Motorcycle Repair: yamaha fj 1100 1985, fj 1100, electrical diagnosis

Question
Hi I have a fj 1100 and it will run some times well then it will die but if you start it with the lights on it will run but as soon as you turn off the lighs it wiil stop same if you have the brake lighs on .i have been told that it might be the voltage regulater but were is it on this model? thanks for any help

Answer
Hi Wayne,

The FJ11 has an alternator mounted behind the cylinder block.

Inside the case is a stator, a rotor with slip rings, a three phase rectifier bridge, and the voltage regulator.

Most common problems:

1) Overcharging, due to a bad connection in between the battery and the (thin) sensing input wire feeding into the alternator. (The thicker red lead is the postitive output to the battery). Check for a voltage difference between the two leads (the thin one and the thick one) with the engine running. Anything over .3V is too high, and you should check all connections in the bike, including the ignition switch.

2) Overcharging due to a bad regulator. If 1) doesn't fix it, replace the regulator with our RG25

3) No charging or intermittent charging: Bad brushes on the slip rings. Check to see if they're too short, or broken.

4) No charging due to bad rotor. The rotor needs to have a resistance of in between 3.8 and 4.2 Ohms. You check this resistance after removing the brush holder, and checking between the copper slip rings. Anything lower or higher means a bad rotor, which will need to be rewound.

Otherwise, get a copy of the service manual and learn about performing electrical tests or have a local tech perform electrical diagnosis for you.

Respectfully,
Mark Shively