The F800 series engine(or even newer that share the same style of generator/rectifier) uses a pretty simple style of electrical power generator. You have a flywheel that contains some magnets that spins around a stator – a device that is composed of some metal parts that have copper windings on them, so that, when the flywheel spins, the magnets induce a magnetic field in the metal parts of the stator, which, in turn, create alternating current in the windings. The windings are always a multiple of 3, and are such connected that thy are connected so the output is a 3 phased current, hence the 3 yellow coloured wires. That is the generator output.

The generator part is pretty easy to diagnose, use a digital volt meter (DVM) and set it to the diode function. Check the continuity between each yellow terminal and the bike’s ground, there should be none! If one or more winding/s show a direct contact between it and the chassis, it means that the winding is shorted, and no longer produces current. That usually means a burnt stator, blacked on one third of it’s mass. It usually happens because of multiple heat/cooling cycles of the engine, and the email coating on the wires wears out because of the friction, thus, getting connected to the metal core.

The second way to diagnose a bad stator is my verifying it’s resistance between the coils. Set the DVM to the ohms scale and measure the outputs, two by two, and compare the resistance values. Something like 1.2-1.3 ohms is OK.

The second part of the charging system is the rectifier, but you can also test that one on the bench.

Using the DVM again on the diode function, measure and compare the values like so:

• the minus DVM terminal on the phases, yellow wires, one in turn, and the positive DVM terminal on the rectifier green wire, either one, check the values; something like 560-600 is acceptable
• the plus DVM terminal on the phases, yellow wires, one in turn, and the negative DVM terminal on the rectifier red wire, either one, check the values; something like 560-600 is acceptable

The 560-60-700(depending on the rectifier you are testing) are actually millivolts, so 0.56-0.6-0.7V.

IF all the readings are the same, it most likely means your rectifier is healthy. If one reading is 0 or close to 0V, the rectifier needs swapping.

Apart from these issues, I have encountered, along the time, these:

• bad/corroded ground between the motor and the wiring harness
• bad connection between the stator output and the rectifier; a good fix is soldering the wires in place.

Good luck!