2004 Official factory service manual for Yamaha YFZ450S ATV Quad., Page 331Get this manual
LIGHTING SYSTEM EBS01067 EAS00776
TROUBLESHOOTING Any of the following fail to light: headlight, tailbrake lightCheck: 1light switch 2lighting coil resistance 3wiring connections (of the entire lighting system) NOTE: Before troubleshooting, remove the following part(s): 1seat 2fuel tank cover 3side covers (left and right) 4front fender Troubleshoot with the following special tool(s)2Lighting coil resistance Disconnect the A.Cmagneto coupler from the wire harness Connect the pocket tester1) to the lighting coil terminals as shownPositive tester probe yellow Negative tester probe ground 2