Headlight brown down

[chaklid]

helloo y'all,

am experienced, but cannot figure out why when i first start up the bike the head light comes on and, both, high and low lights burn normally. Then, after riding and came home, i noticed that the high beam was browned out or very dimm. the next day when i first start up again, both high/low beams lights are normal, then, same thing, normal, bright low beam, but dimmed, weak high beam. ???? anything i'm missing??

thanks

chakli

1979 SF

[John]

Maybe check for corrosion in the dimmer switch in the left handlebar controls. Could be heating up and creating high resistance.

[ManagerMike]

The headlamp relay would be my first suspect, the contacts are likely heating up and creating a voltage drop. Since the high beam draws more current than low, it would be affected to a greater extent.
Bypass the relay and see if your symptoms go away, I replaced mine with a standard bosch type relay from napa, ten bucks or so.

[crazy steve]

I'd look at the RLU (reserve lighting unit); this is supposed to turn on the opposite filament if the other one fails. It will turn on the high beam IF the low beam fails, and is designed to dim the high beam down to roughly the same level as the low beam when low is selected. You shouldn't have both filaments energized at the same time in any case, it should be one or the other.

Running both filaments at the same time will shorten lamp life, and may overload the headlight ground.

[crazy steve]

I'll also note that federal regs specifically prohibit having both headlight filaments on at the same time, with the only exceptions being momentary operation of both while switching between them or 'signaling' i.e. flashing your highs when passing, etc.