Onan RV Generator Only Sparking When Starter Released


If you’re having the hardest time getting your Onan RV generator to start right after winter, then you’re in good company.

Doing your due diligence, you decide to check the spark plugs. You ground the threads out on the engine and try the starter — nothing.

At least nothing until you release the starter switch. That’s when you see a couple sparks as the crankshaft slows down and stops.

I experienced this exact same problem and while it could be any number of things — I highly recommend that you check this one possibility out before you start chasing your tail with the battery, spark plugs, ignition coil and more.

It is not uncommon for an Onan RV generator will fail to spark when the ignition rotor wings have been sheered off. This typically happens due to mice building nests inside of the generator during the winter. Repairs generally cost $350 for parts and labor.

Why Does My Onan RV Generator Only Spark When the Starter Switch is Released?

If you have the same luck as me, your generator became the winter residence for a family or two of your local mice. These tenants didn’t pay rent and they left you with an annoying cleaning project when they left.

The problem is that you likely didn’t get all of nest out the first time around and went to start your generator, and that has resulted in some damaged internal components when your crankshaft started trying to spin.

Yes, your spark plugs are likely just fine. So is your ignition coil. So is your battery.

The problem lies right where I’m indicating in the picture below. The ignition coil. To be more precise — what’s under the ignition coil.

Picture of me pointing to the ignition module on the Onan 4000 RV generator.

The trick to removing the ignition coil and module is to remove the two screws holding it on and then to get the fan blade underneath to line up properly in order to remove the ignition module. If you don’t line up the fan correctly, you won’t be removing it.

There’s one place in the rotation of the fan wheel that has a larger cutout in the blades for the ignition module to pass by.

You’re welcome for this little bit of knowledge because I was using some pirate language for 15 minutes until I figured that out.

To line up the fan blades correctly, use a hex key to spin the exposed end of the crankshaft. Make sure the spark plugs are removed so that you don’t have to contend with any compression.

With the blades lined up, pull up on the ignition module and set it aside.

Use a flashlight to shine down inside and see if you have a mouse condo inside. Remove it if you do.

Now, check the integrity of your ignition rotor.

The ignition rotor is a plastic piece that encircles the crankshaft and has two wings sticking up a few inches that are 180 degrees opposite of one another.

I only recovered one of the wings of the ignition rotor.

One wing has a north polarity magnet, and the other a south polarity magnet.

As the crankshaft spins around, these magnets circle around with it and tell the ignition coil which spark plug to fire as they swing under the ignition module.

The problem is that with the mouse nest inside, the plastic wings on your generator likely sheered right off as they met the resistance of the nest at such high speeds.

Image of the manual for the Onan 4000 RV generator showing a diagram of the ignition module and ignition rotor.
The ignition rotor is banded around the crankshaft and has two “wings”. In the picture the wings are in the 12 o’clock and 6 o’clock positions. Unfortunately, it’s made of plastic and is prone to breaking if a mouse builds a nest in your generator over the winter.

When you look down with a flashlight, you’ll likely just see the crankshaft and a plastic band around it. When you spin the crankshaft slowly, you’ll see that along that band there are rough spots that are 180 degrees opposite from one another. This is the base of the wing where it fractured off of the band.

Repairing the Ignition Rotor on an Onan RV Generator

I wish I could tell you that I fixed this myself and give you precise info on how to do it. In all honesty, I found it much more cost effective and easier to have it serviced by my local small engine repair shop.

I just didn’t feel like tearing this heavy generator in two pieces by myself.

In total, the parts were $100 (brushes, ignition rotor) and labor was $250 (cleaned split rings, replaced brushes, replaced ignition rotor, and dealt with the tearing apart of the generator). Spring of 2022 is the time this article was written, for price context.

When you remove the ignition module and use a flashlight to look at the crankshaft, you’ll see the brown band around it and if you’re having the same problem as me you’ll see where the wing sheered off of the ignition rotor.
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Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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