Yamaha VIKING 2015 Owner's Manual page 66

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 When the knob is set to "DIFF LOCK" or "4WD", the differential gear lock indicator and indica-
tor light will flash until the differential gear is completely locked or unlocked.
 When the indicator and indicator light are flashing, turning the steering wheel back and forth
will help the differential gear lock to engage or disengage.
 Riding before the differential gear is properly engaged or disengaged (e.g., when the indicator
and indicator light are flashing) will cause the vehicle speed to be limited until the differential
gear is completely engaged or disengaged.
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