Howto: Testing a motorcycle stator

1. What is a stator?

A stator is essentially an electrical generator. It is also referred to as a “magneto” or “stator coil”.

2. What does a stator do?

A stator’s primary function is to convert mechanical motion (usually rotational motion from the crankshaft) into electrical energy though the creation of a magnetic field. Stators work together with a flywheel; a rotating bowl attached to the crankshaft which has magnets attached. Stators do vary in design but essentially do the same thing. The rotation of the magnets around the stator is what creates the magnetic field.

3. What does it look like?



4. Where is it found on my motorcycle?

It is found on the opposite side of the bike to the oil reserve, in virtually the same location, behind a protective cover. The cover may have a slight amount of oil in it but should mostly be empty, so accessing a stator is usually achieved by taking out the cover plate bolts. On some bikes, the stator is enclosed by the flywheel which may need to be removed first before the stator can be accessed.



5. How does a stator work on a motorcycle?

Simply put, there are two main types of electrical current; AC (alternating current) and DC (direct current). Stators produce AC current. Batteries, which operate by storing and releasing DC current, cannot be recharged by AC current. The stator sends the AC current to the regulator rectifier, which then converts the AC current into DC current so that the battery can be recharged. Without the stator, the battery would quickly run out of power (as the battery powers all the motorcycle electrics). Modern motorcycles can be a little more complicated than this, but the basic principles would be very similar.

6. How do I test my stator?

There are several tests that can be done to ascertain the condition of the stator. These will require a multimeter, which can be purchased relatively cheaply from a number of different stores. Make sure you purchase a multimeter with both AC and DC capability.

a) Online Battery recharge test

First, set the multimeter to 20V (DC). Look for “20” on the dial which will be near a V, on the right hand side of the V will be a continuous horizontal line on top of a dashed line. With the motorcycle running in idle, place the positive (red) probe of the multimeter on the positive terminal (+) of the battery and the negative probe (black) onto the negative terminal (-) of the battery. You should get a “float” voltage reading of somewhere between 13.2V and 14.2V, which should also be relatively consistent. Battery manufacturers disagree on exactly what this voltage should be, but as long as it’s above around 13.2V and holding relatively consistently, the stator is most likely working correctly in this recharging function.

As you rev the bike you should be seeing an increase in voltage across the terminals. Naturally, as you spin the crankshaft faster, the magnetic field is stronger, generating more current.

b) Offline continuity test

Some electrical tests measure something called resistance, which is shown by the unit Ohms (Ω). In a nutshell, resistance is a measure of a materials ability to oppose the flow of electric current.

You will need to reference your motorcycle user manual to understand the resistance range typical for a stator on your particular motorcycle. From the stator, trace the wire back to where it connects to the regulator rectifier and there should be three distinct wires (representing the three stator “phases”). Grab a white ink pen and dot the wires on each side so you remember which came out of which (it may not matter much but if you are troubleshooting down the track you don’t want to go changing anything and making it harder for yourself).



Set your multimeter to Ohms (Ω). Some multimeters have the ability to read resistance at different levels, in this case I set mine to “200”, in the range near the Ohms symbol Ω. This multimeter also emits an audible continuous beep when it gets a resistance measurement, some do not.

All you have to do now is stick one multimeter probe in one wire, and the other in a different wire, and you will get a reading – in this case 0.9. Each bike will be slightly different, but you are checking for consistency. If you labeled each wire A, B and C, A-B, B-C and A-C should all give you the same reading.



c) Offline short test

Keep the multimeter on the same setting as (b). Swap the black probe between each wire (A, B and C) while keeping the red probe on the negative (-) terminal of the battery. You should get an open circuit reading (ie no reading on the multimeter). If you get a reading, it means one of the phases of the multimeter has shorted out to ground and you need to replace your stator.



d) Online voltage test

Switch your multimeter to VAC (V~ is the symbol for AC Volts). This multimeter will be set at “200”. Turn the motorcycle on. Measure across wires A-B, B-C and A-C and ensure they are all consistent (should be within approximately 5% of each other). Now raise the RPM of the bike to around 5,000rpm and perform the test again, noting the voltage is much higher but the consistency should still be there.

Happy troubleshooting!!