When a fresh spark plug still gives you weak or no spark, the ignition coil is the next thing to check. Here is how to test it, how to set the air gap, and how to catch a coil that only fails once it is hot.
The 30-second answer: When a new spark plug still gives weak or no spark, the generator ignition coil is next. Unplug the kill wire first to rule out the oil sensor, set the coil-to-flywheel air gap to about 0.012 inch with a feeler gauge, then check primary and secondary resistance with a multimeter. An open reading means the coil is dead. Replacements run $15 to $40.
The short version
- Do the free checks first: kill wire, low-oil sensor, plug boot, and flywheel key.
- The air gap matters. A coil that’s too far from the flywheel magnets makes a weak spark.
- A multimeter finds dead coils, but some only fail when hot, so test spark at the moment it quits.
- Most coils cost under $40, which makes this one of the cheapest electrical repairs on a generator.
How Do You Know the Generator Ignition Coil Is Bad?
The coil, sometimes called the magneto or ignition module, sits next to the flywheel. As the magnets on the flywheel sweep past it, they induce a current in the coil’s primary winding, and the coil steps it up to the roughly 15,000 volts the spark plug needs. When a coil fails, you lose spark entirely, get a weak orange spark that won’t ignite fuel, or get a spark that works cold and quits after the engine heats up. Out of every ten generators I’ve seen with “no spark” after a fresh plug, about three turned out to be the coil, three were the kill circuit or low-oil sensor, two were a bad plug boot or wire, and the rest were a sheared flywheel key or a bad air gap. That’s why I never condemn a coil until I’ve worked through the cheap stuff first.
Step 1: Rule Out the Kill Circuit
Almost every generator has a thin wire running from the coil to the on/off switch and the low-oil sensor. Those devices shut the engine down by grounding the coil. If the oil sensor misreads or the switch fails, the coil is fine but has no way to make spark. This test takes two minutes:
- Find the kill wire coming off the coil. It’s usually a single thin wire with a spade connector.
- Pull it off and tuck it where it can’t touch metal.
- Pull the plug wire, put the plug into the boot, ground the plug’s threads against bare engine metal, and crank while you watch the gap.
If you suddenly have a strong blue-white spark, the coil is good and the problem is the switch or the oil alert. Move to low oil shutoff with oil full. For the full no-spark checklist, see generator has no spark.
Step 2: Set the Air Gap
The coil has to sit close to the flywheel without touching it. Too far, and the magnetic field is too weak to make a good spark. This most often happens after the flywheel has been removed for a key replacement and the gap wasn’t reset.
- Rotate the flywheel so the magnets are directly under the coil’s legs.
- Loosen the two coil mounting screws.
- Slide a feeler gauge, or a folded business card in a pinch, between the coil and the flywheel. The target is about 0.010 to 0.014 inch, roughly 0.25 to 0.35 mm. A business card is close to 0.012 inch. Check your engine’s spec if you have it.
- Let the magnet pull the coil down onto the gauge, tighten the screws, then remove the gauge and spin the flywheel by hand to confirm it clears.
If the flywheel has moved on the crankshaft, the timing is off and the gap won’t save you. Check the flywheel key next.
Step 3: Test the Generator Ignition Coil With a Multimeter
You need a basic multimeter with an ohms setting. The coil should be disconnected from everything: plug wire off, kill wire off.
Primary winding
Touch one probe to the coil’s metal laminations (the ground) and the other to the kill terminal or primary lead. Small engine coils usually read well under about 1 ohm, so it’ll look nearly like a short. That’s normal. An infinite reading means the primary is open.
Secondary winding
Touch one probe to the laminations and the other to the metal end inside the spark plug boot or the high-tension lead. Expect somewhere from a few thousand ohms up to roughly 10,000, depending on whether the boot has a built-in resistor. An infinite reading means the secondary is open, and the coil is done.
I want to be honest about the limits here. Specs vary by engine and manufacturer, so treat these as ballpark values. And a coil can pass an ohm test and still fail under heat or at running speed. That’s why the next section matters.
The Coil That Only Fails When Hot
A classic pattern: the generator runs fine for 20 minutes, quits, and won’t restart until it has cooled off for half an hour. A coil tested cold will pass. With an inline spark tester on the plug wire, run the machine until it dies and check right away. No spark, then spark returns after cooling, points to a heat-soaked winding breaking down internally.
The lesson: if you’ve got a run-then-die pattern, test for spark the moment it quits, not after it cools. An inline spark tester runs about $8 and is the best money I’ve spent on small engine diagnostics. Also read generator won’t restart when hot, because vapor lock and a bad coil look the same from the outside.
Cost and Time
| Item | Typical cost | DIY time |
|---|---|---|
| Inline spark tester | $6 to $12 | 5 minutes |
| Feeler gauge set | $6 to $10 | 10 minutes |
| Ignition coil (aftermarket) | $15 to $30 | 20 minutes |
| Ignition coil (OEM) | $30 to $60 | 20 minutes |
| Shop repair | $90 to $160 | Drop-off |
Replacing the coil is simple: remove the two screws, swap the part, reset the air gap, and reconnect the kill wire. Use a fresh plug while you’re there (generator spark plug has the gap and heat range basics).
Video walkthrough
Frequently Asked Questions
Can a bad ignition coil damage the generator?
Not by itself. A dead coil just stops the engine from running. I do see people crank for long stretches with no spark and flood the engine, so stop cranking once you’ve confirmed there’s no spark.
What does the right spark look like?
Strong, snapping, blue-white. A thin orange spark or one that jumps only in a dim spot usually won’t light fuel under compression. I always test with the plug out and grounded.
Do I need a special tool for the air gap?
No. A business card works fine, though I prefer a feeler gauge because I can be sure of the number.
Is the coil the same as the magneto?
On small engines, yes, people use the terms interchangeably. The coil and the flywheel magnets together are the magneto system.
Should I replace the spark plug at the same time?
I do. A fouled or worn plug can mimic a weak coil, and it’s a $4 part. If the plug is oily, I’d also check the oil level (overfilled with oil).