Electrical gremlins can be challenging to tame, but you’re in the right place.
An ATV will not start with a bad stator. The engine requires an electrical current from the stator to power the coil and fire the spark plug. Symptoms of a faulty stator may include:
- No start
- Intermittent start
- Hot start issues
- Fouling plugs
- Flat battery
In this post, you’ll learn how a faulty stator also known as an AC Magneto affects your ATV, how to test it, and how to replace it.
It is possible for the engine to start if the stator is failing. However, it all depends on how it’s failing. An intermittent failing stator will cause the engine to run like a pig or not at all.
An inconsistent voltage supplied to the coil will result in:
- Misfiring plug
- Black smoke
- Fouling plug
You get the idea!
What Does A Stator Power?
There are two flavors of stator, single or 3 phases. The 3 phases are the most common. Each phase of the stator is comprised of copper wire wound around an iron core. The stator is fixed to the engine, and as the rotor (some may call it a flywheel) with magnets attached rotates around the coils, voltage is created.
All engines need electrical current in order to function. Even a basic ATV without lights, battery, or starter motor still needs voltage to power the coil. The coil as you know transforms voltage into a spark at the plug.
More sophisticated ATVs will have larger demands – ECU, battery, starter motor, electric steering, dash lights, sensors, coolant fan, lights, winch, etc.
While your ATV may have a battery, all voltage is ultimately provided by the stator. On most ATVs, battery power is used to operate dash lights and crank over the engine.
The stator does all the heavy lifting. The cranking rotor excites the stator which generates voltage to power the coil via the CDI box (Capacitor Discharge Ignition) or ECU (Engine Control Unit), whichever your ATV has.
After the engine starts, the stator provides all the power required, including topping up the battery voltage spent cranking the engine over. And so you can see how important a working stator is, that said let’s get down to checking yours.
Some bikes run a basic CDI system and more sophisticated fuel-injected bikes will have an ECU.
Obviously, wiring is model-specific, but the diagrams here give you a feel for the importance of the stator.
How To Test The Stator
Stators do a very important job and are under a ton of stress, so you won’t be surprised to learn they fail all the time.
Common stator problems:
- Open circuits
- Damaged or corroded wiring
- Power-washing – over doing it with the power washing is a common root cause of problems, it forces water in and can short out circuitry or cause corrosion
- Rotor fault – magnets falling off is popular with some brands
- Wiring loom – rodent damage
- Block connectors – loose/corroded block connectors
Three stator tests are common:
- Dynamic Battery Voltage Test (engine running)
- A Static Resistance Test (engine off)
- A Dynamic Voltage Test (with engine running)
1 Dynamic Battery Voltage Test
This test is called dynamic meaning your engine will be running. This is the easiest and fastest stator test of all, that said, it’s not conclusive. This test won’t suit if your engine isn’t running, if that’s the case, go ahead and move to the static resistance test below. You’ll need a voltmeter, a helper would be great and you’ll likely need to remove your ATV seat to gain access to the battery.
The process is as follows:
- Start your engine.
- Have helper hold throttle at approx 3000 rpm.
- Connect voltmeter as per picture below Red probe to + battery terminal and black probe to – battery terminal.
A healthy charging system will read 13-15.5 volts. This ATV is charging while not quite 13 volts, it’s working OK.
If you have a reading between 12 and 12.65-volt range, your charging system isn’t working and the stator may indeed be at fault. But it isn’t the only possible cause. Here are a few of the more common causes of a no charging issue:
- Loose battery terminals
- Dirty battery terminals
- Faulty battery
- A blown fuse
- Loose wiring
- Faulty Rectifier / regulator
- Faulty stator
2 Static Resistance Test
The tests called static means your engine and ignition switch are off. This test will suit you if your bike won’t start and you’ve checked for spark at the plug and found nothing, or you found your bike isn’t charging and you have checked all the low-hanging fruit like loose battery terminals, etc.
For this test, you’ll need a voltmeter (DVOM), and you’ll need to access the stator wiring loom.
It’s easy to locate, as it’s the loom that exits the engine side cover.
You will need to follow the loom to a block connector and you’ll need to disconnect it, yours may be further back towards the rear of the ATV, but you get the idea.
You’ll be testing the stator/engine loom side, and as your engine probably isn’t running, we’ll perform the static test first.
This is a two-part test, and both tests are simple to execute. I’ll assume you have located and disconnected the stator terminal. The block terminal connector may contain three wires or more.
You’ll be looking for three identical wires or three wires grouped.
Some terminals will house just the stator wiring in the terminal, and others will integrate the pickup wiring. I’ve included both in the picture.
Let’s name the three stator terminals A, B, and C; the designation and order is unimportant. You should know that we only ever test resistance on a dead circuit, meaning the terminal must be disconnected. Attempting to test resistance on a live circuit may damage the voltmeter or the circuit.
- Set the DVOM to read resistance (Ω) may also be written as “ohms”.
- Using your meter, read resistance between A and B, B and C, and finally, A and C.
The reading should be in the region of .1 to 1 ohm. Each manufacturer will have its own spec. But as a rule of thumb, resistance should be low and should not read open.
An open (O/L) meter reading on one phase is the most common result when testing a faulty stator.
Testing short to ground
This is the second part of the static test. Attempting to test resistance on a live circuit may damage the voltmeter or the circuit.
- Place the ground probe on chassis ground
- Probe A, B, and C, in turn
- All should read open circuit
Any reading on the meter other than an open means you have a short to ground which means your stator is faulty and will need to be changed.
3 Dynamic AC Voltage Test
The dynamic test will require a running engine and so may not suit all readers. Note the reading of voltage in this test is AC (Alternating Current), not the more common DC.
- Set your meter to read AC voltage (~) and the engine running
- Probe A to B; A to C; B to C
A reading of 15 to 60 volts AC can be expected, crucially the voltage should not be less and should rise with RPM.
Fitting a stator is straightforward, but you’ll need a special crankshaft rotor tool, and it may be engine-specific.
To replace the stator:
- Remove side cover – may need to remove gear selector footrest, etc
- Remove the side cover – unfasten bolts in a star pattern to avoid straining the cover
- Loosen rotor nut – may need a rotor holding tool
- Using rotor wheel puller, pull rotor
- Remove nut and remove rotor – be mindful of Woodruff key
- Remove the stator fasteners & remove stator
Align the rotor key-way with the crankshaft key and torque to specification. (You’ll need to use the rotor holding tool and you’ll need a torque wrench tool). Incorrect torque can cause the rotor to come loose, or over-tightening can crack the rotor.
The crankshaft is a super important component, over-tightening can damage it so please do get the correct torque spec and use a torque wrench.
Ensure the wiring isn’t pinched by side cover and the grommet fits correctly. Replace the side cover gasket and torque to spec and in a star sequence.
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