71 Challenger - no low beams, only high beams (RESOLVED)

[Ricomondo]

I'm helping a friend with his 71 Challenger issue:

Started out with low beam headlights went out - car already had new dimmer switch, and all other lights work; dash, tail, parking.
But when the dimmer switch is activated on high beam, all 4 lights work.

Thought maybe it was the headlight switch, so he bought a new dash switch and I swapped it out for him. The low beams then worked.

Fast forward to last night he was at a cruise night and when he turned on the headlight switch, the same issue, no low beams, so he had to drive home with all 4 lights on high beam.

Before I trek out to his place, looking for suggestions?

Could the low beams on both side be bad? If so, why would they go on when the high beam is activated?
Could the new dimmer switch be bad?

TIA!

[71vert340]

 Could be a bad dimmer switch. Here's where I always start on electrical problems. With the electrical schematic and a multimeter. I know its a hassle and time consuming but you can find it. By the way, rather than driving home with all 4 headlights on and blinding other drivers, reach in behind the headlights and unplug a couple of them. I had to do this once on a car.
Terry W.

[Jay Bee]

Easy to read colored wiring diagrams here (just in case you weren't aware of them).

https://forum.e-bodies.org/electrical-and-audio/11/colored-wiring-diagrams-70-cudachallenger/1349/

[Ricomondo]

Thank you everyone -
I brought my FSM and tester to his house, and after some testing we wound up replacing both low beam headlights and that solved it!

Thanks again!

[anlauto]

Thanks for the update, odd that both headlamps had the low beam filament burnt out at the same time...

[71vert340]

Thank you everyone -
I brought my FSM and tester to his house, and after some testing we wound up replacing both low beam headlights and that solved it!

Thanks again!

Glad it was something simple.
Terry W.

[Ricomondo]

Thanks for the update, odd that both headlamps had the low beam filament burnt out at the same time...

Absolutely! - But I'm glad I was able to help him out. He's in his early 70's and not much of a "car guy" when it comes to mechanical stuff. He wasn't even sure when the last time the low beam bulbs were changed. They were quite crusty at the plug ends. Gonna change out the highs next time I'm over so they are all brand new and the same brand. Plus totally forgot I had a FSM until someone posted the schematic above...lol.

thanks again everyone!