Hi, Anonymous if your headlight is not working, intermittent, only one beam works, keeps burning out, blinking, dim or surges from bright to dim to bright the first thing you need to check is the headlight bulb for damaged filaments then check your light circuit fuse, then check your headlight bulb socket ground wire for a clean tight connection, that being said the usual suspects are:
1. Faulty ground or power supply wire to the headlight bulb socket.
2. Worn down solder contacts or loose fit of the bulb base in the socket due to vibration.
3. Worn, chafed, or broken wires in the light circuit.
4. Corroded, broken, or loose pins/sockets in wire connectors.
5. Faulty hi/low beam switch.
6. Faulty ignition switch.
7. Fuse is good but no continuity on both sides.
8. Faulty headlight relay.
9. Faulty wiring between battery positive and headlight bulb socket.
10. Weak charging alternator/generator/lighting coil
11. Faulty voltage regulator/rectifier ground or overcharging.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day.
https://www.twtex.com/forums/threads/2009-fat-bob-fxdf-electrical-issue.94756/
http://www.mediafire.com/file/y8au34oc778d7oc/2009_Harley_Dyna_Service_Manual.pdf/file
https://partsfinder.onlinemicrofiche.com/ronnies/showmodel.asp?make=hdmc
https://ownersmanuals2.com/harley-davidson/fat-bob-2009-owners-manual-66578
https://www.6ya.com/
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