Yamaha YBR 125 Service Manual page 210

2.The brake lamp is not lit
1.Bulb and socket
Check if the bulb and the socket are conducted.
2.Brake switch
Remove brake switch guide wire from the bundle
connector.
Check if the switch between the "Yellow &Brown"
and "Black& Black" is conducted
3.Voltage
Connect min. Multitester(DC20V) to socket
connector.
Positive guide wire of multitester→Yellow guide
wire
Negative guide wire of multitester→Black guide
wire
Turn the main switch to "ON".
Pull brake lever or step brake pedal.
Check if the voltage of "Yellow"guide wire at the
socket connector gets to 12V.
The circuit is normal
Meet normal value
Signal system
No conduction
Abnormal
If the brake switch is failure,replace it
Out of normal value
There is poor layout from the main switch to the
socket connector,repair is needed.
7-34
ELEC
Replace bulb or socket