Hi - Just thought I'd lay this on the group with the hopes it helps a few folks with diagnostics.


Stator coil assembly removed from a 2003 XT225 with 12K miles on it. Note burned coil wrappings. It also has a noticeable burned smell.

Last season my bike would cut out climbing short steep hills. It was like someone had just turned off the key. This happened relatively infrequently. The bike also refused to start hot via the starter motor. It would pop start reliably when hot. It would always start when cold. This was very frustrating to say the least. It turned out to be the stator assembly. Replacing this has fixed the problem(s) and also improved the bike's performance.


The stator has 15 alternator coils and a charge coil shown at 12 O'clock.

The stator assembly consists of two types of coils. 15 alternator coils and 1 charge coil. The alternator coils are connected in a 3 phase configuration and supply power to the accessories and battery charging circuit. The charge coil is part of the magneto style ignition which produces the spark through a CDI unit. It's done this way for reliability. You could have a dead battery and still pop start the bike. In short, the spark comes from its own independent system.

There are two windings in the charge coil. One is referenced to ground and the other floats. These come out through a water tight connector (under seat near regulator) on yellow, green and brown wires. The manual gives the following resistance values for a good charge coil:

Ground to Yellow -> 20-30 ohms
Brown to Green -> 584-876 ohms

In my case the charge coil was working but had shorted windings that changed based on temperature. I gather the enamel insulation had broken down on adjacent windings. Since copper expands when heated it seems reasonable that a short could come and go with temperature. Expanding winding (exposed copper) pressed up against one another. Since only a portion of the windings shorted, the charge coil produced its pulse but at a lower value. Not enough energy, it seems, to provide a hot spark at cranking speed. A pop start, however, turned the crank fast enough for the engine to fire. Electricity is produced in a coil as a function of the rate of change of the magnetic flux. A faster change produces more electricity below core saturation.

Anyway that's my story and an amateur's explanation of the details. Jeannie's bike wouldn't start the other day owing to no spark. Her charge coil reads open circuit on both windings (bad scene for a woman). Wires are good going into the engine. A look inside the engine will be revealing.

I may attempt dissecting my charge coil and attempt to rewind it. It's one of those rainy day projects that may never see completion.

Thanks for the info, hopefully your new stator will last longer then this one did, i know you change your oil very frequently at every 500 miles, so that can not be the reason for it to have gone bad.

Do you have a pic of the new stator compared to this burned up one?

150Ron - Parts are on order. I will photograph the new stator. It seems reasonable to assume the coatings around the coils will stain with carbon in the oil over time.

The plastic sleeve around the wire between the stator wiring harness and rubber grommet just crumbles when pressed on. Wire insulation (leads external to the coils) is pliable and in good shape. I mention this as an example of how different materials break down over time in heat and hot oil.

In our case, the bikes see little road mileage with an average trail speed of 10 MPH. Lots of 1st and 2nd gear stuff with higher engine RPM at lower air flow. It all adds up to increased heat, wear, tear and broken parts.

I did some dissecting of the charge coil today and a rewind isn't a practical option IMHO. The pole piece is welded to the stator ring. That means removing/replacing the coil bobbin would be difficult if not destructive. The wire used is very fine meaning there is a lot of it and the winding job would be a very delicate process.

New stator price range is $200 - $283. Not expensive as stators go. Some for ATVs run around $700.

If anyone here has rewound one, I'd surely like to have the process revealed.

That is actually a really good price, i know some companies like baja designs rewind stators, for around $150+ shipping and other charges, so its better to just get a new one if its around $200 or so.

And yes, beind in 1st and 2nd gear and at 10mph does not cool the motor that much at all, i do lots of riding like that too, specially when climbing, always in first gear, as its very steep and 2nd gear does not have the power, that is why i choose full synthetic oil, as its suppose to keep the temps cooler.

i just replaced the stator in my ttr250 today. took ablout a half hour

Hi Muniac,
Thanks for taking time to provide a solid narrative with great photos. It sounds like you (and others) attribute the failure to high temperatures - most likely from higher rpm riding - is that correct?
Will you change anything (riding habits? oil type? bolt-on cooler?) to avoid another failure in another 12K miles?
Or do you figure this was this a one-off fluke?

Jeannie's bike wouldn't start the other day. Luckily this happened before the 500' drop into the canyon. Working through a diagnostic revealed her charge coil was open on both windings.


New pulser and stator installed in 2003 XT225. Jeannie's bike.

Her bike is about 1000 miles behind mine. Finished up the repairs about an hour ago and the bike is running well again. The charge coil supplies the power for the magneto style ignition. Of interest is her bike not only came back to life but did so with noticeably improved power. I gather the spark was weakening slowly over time and we didn't realize how the performance had deteriorated.

Summarizing bad stator symptoms below:

1) Coils can deteriorate slowly over time.
2) Sudden failures are possible.
3) Intermittent failures can occur which are temperature dependent.
4) A bike may start when cold, run OK but not start when hot.
5) Spark can cut out and stall the bike during "hot" climbs.
6) Slight cooling off periods can restore spark.
7) A bike that doesn't start when hot may pop start.

Rick - Glad to help out. High engine revs at slow speeds runs an engine really hot. In our case, many trails we ride creates this condition. Heat is usually the enemy of most electronic equipment.

I think the epoxy and charge coil wrapping get stained with carbon from the oil. Carbon is a conductor so its presence within windings would cause problems. The charge coil is wrapped unlike the other 15 alternator coils which are epoxy dipped. See photo above. It's logical to assume the charge coil would react differently to oil impregnation. The charge coil windings are very fine wire. Another difference from the alternator coils that may lead to unique failures.

No changes planned. The riding we do is a function of how to safely move through trails and terrain. I don't think this is a fluke given it happened on two bikes. Trail riding is abusive and parts will fail. Overall the XTs have done very well by us. As the bikes age, I'm learning what the mean time before failure is on various parts. Full synthetic oil is still a good option. Just need to factor all of this into routine maintenance and keep on rolling.

Hello,

Thanks for info. Will you please say where you got your stator?
Was it Yamaha or aftermarket?

Thanks,

Heirhead

Parts were procured from Power Sports Plus and service was OK. Here's what I replaced:

1) Stator Assy..... 4JG-85510-01-00 $199.65 (List $283)
2) Pulser Coil..... 4JG-85580-01-00 $ 44.47
3) Gasket.......... 4BE-15451-03-00 $ 5.96

Prices were reasonable and all the above were OEM Yamaha parts. I also replaced the cheap engine case screws with SS socket head cap screws.