Low beams not working

[Mr Diesel]

1983 GMC with quad headlights.

The other day the low beams stopped working. They DO work when I turn on the high beams, though. All 4 lights come on when I turn on the high beams, but when I dim them to low beams I get nothing. Replaced the dimmer switch on the floor and no change. What is confusing me is how they come on with the high beams. There is only one positive wire going to low beam so I can't figure out why that wire is hot only on high beam. Any ideas?

[VileZambonie]

Did you check the bulbs? They are dual filament

[Mr Diesel]

Yep. Swapped in another set of those too. Still nothing on low.

BTW, how can they be dual filament? I see only two wires, a green positive wire and a black ground wire on them. Wouldn't a dual filament need two positive wires?

[jaredts]

I suspect Vile missed the quad headlights.  You should probably try and cipher the schematics in the tech section.  I don't know the circuit, but my guess would be that there's a low beam relay that's gone out and the high beam relay still works.

[Mr Diesel]

Hmm. I was under the assumption that there were no relays, just power running through the switch and to the lights. I will look at my schematics!

EDIT:

Okay, I figured it out. You were right, Vile. The low beam elements were burned out.

I was thinking the lower two lights were the low beams and the upper two high only, but it is the opposite. I had swapped out the lower two bulbs, not realizing those are high beam only. DUH. Now I know how quad headlights work. No wonder I was scratching my head all confused, I swapped out the wrong two headlights! Let's pretend this never happened... 

[VileZambonie]

A lot of people make that mistake. Glad it fixed it. H4656 Dual filament peeps