Alternator (Stator) Failure

Now for the big gremlin which besieges all CX Turbos. If you manage to squeeze more than 20,000 miles of service from your stator, you are lucky. The stator is the coil of wires within the rear of the engine which generates electrical power for the entire bike. Most stators burn out between 15k and 20k miles. With a factory stator costing about two hundred dollars, and labor for a complete engine removal running around four hundred dollars, replacement costs can become very significant for those CX Turbo's which get ridden heavily. I am the original owner of a 1980 CX500 Deluxe. The stator in that bike did not burn out until 125,000 miles (in Florida while on a trip, of course). What causes the stators in the Turbo's to fail so often? Many factors are involved. Some steps can be taken to possibly stretch the interval between failures, but sadly the overall design of the charging system is poor. Sources for the following information are from Honda, my own personal experience and many others who have tried to alleviate this problem.

Fundamentally, immersing a stator in a hot oil bath is the worst thing for it. Choose any motorcycle which has an air cooled generator, meaning not within the oil bath. How often do you hear of one of these failing? Seldom. Consider the early 1980's Suzuki GS series. These bikes were all notorious for smoking their hot-oil-bath alternators. In 1986, Suzuki's GSX-R750 broke the GS trend in using an air cooled, automotive style alternator bolted to the top of the engine. Immediately, stator failures became rare, as the air cooled stators were utilized in all Suzuki sportbikes. Then, in 1996, Suzuki came out with the newly redesigned GSX-R750T. Engineers decided to return the stator to it's original location inside the engine. In a move acknowledging the problems with this design, Suzuki engineers placed two high- pressure oiling orifices pointed directly at the stator, in an effort to sink the heat build-up from the stator. In fact, at least one within our circle has suggested drilling out a small hole in a key oil galley passage above our stators as a means to improve cooling. I, for one, am not tempted to use my DeWalt for drilling holes deep inside my engines!

I hear many of you saying, "Well, I have a Yamahondaki XTC900RRRRR which has its stator in an oil bath, and it's doing fine at 40,000 miles." Why? Consider how much hotter your Turbo runs than your Yamahondaki. Most of those hotter-than-hell exhaust gasses which pass out the tailpipe of your Yamahondaki are retained for use in the turbocharging process. In our Turbo's, the heat energy is used to spin an impeller within the turbo housing. The other side of this impeller forces the intake charge of fresh air back into the engine at up to 16 psi above atmospheric pressure. The computer is dumping more fuel into the intake tract to keep the stoichiometric ratio proper. The result is a bigger bang in each cylinder for each 720 degrees of crankshaft revolution, and a bigger grin on your face while it's happening. However, a bigger bang creates more excess heat which now is recycled back into the turbocharger and sinked into the engine to be removed by the radiator and, you guessed it, the oil, in a snowballing cycle. The heat being removed by the radiator blows right past the hellish hot turbocharger and right past the engine. The extra heat in the oil works hard to suck the life right out of the stator. So you see, our engines have heat working for them, but also against them, both from the inside and out. This is one reason your Yamahondaki stator may last a long time even though it is in an oil bath.

Another area of the CX Turbo's charging system which may contribute to stator failure is the design of the regulating system. Any current produced in excess of that which is needed to charge the battery and run the bike's electrical system is shunted to ground by the regulator. The stator in our CX Turbo's can supply twenty-eight amperes from 5,000 RPM. Let us suppose at 5,000 RPM the bike's sum current demand is ten amperes. The difference in the Turbo's regulator is that it requires the stator to supply the maximum current it is capable of providing at any given RPM, regardless of current demand. This means that if the current demand is only ten amperes at the rated 5,000 RPM, the remaining eighteen amperes are shunted to ground. Incidentally, this characteristic of the Turbo's charging system leads me to surmise that operating electrical accessories such as an electric vest does not lead to additional stator stress, and thus stator failure. The current which the stator provides the accessory would otherwise be shunted to ground in the absence of such accessory. Explaining why this shunting of current to ground is bad explains, in part, the abysmal life expectancy of a CXT stator.

Current flowing through a resistance (the stator coils) generates heat within that resistance. Since the stator is required by the regulator to supply maximum current at any moment, it is also subjecting the stator to maximum heat. Recall that the stator coils reside within an extremely hot environment to begin with. Considering all the heat stress the coils are subjected to externally via the oil, plus all the internal heat generation caused by the mode of regulation, one can begin to understand why our stators burn up. Yet, what specifically happens to a stator when it "burns up"?

There are three coils which make up our stator. A coil consists of copper wire with a very thin coating of insulating varnish. Once the coils are wound, they are coated with a type of epoxy. The three ends of the coils are then spliced into the yellow ACG wires which exit out the top rear of the engine. The actual spliced area inside the engine near the coils is crimped to a metal strain-relieving arm. Eventually the insulating varnish may succumb to heat stress and develop cracks. The stator can fail in one of several ways. First, the cracks can allow two coils to short each other out, or one coil within itself. Also, a crack can allow a coil to short to ground. (The whole stator assembly is wound on a really effective ground!) Alternately, a wire could open, completely halting the flow of current through that wire. My personal experiences have been of the short-to-ground variety. The epoxy-sealed nature of a stator has precluded my pinpointing exactly where a coil shorts to ground, but one of my observations is common to all stators I have seen shorted to ground.

The splice from coil wire to the yellow ACG wire resides in a tightly crimped metal strain relief. All these junctions I have observed to be severely charred. I would not be surprised if this is one area where either an open or a short to the strain relief occurs. Additional heat builds up at this junction because all three wires carrying maximum amounts of current are all squeezed tightly together in this one small area, thus diminishing further the rate of heat dissipation at this point.

How can one address the whole issue of stator failure? Short of a complete redesign of the charging system, not much. One can lower the resistance of key electrical connections, which I will discuss shortly. Some people suggest purchasing an aftermarket rewound stator, and at half the cost of an OEM unit, it certainly is an attractive option. I currently have 15K miles on such a unit. Yet, I hold little hope for this to somehow miraculously solve the problem. I feel a big part of the real issue is the total environment the stator is forced to reside in, not just the design of the stator itself. So anything the aftermarket may do differently in building their stator addresses only part of the issue. With that said, I will only partially contradict myself by saying that IF you could find a rewinder who does more than just claim he uses wire with thicker insulating varnish, and who can reasonably show you and prove to you that he has somehow adequately addressed that crimped strain relief, I would say you have found someone who may be on the right track.

Some rewinders claim that double-dipping the coils in epoxy is the solution. Yet, the epoxy is only a sugar coating, its purpose to keep the oil from the coils and keep the coils tightly together. It yields no benefit to what lurks deep within the coil, where wires can short together. In fact, a reasonable argument may be posed that a thicker coating of epoxy will retain more heat within the coils. Some people also believe that using fewer turns of a slightly heavier gauge wire may solve the problem. Besides the obvious reduction in current generation, even this suggestion does not address the aforementioned problem areas. I have read of a couple of people who have rewound their stators thusly. However, sadly, no follow-up on extended stator longevity attributed to this modification has ever been forthcoming. I would love to hear of anyone in the United States who claims to have, say, 40K miles on a stator rewound with heavier gauge wire, which failed at the normal 15K to 20K miles with stock gauge wire previous to the rewind. This may be meaningful.

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