Turn signal and brake light not working

ZAC CONNER

MEMBER
2010 FORD F-150
5.4L
V8
4WD
AUTOMATIC
110,000 MILES

Recently bought, has a brake light and blinker light that does not work. Consequently, the brake and blinker light also doesn’t work on that side if I have a trailer hooked up. I replaced the bulb, checked the fuses and relays and they were all working. But when checking the power going into one of the relay ports. One was working as it should, the other wasn't receiving any volts. (On a side note, after taking the relay out for the working blinker light, it still worked without the relay in. And I don't know why) I later made a discovery, with the running lights on, the nonworking light begins to work but it is very dim, and is out of sync with the rest of the blinkers. This leads me to believe it is a grounding issue, but I can’t seem to find where the faulty ground is at. Any tips or help on fixing this would be greatly appreciated!

5 Replies

KASEKENNY

MECHANIC
18,907 POSTS

Which side is this happening on?

I am attaching the wiring diagram where I highlighted the wiring for the left side. It is not much different if this is the right side.

However, I suspect you have an issue with the smart junction box. I suspect you are finding that the lights still work on the side that is working, when you pull the relay because you are pulling the trailer tow relay for that side and not the actual turn signal relay.

This vehicle uses what is called a FET and not a relay. There is a circuit board on the SJB that decides when to send voltage and the FET is there to simply control the current flow.

So again, I suspect the issue is in the SJB. So let's test voltage coming out and if it is a good 12 volts on the wiring going to these lights then you are probably correct that the ground is the issue.

https://www.2carpros.com/articles/how-to-check-wiring

Let me know what questions you have.

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Friday, February 19th, 2021 AT 1:29 PM

ZAC CONNER

MEMBER
4 POSTS

Thanks for the help. I tested the voltage in the SJB and it was giving me a solid voltage. I switched the bulb sockets around, that didn’t work. I started following wires back and found a rusted ground wire going into the frame. Cleaned it up and now everything is working. Maybe I wasn’t looking at it right, but it appeared that multiple lights were being run off that ground (hard to tell without cutting the wiring harness up and I'm no electrical expert). But if that's the case, and that ground was the issue, why where the other lights working?
The main thing is that everything is working. But i'm hoping to learn something extra. I have not done hardly any electrical work and i'm not fluent at reading a wiring diagram, lol.

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Tuesday, February 23rd, 2021 AT 7:46 PM

KASEKENNY

MECHANIC
18,907 POSTS

If you can give me the general location of the ground, I can probably track it down and find out what is run off of it. However, the other lights would work because many of them use different grounds and even redundant grounds. So if one fails or corrodes, it will still find a path to ground. Electricity with take the easiest path to ground so if one is corroded, it will simply be routed to the other if it has a second. However, I suspect the reason for this is there are different grounds for the lights that were working.

Thanks for the update and I am glad you got it resolved.

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Wednesday, February 24th, 2021 AT 7:15 PM

ZAC CONNER

MEMBER
4 POSTS

As you seen from previous post, I had the issue resolved. Today halfway through my drive, it stopped working again. I went back and cleaned all the connections and grounds like I done last time but it didn't fix it like last time. Do you think it is a connection/ground issues? Or does this sound like a problem that can happen with the Smart Junction Box?

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Thursday, March 4th, 2021 AT 3:17 PM

KASEKENNY

MECHANIC
18,907 POSTS

I would lean towards the Smart Junction Box but it could be a ground wire and not a connection issue. What I would do is check the resistance of that ground wire just to make sure the wire does not have high resistance. It should be around a half ohm of resistance.