Rear drivers side signal/brake light not working properly?

2011 FORD F-150
113,583 MILES • V6 • 4WD • AUTOMATIC
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JB42
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My rear driver's side turn signal and brake light only work if the head lights are off. Blinkers do blink fast even after changing the bulbs and I have no running lights on the rear drivers either. What would be causing this?
Oct 27, 2022 at 10:37 AM
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JACOBANDNICKOLAS
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Hi,

When the headlamps are on, does the front signal lamp work? Also, does anything work on the left rear?

I attached a pic from the wiring schematic. There is a fuse designated for the left turn/brake lamp. See pic below.

The more I think about what is happening, it isn't making sense. The two circuits are not tied together. Check the fuse I mentioned. If that is good, we may have either a ground-related issue or the body control module (BCM) may be faulty.

I found ground 202 (G202) is for the headlamp switch and the multi-function switch. See pic 3 for location. I have a feeling that the ground is weak and when you turn on the headlamps, the ground is too weak and causes failure to the left rear light assembly. Check that ground for any damage, corrosion, or wiring issues.

Let me know.

Take care,

Joe

See pics below.
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Oct 27, 2022 at 6:53 PM
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JB42
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Yes, when the headlights are on the left front and mirror turn signals work. Nothing works on the left rear when head lights are on including left rear running lights.
Oct 28, 2022 at 11:06 AM
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JACOBANDNICKOLAS
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Hi,

Were you able to locate the ground I mentioned to inspect it?

Let me know. Also, let me know if you have questions or if I can help in any way.

Take care,

Joe
Oct 28, 2022 at 9:55 PM
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JB42
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Located the ground, no corrosion or breaks that I could see, took it off and cleaned it just to double check. Still have the same issue.
Oct 29, 2022 at 2:35 PM
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JACOBANDNICKOLAS
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Hi,

If that checked good, we either have a faulty body control module. Although the headlamps and the rear light are not tied directly together, they both use the BCM to power the different lights. So, I suspect something may have failed internally.

If I had to guess, it is at the BCM. What I suggest is to go to the BCM and check the gray wire with a green tracer for power when the headlamps are on. It will be pin 12 in the pic above in my last post. If there is power at the BCM pin 12, inspect the connector for damage. However, I don't think that is the issue.

I attached the wiring schematic for the headlamps. Take a look through them and you'll see what I mean. The two symptoms don't seem to fit together.

Let me know your thoughts.

Take care,

Joe

See pics below.
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Oct 30, 2022 at 7:39 PM