Signs of bad ignition coil on motorcycle

A coil is a critical ignition system component and your bike won’t run without them. A coil works hard, and for the most part, they are reliable, but they do fail. You are in the right place, I’m a mechanic and very shortly, you’ll know if your dirt bike coil is good or bad.

The top four symptoms of a bad dirt bike coil include:

1. No start
2. No spark from the spark plug
3. Hard hot starting
4. Shuts down when hot

In this post, you’ll learn what the symptoms of a faulty dirt bike coil are, how to diagnose a faulty coil, and of course how to fix it.

Symptoms of Faulty Dirt Bike Coil

Here we’ll look at the most common symptoms of a bad dirt bike coil in a little more detail. But just before we look at them, it is worth knowing that coils commonly fail in one of two ways. The symptoms associated with each type of coil failure are different.

1. Total failure – (stops working altogether)
2. Fails when hot – (works when the engine is cold)

Common dirt bike faulty coil symptoms are as follows:

No start – no start is the number one symptom associated with total coil failure. That said a dirt bike may not start for a ton of reasons and so this common symptom isn’t conclusive evidence of coil failure in itself.
No spark from the spark plug – no spark at the plug is if you like the first solid clue we are dealing with an ignition system fault and of course, as you know the coil is a major player in the ignition system so we’ll already suspect we could be looking at total coil failure. But as before we’ll need to check that coil first.
Hard hot starting – commonly associated with a failing coil as opposed to a totally failed coil. This type of intermittent failure can be irritating to catch as it isn’t always present and in some cases doesn’t even happen on every hot start.
Shuts down when hot – commonly associated with a failing coil. May be intermittent or regular as clockwork when the engine heats up. I’ve covered this exact issue previously and you can check it out here – Dirt bike won’t start when hot

While it’s true the coil is a major critical component of the ignition system that commonly fails, it must be said it isn’t the only component of the system that can cause issues. The presence of one or more of the above symptoms is not conclusive proof of coil failure.

And so before reaching for a new coil, let’s quickly test it and we’ll do exactly that right now.

How to Test Dirt Bike Coil

Checking spark at the spark plug is among the very first checks we make when we suspect a coil issue. Testing the coil though can be hit and miss without the correct tools. In the field, we can run a MacGyver-style hack by grounding the spark plug on the engine and checking for signs of life (we’ll cover that below).

The problem with this type of testing is, that it doesn’t load up the ignition system. And loading the system up and stressing is exactly what we want to do when testing.

First, let’s cover the MacGyver hack for in-the-field spark checking.

Checking spark MacGyver style as follows:

• Remove plug cap
• Remove the spark plug
• Refit the cap
• Ground the plug on the engine
• Crank over the engine
• Check plug for spark

Two results are possible:

1 No spark at the spark plug – Check the grounding. (Grounding the plug means making the metal body of the spark plug make good contact with bare metal on the engine.) If the plug isn’t grounded correctly the plug won’t fire and that often leads to misdiagnosis. If you are happy with the grounding, the plug should be swapped out with a spare to eliminate it as a possible cause of no spark

2 The plug sparks – Great! But remember this type of spark testing doesn’t stress the ignition system and so finding a spark isn’t conclusive.

The correct test is the inline ignition system test and we’ll cover that next.

The inline tool is especially useful when searching for intermittent hot start issues as the engine will run with the inline tool in place and of course, the tools inspection window may be viewed for signs of spark failure as the engine warms up and approaches its failure point.

The inline ignition system test is as follows:

• Remove plug cap
• Connect plug cap onto the inline tool
• Connect the inline plug cap to spark plug
• Crank engine
• Check for spark

Two results are possible:

1 The plug sparks – If you have a spark when testing with an inline tester, your ignition system is good. That said, if you still feel you have an ignition system issue, you can check the following:

• Check the correct type of spark plug fitted (heat range)
• Check plug condition and gap
• Check for grounding ignition system wiring (Kill switch)

2 No spark at the spark plug – It is very likely your coil is the root cause of the no start but there are as said other components that could cause no spark. To produce conclusive evidence we’ll need to check coil resistance.

A DVOM is also known as a voltmeter, but we won’t be checking voltage, we’ll be checking coil resistance. You can find a DVOM I use here on the ATV tools page.

Coil Test

When testing a coil we’ll run three tests:

1. Primary coil resistance test
2. Secondary resistance test
3. Plug cap resistance test

Before running this test you’ll need to isolate your coil by removing all block connectors and removing the coil cap. Running a resistance test while the coil is connected to the bike’s wiring circuit won’t work. Resistance must not be executed on a live circuit.

1 Primary coil resistance test as follows:

• Remove all coil wiring
• Set your meter to resistance Ω
• Test the primary windings by placing the probes as per the picture

The resistance readings 1.16 to 2.64 are typical but your bike’s spec will likely differ, check your online users manual.

2 Secondary resistance test as follows:

• Meter set to resistance Ω
• Test the secondary windings by placing the probes as per the picture

The resistance readings 8.64 to 12.96 are typical but your bike’s spec will likely differ, check your online users manual.

3 Plug cap resistance test as follows:

• Meter set to resistance Ω
• Test the secondary windings by placing the probes as per the picture

The resistance readings 1.16 to 2.64 are typical but your bike’s spec will likely differ, check your online users manual.

If your coil is within your manufacturer’s spec, your spark plug is correctly gapped, and it is the correct type of plug (heat range) then you are all good on the ignition system.

If your bike fails to start, check the following:

Air filter – a blocked or mouse infested air filter is top of the list for good reason

Following correct starting procedure – choke, kill switch, etc.

Flooded engine – constantly attempting to start a hard starting bike may cause flooding.

Fuel-related issues – bad gas, fuel starvation, dirty carb, etc. I’ve covered a hack for checking for fuel issues quickly in this post – ATV won’t start. Sure it’s an ATV but the principle is identical.

Valve lash – the engine must breathe and the valves allow air/fuel mix in and spent gases out.

Timing out – timing is important, spark must occur at a critical moment.

Compression issue – poor compression will cause a no-start.

Other Causes of No Dirt Bike Spark

Here’s a shortlist of other root causes of no spark:

• Faulty stator
• Faulty pickup
• Faulty ignition
• Faulty CDI
• Kill switch short

You may find the following posts helpful:

Will dirt bike start with low compression?

Dirt bike troubleshooting