Yamaha YZF R125 lighting system ~ circuit

⚠️ Safety First: Do not replace fuses with higher-amp versions. If a 15A fuse keeps blowing, there is a short circuit in the loom that must be fixed to prevent a fire.

1. The "Engine Running" Logic

On the R125, the headlights do not turn on with the ignition key alone. The ECU waits for a signal from the stator that the engine is spinning before it triggers the Headlight Relay.

• The Test: If your sidelights work but the main headlights don't (even with the engine running), listen for a "click" from the relay located behind the front fairing. No click usually means the relay is dead or the ECU isn't sending the trigger signal.

2. Identifying "Dipped" vs. "Main" Beam Issues

• Dipped Beam: Usually stays on constantly once started.

• Main Beam: Triggered by the handlebar switch. If the blue "high beam" light on the dash comes on but the light doesn't, the issue is likely the bulb or the connector.

• Yellowing Connectors: Check the plastic H7 bulb plugs. The R125 generates significant heat here, and it’s common for the plastic to melt, causing a poor connection.

Electrical Contact Cleaner: The R125's handlebar switches are exposed to rain, leading to internal corrosion. Spray this directly into the high-beam toggle switch to restore "sticky" or non-responsive buttons.

The lighting circuit on the R125 is split into two halves: the DC side (sidelights/dash) and the AC-triggered side (headlights).

Component	Wire Color	Function
Main Headlight Relay	Red/Yellow (R/Y)	Power feed from the Fuse Box.
Relay Trigger	Yellow/Black (Y/B)	Signal from the ECU (only live when engine runs).
Dipped Beam	Green (G)	Power to the low-beam bulb filament.
High Beam	Yellow (Y)	Power to the high-beam bulb filament.
Common Ground	Black (B)	The return path to the chassis/battery.

Troubleshooting with the Map

1. Check the Green Wire: With the engine running, use your multimeter to probe the Green wire at the headlight plug. If you have 12V there but no light, your bulb is blown.

2. Check the Relay Trigger: If there is no power at the Green wire, probe the Yellow/Black wire at the relay. If this doesn't show voltage while the engine is running, your headlight relay or the ECU trigger is faulty.

3. Grounding Issues: If the Black wire shows any resistance to the battery negative terminal, your lights will be dim or flickering. Clean the main ground point on the frame.

On the Yamaha YZF-R125, the AC (Alternating Current) side of the lighting circuit is what controls the main headlights. Unlike the sidelights or indicators, which run directly off the battery's DC power, the headlights are tied to the engine's rotation.

How the AC Side Functions

The "AC side" isn't actually sending raw AC voltage to your bulbs (the regulator converts it first), but it is AC-dependent. The ECU waits to see an AC pulse from the stator—proving the engine is spinning—before it allows the headlights to turn on.

This is a protective measure to ensure the headlights don't drain the small battery while you're trying to start the bike.

The Wiring "Motor Map" for the AC Side

If your headlights aren't coming on but the bike is running, you need to trace these specific wires:

Yellow/Black (Y/B): This is the ECU trigger wire. It sends a signal to the headlight relay only when the engine is running.

Red/Yellow (R/Y): This brings power from the Lighting Fuse to the relay.

Green (G) & Yellow (Y): Once the relay "clicks" over, it sends power down the Green wire (Dipped Beam) or the Yellow wire (High Beam).

Electrical Contact Cleaner: The headlight relay is often tucked behind the fairing where it gets hit by road spray. Use this to clean the pins if the relay isn't "clicking."