Time to whip out a multimeter and start poking around. If memory serves, there should be three wires at the bulb socket. One ground wire, one power for low beam and one power for high beam. First step is to figure out which wires are which. You can skip this if you have the wiring diagram for your model/year ride.
- set your multi-meter to as close to 12 volts without going under
- pull the bulb
- set up the bike so the headlights would be on low, if the bulb was in the socket.
- touch the negative lead of the multimeter to the negative battery post
- touch the positive lead of the multlmeter to each wire in the bulb socket
One should read 12 volts. That wire is the power wire for the low beams. The others should have nothing.
- touch the positive lead of the multimeter to the positive battery post
- touch the negative lead of the multimeter to remaining two wires in the bulb socket.
One should read 12 volts. That is the ground wire which makes the remaining wire the power wire for the high beams. Now for the 'fun' part: a voltage drop test.
- plug the bulb back in
- set the lights to high
- take a small safety pin or needle and back probe the headlight connector for the high beam wire.
- touch the negative lead of the multimeter to the negative battery post
- touch the positive lead of the multimeter to safety pin
You should read 12 volts if everything was working properly. But since its not, you'll either read somewhere between 0 and 12 volts. If you read zero volts, either you didn't stick the safety pin in deep enough to make contact with the wire in the connector or there is a break in the wire between the connector & the positive battery post. If you read some amount of voltage, then the wire is damaged/corroded somewhere between the connector & positive battery post. Either way, to find the problem in the wire, walk up the wire, back-probing at any connectors until you get a 12 volt reading. The problem will be between the first place you read 12 volts and the last place you got a less then 12 volt reading.
Hope that helps... Good luck...