Turn indicator ALSO headlight indicator? Dumb question maybe...

[joshd1971bronco]

Putting the finishing touches on my ‘76 frame-off. I installed the American Autowire harness from WH. I’m pleased with the harness, well labeled and quality connectors. Powered it up and everything appears to work EXCEPT when I turn the headlights on, the left turn indicator lights up (stays solid).

Interestingly, this indicator does blink when I turn the left blinker on - whether headlights on or off. It’s almost like it’s wired to use the left turn indicator as a ‘headlights on’ indicator as well as a turn indicator.
Everything else works as expected - brights, park lights, exterior bulbs, etc.
Did the truck originally do this? I don’t remember it acting this way.
Other pics just cuz

 

[Broncobowsher]

Bad ground on the front turn signal bulb.
Take both front turn signal bulbs out and check again. If the light is out, bad ground/wiring.

Since this is aftermarket wiring and I am not 100% how the harness is built, if the light is still on with both turn signal bulbs removed, unplug the headlights. I don't know if they share and turn signal grounds.

What happens: You have a bulb with 2 filaments that share a single terminal. The outer sleeve of the base that goes to ground. If that outer sleeve doesn't ground correctly, electricity looks for it's own ground. So power through the first filament, to the common terminal that isn't a ground, backwards through the second filament. Now the second circuit (in your case the turn signal) is getting power from the light bulb that has a bad ground. The lights will still work because there is still enough of a voltage drop to light up the bulb.

Another thing, LED bulbs. These can do weird things. Since they are diodes there is a polarity sensitivity to them and you have 2 in a single case. And the low power demands mean they can light up rather well on trace voltages. There is a lot of different build qualities in them so no generic answer will cover all of them. I recommend going to regular bulbs to diagnose the circuit and go back to LEDs once everything is working right.

 

[joshd1971bronco]

Thanks for the detailed reply. I’ll do the tests tonight and report back.
I’m NOT using LED bulbs anywhere - in the cluster or on the exterior.
And to be clear, all the exterior bulbs - turn signals, headlights, brake lights, are functioning correctly.

 

[B RON CO]

Hi, X2 on the grounds. The headlight bulbs ground through the black wire on the plug and the
parking lamp/ directional grounds through the bucket mounting screw, along with the side lamps and tail lights.
Make sure the screws make good metal contact.
Also make sure the negative battery post is grounded to the engine and the sheet metal body. I go from the negative battery terminal to the engine block and radiator support, and from the engine block to to firewall.
Good luck