I was inspecting my lights the other day and I realized that although my regular headlights were working fine, one of them would go out when I turned my brights on! This freaked me out at first, I thought I had gremlins. After playing with it I realized that the primary headlights are dual element, and switch over to a dimmer secondary element when the brights are turned on. Turns out this secondary element in one of my headlights is toast, hence that light going out when the brights come on.

What is the purpose of this design?

 

I'm assuming the purpose is to create brighter lighting when on brights - if you disconnect you hi beam lights and flip the switch to hi, the low beam lights actually shine brighter and higher in the air. Its always been this way as long as I can remember with GM 4 light setups.