Yamaha YZF R125 starting starter motor system

⚠️ Safety First: Always verify that the bike is in Neutral before testing the starting circuit. If the neutral switch or sidestand sensor is faulty, the bike could lunge forward while you are testing the starter relay.

1. The Starting Logic (Ignition Cut-Off System)

The R125 has a safety "logic" that prevents the starter from engaging unless specific conditions are met. If the bike won't crank, check these first:

• Neutral Switch: The bike must be in neutral (Green 'N' light on).

• Sidestand Sensor: The stand must be UP if the bike is in gear.

• Clutch Switch: The lever must be pulled in if the bike is in gear.

• Kill Switch: Ensure it hasn't been accidentally bumped to the "OFF" position.

2. The Starter Relay (The "Click" Test)

The starter relay (solenoid) is the heavy-duty switch that sends high current to the starter motor.

• The Symptom: If you hear a loud click but the engine doesn't turn, the relay is likely working, but the battery is too weak or the starter motor is jammed.

• The Test: Use a multimeter to check for 12V at the small signal wires when the start button is pressed. If the relay gets the signal but doesn't pass power to the thick starter cable, the internal contacts are burnt.

R125 starting circuit wiring diagram 

Heavy Duty Starter Relay: If your relay has failed, replace it with a heavy-duty version. The R125’s original relays are known to fail if the bike is frequently started with a low battery, which causes the internal contacts to "arc" and weld shut.

The wiring on the Yamaha YZF-R125 starting circuit can be confusing because the colors often change slightly between the generations (V1 vs. V2/V3).

Here is the "Motor Map" for the critical wires you'll need to probe with your multimeter to find a break in the circuit.

The Starter Relay (Solenoid) Connector

The relay usually has four wires in its plastic plug (separate from the two thick battery cables):

• Red (R): Main power feed coming directly from the battery/fuse. It should always have 12V.

• Blue/White (L/W): The trigger wire from the Start Button. This should show 12V only when the button is pressed.

• Black/Yellow (B/Y): The safety ground wire. This connects to the Starting Circuit Cut-off Relay.

  • If this wire doesn't have a ground connection, the bike thinks the stand is down or it's in gear, and it won't crank.

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Safety Switch Colors

If the relay isn't clicking, one of these switches is likely blocking the signal:

• Neutral Switch (Sky Blue - Sb): Sends a ground signal when the bike is in Neutral.

• Sidestand Switch (Blue/Black - L/B): Provides the ground path needed for the ignition to stay live.

• Kill Switch (Red/White - R/W): Carries the main "Run" power to the ignition coil and starter circuit.

Video of the starter sprag clutch gear below