⚠️ Safety Warning: When performing the dynamic (running) test, the stator produces high-voltage AC electricity. Do not touch the metal multimeter probes or the connector terminals while the engine is running to avoid electric shock.
Yamaha YZF-R125 Generator (Stator) Testing Guide
If your battery is constantly draining, failing to charge, or your lights are dimming, the generator (stator) might not be producing enough power. You can diagnose this using two types of tests: a Static Test (Engine Off) and a Dynamic Test (Engine Running).
1. Preparation
Access: Locate the stator wiring harness. It typically consists of three White wires in a single plastic connector block leading from the left-hand engine cover.
Tools: A digital multimeter is required.
2. The Static Test (Resistance & Ground)
Perform this test with the engine and ignition completely OFF.
Internal Resistance: Set your multimeter to the Ohms (Ω) scale (200Ω range). Unplug the connector and measure the resistance between all three combinations of the white wires (1-2, 1-3, and 2-3).
Spec: A healthy R125 stator should read between 0.22$\Omega$ and 0.34$\Omega$ at 20°C.
Failure: If any pair shows "OL" (Open Loop) or significantly higher resistance, a winding is broken.
Short to Ground: Touch one probe to any of the white wires and the other probe to a clean, unpainted part of the engine case or the battery negative terminal.
Result: The meter should show no continuity (Infinite resistance/OL). If you see any numerical reading, the stator is "shorted to ground" and must be replaced.
3. The Dynamic Test (AC Voltage Output)
Perform this test with the connector unplugged and the engine RUNNING.
Setup: Set your multimeter to AC Voltage (at least 100V AC range).
Procedure: With the engine idling, measure the voltage between the three combinations of white wires (1-2, 1-3, and 2-3).
Idle: You should see roughly 20V AC or more.
High RPM: Rev the engine to approximately 5,000 RPM. The voltage should climb significantly, often reaching 50V to 70V AC or higher depending on the specific model year.
Consistency: All three phases must produce nearly identical voltage. If two wires show 60V but the third shows only 10V, the stator has a failed internal phase.
4. Final Diagnosis
Stator Tests OK? If the AC voltage and resistance are within spec, your charging issue is likely the Regulator/Rectifier or a dead battery.
Low AC Output? If the resistance is fine but the voltage is low, the magnets in the rotor (flywheel) may have weakened, though a burnt stator winding is far more common.
Quick tests , for checking the alternator .
Easy tests , first you check between each coil of the generator , the white wires .
Then between the white wires and earth .
finally and especially if you have no spark , test the Pick up coil , the separate block with pink and white wires .
Test between each of the generator white wires , each coil with you meter set to ohms (auto) so three tests . its should have a set resistance , see results above )
Then test between each coil , white wire and earth or body of the stator . There should be no continuity , so set multimeter to test continuity and listen for buzzer , if the buzzer rings you
have a coil directly shorting to earth and its therefore faulty .
See another video of this here
How the stator and rotor generator works on a YZF R125
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