is suffering from a weak spark.

If the engine takes a little bit longer to fire up when turning the key and getting the engine to turn over. This is another sign of possibly a bad coil, or missing spark on one cylinder.

Cracked or Broken Coil

Like everything that is on a boat engine, the coil inside your outboard is sealed component. To run properly, the coil must remain intact and crack-free.

If you examine the coil and find it is cracked or looks melted. This is a sign that the coil has gotten, hot, shorted, or overheated at some point in time. Anytime this is noticed, you must replace the coil.

No Spark at the Spark Plug Gap

When the coil fails completely, you won’t get any more voltage to the plugs.

Because there will be a lack of energy to the plug, there will be no spark at the gap and your outboard motor will generally not start. And if it does, then it is going to be shaking or misfiring pretty badly.

Engine Misfires

Let’s say that the coil produces just enough voltage to start the engine and keep it running, but not enough to maintain the operation.

In these cases, the engine might run erratically.

You could also notice that this occurs sporadically. Maybe one day, the engine is giving smooth acceleration, but the next, it becomes rough.

This problem indicates that the outboard coil is not maintaining a steady voltage output.

How Do You Test a Coil on a Boat Motor?

While these problems show you where to look for faulty parts, the symptoms alone can’t diagnose a bad coil. Instead, follow these steps to determine if you have a bad coil on an outboard motor.

Step 1

Disconnect the wires from both the negative and positive terminals on the coil or unplug the plug going to the coil.

These terminals connect to the primary circuit, which is connected to the marine battery to create a charge.

Step 2

Disconnect the boat’s spark plug lead from the spark plug.

This spark plug lead connects to the secondary circuit, which sends an electrical charge from a coil to the spark plug. This voltage gets transported to the spark plugs, where it fires up the engine.

You don’t want to test your outboard coil with these wires connected.

Step 3

Turn on your voltmeter. Make sure it is set to OHMS, not DC or AC voltage. You want to test the resistance, not current or amps.

When working with the outboard motor, you want to turn the dial to 200. Don’t set it at 2,000 or 20,000, as these are just too high, with a good meter, you will have an auto setting. We recommend using the auto if your meter has it.

Step 4

Place a probe from your meter onto the positive terminal, and another probe on the opposite terminal to test the primary circuit.

The appropriate value of an outboard should fall between 0.02 and 0.04. If you are getting a reading outside of this range, it’s likely that the coil is faulty.

Now it’s time to test the secondary circuit. Take the negative probe, which is usually black, and place it on the negative terminal.

Take the positive (red) probe and place it on the coil tower. Your meter should read between 8 and 11. If you get a different reading, the coil is bad.

It’s also really important to note that this is a general test. As outboards advance, the coil design’s will change. So, depending on the brand, model, and style of the coil that you have.

It’s best to find the testing procedures from the engine manufacturer to ensure you are getting the proper readings for that exact style of coil.

These coils also have outboard readings that you can test as well. But the testing procedures vary depending on the brand and model, so getting the testing procedures for your engine’s coil will give you the definite results!

How Do You Know if Your Outboard Spark Plugs are Bad?

If the coil is operating as it should, your problems could be related to bad spark plugs instead.

The marine spark plugs are another vital component for a smooth operation. The plugs take the high voltage electricity from one end and turn it into a spark at the other end.

This spark ignites the fuel/air mixture inside the engine to create the combustion required to power your vessel.

To determine if your marine spark plugs are bad, you must examine each one. Inspect the plug for damage. Look at the insulator tip, upper and lower insulators, as well as the boot.

You can also look for signs of spark plug fouling. When the plug doesn’t burn fuel normally, damage is done to the plug and the engine will not start.

Normal spark plugs are dry and feature a grayish color. If the spark plug is wet, it might indicate that water has made its way into the fuel.

A white residue on the plug reveals that the plug is too hot. Additionally, a plug that appears eroded might also indicate that it is too hot.

Otherwise, you might have a black plug covered in soot, which means that the fuel contains too much oil.

If you see the ceramic of the coil broken or you find a really dark rusty or burn ring around the base of the plug. It’s time to change out the plugs.

You can also ohm out the plugs with our meter and make sure that the plug is not dead shorted to itself. If you get continuity from the tip of the plug to the metal threads of the plug, the plug is shorted and needs to be changed.

Keep in mind though that some plugs will have a specific amount of resistance here and you will get a killa-ohm reading, which for certain plugs is ok. With bad plugs, this means that you don’t have a bad coil on your outboard motor!

Other Outboard Motor Spark Problems

If you have determined that the ignition is giving you trouble, but you aren’t sure that it is a bad coil on an outboard motor, you might want to consider some other options.

Here are a few spark problems you might face and what to do about each.

No Spark

If there is no spark noticed at the plug, remove it and replace it with a new one. If you still don’t get a spark, you want to check for a faulty stop switch.

Disconnect the stop switch circuit and retest the spark output.

On a smaller pull-start outboard motor, you find the stop button and follow the two stop switch wires back to the origin. One should go to the CDI, while the other goes to the ground. Disconnect the wire leading to CDI and recheck the spark.

Check the ground wires and the electrical connections. Don’t neglect to test the terminals of the ignition system, as well as the main harness plug.

If any of the connections are loose, corroded or dirty, you might face spark problems. Thankfully, this problem simply requires that you tighten and clean the connections.

There are other reasons you might have no spark. Besides these basics, it could be related to a failed flywheel magnet, faulty flywheel sensor coils, a bad stator under the flywheel, a defective capacitor discharge ignition unit, a bad coil, a malfunctioning microcomputer or faulty capacitor discharge module.

You may even face a combination of issues, such as when the stator is faulty and causes a CDI failure.

Use a peak reading voltmeter to diagnose these problems. Using a standard voltmeter isn’t going to measure the output correctly.

Weak Spark

A strong spark is blue. If it doesn’t appear strong or looks orange, you might have a weak spark.

The output is under a load when the combustion chamber faces high pressure or high heat conditions. When this occurs, the weak spark might not ignite at all.

Weak sparks cause trouble starting, as well as running, especially because there’s not enough heat generated with the spark to accomplish a reliable ignition of the air/fuel mixture in the combustion chamber.

The most common reasons boats struggle with a weak spark turn out to be either a faulty stator or defective flywheel magnets.

Intermittent Spark

At times, there might be a spark and at other times, there is none. This signifies a real problem.

However, most of these issues can be difficult to diagnose.

Insulated copper wire can fail, leading to an open circuit, ground leak or short circuit somewhere. This most often happens when the engine is hot but returns to normal when the engine is cooled.

Additionally, the CDM and CDI have many electrical components that can fail without notice. Sometimes, these will work well one minute and fault the next.

To confidently repair this situation, you would need to replace all components, but that can prove to be an expensive option, and often unnecessary.

Sparking at the Wrong Time

If you are facing incorrect ignition timing, this can lead to piston failure. This problem is caused by a sheared flywheel key, poor timing setup, defective trigger coils or a malfunctioning CDI.

To diagnose this problem, you might need to index the flywheel.

If you notice that all of the cylinders are improperly timed, it could reveal trouble with the flywheel key.

For the best results, check the ignition timing the next time you have your outboard serviced.

Cross Firing

With a multi-cylinder engine, you might notice the spark crossfire into the wrong cylinder, which leads to backfiring.

In this situation, it’s best to inspect the plug leads and make sure they are away from one another.

You may also need to index the flywheel to determine the cause of the failure.

Most Importantly Check This!

The most important thing that you should know if you are getting this deep into an issue with your ignition system.

Is that it is time to get on eBay and find a used service manual for your engine. Having the service manual will give you the exact tests that you need to perform and how to properly track down the issues that you are having with your engine.

This will end up saving you a lot of time and head aches for a small amount of money! It’s a great investment if you are tracking down a complicated electrical issue with your outboard.

Check Us Out!

Look around at our YouTube channel for more step-by-step guides and practical tips.

We also have some helpful articles that enable you to learn more about your boat and the engine.

Brian Jones

Brian Jones has spent most of his career as an ASE Certified Master Tech. Today, he spends his time freelance writing and consulting. He lives outside of Dallas, TX with his family and loves anything mechanical.

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