Posted 18 November 2013 - 01:43 PM
This could be a wiring problem with the headlamp circuit.
With the lamps on check for voltage on all three wires with the headlamps on low, do the same with all three on high.
On low you should have 12v on two wires, and close to 0v on the third. (The third will be the low side ground wire).
On high there should be 12v on two of the wires (the low side ground being one of them), and close to 0v on the third. The third now being the high side ground.
Anything more than 1V on the ground side wires indicates a wiring or connection problem to ground. Corroded frayed or broken wiring, poor connection at the head lamp dimmer switch, poor contact in the dimmer switch, poor ground for the dimmer switch, or poor connection in the junction connector where the wiring comes through the firewall.
Measure battery voltage with the headlamps on lo and hi. If the feed wire voltage drops more than 1v lower than battery voltage there is a problem with the power feed circuit. Poor fuse connection, bad fuse, poor relay connection, high resistance in the relay contact, or corroded frayed or broken wire.