No taillights/brake lights with headlights on

1989 FORD F-150
200,000 MILES • 6 CYL • 2WD • MANUAL
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DAVID ENGELMAN
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  • 3 POSTS
Hello. I discovered this morning when my headlights are on neither my taillights or my brake lights work. If I turn my headlights off, my brake lights work fine.
I do have an issue with my turn signals not blinking as well. I will be replacing the turn signal flasher in the fuse box today at lunch. Any chance the two issues are related? If not, what could be causing the original issue?

Thanks in advance.
Oct 1, 2020 at 5:39 AM
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DAVID ENGELMAN
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Update: i changed the flasher. Turn signals flash as long as i'm not pressing the brake. If i do, the flashers do not work. I turned on my hazards and they worked until i press the bakes. Taillights and brake lights still do not work with the headlights on. Does this sound like i have two bad bulbs?
Oct 1, 2020 at 10:32 AM
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JACOBANDNICKOLAS
  • CAR REPAIR CONTRIBUTOR
  • 110,193 POSTS
Hi,

I just went through the wiring schematics and am finding it hard to determine what can cause what you described. Take a look at the two attached pics. They are the schematics for the system.

The one thing I am questioning is the headlamp switch itself. Take a look through these and let me know if they help.

Joe
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Oct 1, 2020 at 7:50 PM
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DAVID ENGELMAN
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Thank you! I ended up replacing both the headlight switch and the turn signal switch. The problem still existed. I was about to give up when I found a wire shorting in a harness under the back by the brake lights. I fixed that and everything began to work. It was an expensive process, but both units needed to be replaced anyway and I learned something, so I still consider it a win.
Oct 2, 2020 at 10:51 AM
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