Problems within the charging system of a Yamaha WR 250 can affect more than just the motorcycle's headlights. A rapidly discharged battery is the most common symptom of a charging system failure; it causes the motorcycle to stop running after a handful of minutes or preventing it from starting at all. Pinpointing which component within the electrical system is the cause of a problem can require at least one hour.
Lift the sides of the seat up slightly to access the pair of bolts that secure the seat to the frame. Remove the bolts with an Allen wrench and pull the seat off the motorcycle. The fuse box will be visible between the seat rails.
Push the fuse box cover's tab inward and lift the cover to open the fuse box. Inspect the fuses visually, looking specifically at the metal bridge within the fuse. Replace any fuses that have a damaged or "blown" bridge.
Test the battery's voltage with a multimeter. Set the multimeter to DC (direct current) voltage scale using the setting dial on the multimeter's face. Place the multimeter's red positive probe on the battery's positive terminal and the black negative probe on the negative terminal. Take note of the battery voltage indicated on the multimeter's display.
Charge the battery with an automatic battery charger if the battery's voltage is less than 12.1 volts DC. Test the battery's voltage again after charging is complete. Replace the battery if the voltage is still less than 12.1 volts DC.
Start the motor and open the throttle halfway, using the twist grip on the right handlebar. This action increases the engine speed. Maintain this engine speed for the duration of this test of the charging system's output voltage.
Place the multimeter's red positive probe on the battery's positive terminal and the black negative probe on the negative terminal to test the voltage that is being transmitted to the battery by the motorcycle's magneto, charging coil and voltage regulator. Note the charging voltage indicated on the multimeter's display. Ideally, the battery should indicate a minimum charging output voltage of 14.1 volts DC.
Stop the motor. If the charging output voltage is less than 14 volts DC, test the magneto, charging coil and voltage regulator to determine which component is faulty.
Locate the magneto's wire harness on the left engine cover and follow it to the charging coil's connector. Pull the connector halves apart.
Turn the multimeter's dial to the Rx100 resistance scale.
Connect the multimeter's red positive probe to the white wire within the magneto's half of the connector. Connect the multimeter's black negative probe to the magneto connector's black wire. Take note of the magneto's resistance measurement.
Repeat the test on the wires in the charging coil's half of the connector, matching the red positive probe to the white wire and the black negative probe to the black wire. Ideally, the magneto and charging coil should have a resistance of 0.288 ~ 0.432 Ω (ohms) indicated on the multimeter's display.
Replace the magneto or charging coil if either components resistance is above or below the 0.288 to 0.432 Ω (ohms) specification range. Replace the voltage regulator, if the magneto and charging coil are both within the specified resistance range.
Consult a qualified Yamaha technician if problems persist after replacing faulty components.