Tail light wiring

Tiny
SSROBERTSON
  • MEMBER
  • 2000 GMC SIERRA
  • 4.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
I am working on a custom truck that has had the taillights shaved. When they did this they removed the tail light wiring harness and junction box. I'm trying to wire in custom lights (led strip) and the problem I'm having is all the wiring shows me to wire everything to the left side. If I do this I'm getting brake lights or left turn signal but not both. If I run a jumper wire to make both work it is back feeding to the dash turning on the turn signal indicator light and an odd buzz.
Friday, May 21st, 2021 AT 7:52 PM

59 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,055 POSTS
Hi,

If it is back feeding, you likely have a ground issue. Power will find the easiest way to ground.

If you look below, I attached the exterior lighting schematic. I highlighted the right side lights and ground. Make sure the ground is good.

Here is a link you may find helpful when testing:

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

Let me know what you find or if you have other questions.

Take care,

Joe

See pics below. Note: I had to cut the pic in half to make them readable. I did overlap them so you can follow from one to the next.
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Friday, May 21st, 2021 AT 9:34 PM
Tiny
SSROBERTSON
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I have hooked this up every way possible, the issue is I can't get it to do both. If I hook it to the left turn signal it that works fine. If I hook it to the brake light wire I get brake lights but no turn signal. Then if I run the jumper it does what is explained above but I get both to work with the turn signal lighting up first and then switches to the brake. Two different led bars have done this the same way. They are meant to plug into a 4 flat trailer harness but I do not have the wires to install a 4 flat trailer adapter since I do not still have the tail light harnesses. So I am trying to hardwire these. I have 5 wires for 5 functions and one of those is a ground.
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Saturday, May 22nd, 2021 AT 6:35 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Have you checked all grounds? For it to back feed to the signal indicators in the vehicle, power is looking for a ground.

Joe
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Saturday, May 22nd, 2021 AT 7:01 PM
Tiny
SSROBERTSON
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I have went as far as grounding the light directly to the frame and still same issue. I wondering if it has something to do with the fact the junction box isn't there, or the fact I'm using LED.
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Saturday, May 22nd, 2021 AT 7:04 PM
Tiny
JACOBANDNICKOLAS
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Hi,

The idea that you are using LEDs can cause issues. They require less power. Are they run in a series with incandescent lights?

Joe
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Sunday, May 23rd, 2021 AT 6:59 PM
Tiny
SSROBERTSON
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There is incandescent in the front of the truck. It is causing a hyper flash, but I do have LED for the front and a LED relay to install still. But I don't feel that should be causing my brake lights to not come on?
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Sunday, May 23rd, 2021 AT 7:06 PM
Tiny
JACOBANDNICKOLAS
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Hi,

I agree with you, but anything is possible. Since things were modified, we can't be sure exactly what was done. Just for the heck of it, install the front LEDs and the relay and see what happens.

Let me know what you find.

Joe
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Sunday, May 23rd, 2021 AT 7:23 PM
Tiny
SSROBERTSON
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The truck wiring seems to be stock under it, just unplugged from the junction box and taillight harnesses. But I will try that anyway to see what happens.
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Sunday, May 23rd, 2021 AT 7:25 PM
Tiny
SSROBERTSON
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Do you happen to have a wiring diagram for the plug that would plug into the junction box from the front of the truck?
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Sunday, May 23rd, 2021 AT 7:28 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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The junction is seen in pic 1 below. I attached all the rear lighting connections and legends for your review.

Let me know if this helps. Also, (and this is an afterthought), I wonder if the vehicle came with factory trailer wiring. Something could be happening there allowing power to back feed.

It's just a thought.

Let me know.

Joe

See pics below.
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Sunday, May 23rd, 2021 AT 7:50 PM
Tiny
SSROBERTSON
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Thank you, those pin outs may help. I have only used a test light so far to verify wiring. Without having the rear harness in not certain if it had factory trailer wiring. This truck did have an older LED bar on it before but it no longer works correctly. So no idea if it was ever installed correctly.
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Sunday, May 23rd, 2021 AT 8:07 PM
Tiny
JACOBANDNICKOLAS
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Hi,

That is likely the issue. Someone never did it right the first time, and now you are trying to figure out what the heck they did. It's frustrating, but we are the doers. Take your time and you'll figure it out.

Just for curiosity, when you say the old one didn't work correctly, what was happening? I'm wondering if the lack of a resistor may be causing the back feed.

Let me know.

Take care,

Joe
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Monday, May 24th, 2021 AT 7:17 PM
Tiny
SSROBERTSON
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Actually after working on it some more today I installed the front LEDs and the LED flasher. I ended up checking the wiring in the back and I actually found the trailer wiring harness. I wired it in and I now have all functions, but now the issue is when I turn on the running lights the turn signals stop working. I went back to the old lights in the front and they started working but now with hyper flash. One step forward and one step back.
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Monday, May 24th, 2021 AT 7:25 PM
Tiny
JACOBANDNICKOLAS
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Hi,

If you reinstalled the incandescent lights in the front, the hyper flash is due to the resistor. The system thinks there is an open. If you reinstall the old relay, do they work properly?

As far as finding the trailer wiring, I suspect (but can't be 100% sure) that is where the problem is coming from. Ugh! I wish I could see exactly what has been done. LOL

Do me a favor. Have a helper go through all the lights with you. At the trailer wiring harness, check which wires are getting power based on what is being requested.

I attached the wiring schematics for the trailer wiring harness. Check to see if things match up. Also, note that the signals and parking lights should be powered from two different sources. One other thing. Do the hazards flash when you have the parking lights on?

Focus on the yellow, dark green, and brown wires in the harness.

Let me know.

Joe
See pics below.
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Monday, May 24th, 2021 AT 8:10 PM
Tiny
SSROBERTSON
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The trailer wiring is the only thing that's giving me turn signals and brake lights both. All the wires seems to be responding correctly. The old relay being put in makes the turn signals work with the running lights on. The only other thing I noticed is with the all LED lights and relay, when I had the turn signal on with the running lights and it wouldn't flash I used a test light to check some wiring and when I put the test light to the turn signal it flashed.
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Monday, May 24th, 2021 AT 8:23 PM
Tiny
JACOBANDNICKOLAS
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I'm not sure what you checked when you stay you put the test light to the turn signal, but I can say that the test light had to be grounded. So, it sounds like you provided a ground.

Let me know.

Joe
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Monday, May 24th, 2021 AT 9:02 PM
Tiny
SSROBERTSON
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The left turn signal in the trucks trailer wiring. And it definitely provided a ground. But trying to ground that harness with another ground does not fix the issue. I'm wondering if the resistance from the light bulb in the test light made it work?
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Tuesday, May 25th, 2021 AT 2:18 AM
Tiny
JACOBANDNICKOLAS
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Anything is possible. However, the resistance is slight in that bulb.

I know people have had to put resistors in line for LEDs, but not at the ground. If the test light caused it to work because of ground, you should be able to run a jumper wire in its place to the same location and it work. Honestly, that doesn't make sense.

Let me know. It sounds like you have your hands full with this one. Nothing is making sense. LOL

Joe
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Tuesday, May 25th, 2021 AT 8:07 PM
Tiny
SSROBERTSON
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My hands are definitely full. I have tried a straight ground and it blew the fuse. I'm going to try a LED resistor inline and see what happens.
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Tuesday, May 25th, 2021 AT 8:36 PM
Tiny
JACOBANDNICKOLAS
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Let me know what happens. I'm not sure why a jumper wire would cause the fuse to blow if the test light didn't. Are you certain you connected the jumper to the ground circuit and not a power supply?

Joe
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Tuesday, May 25th, 2021 AT 9:21 PM

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