Tail light wiring

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.

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.

Have you checked all grounds? For it to back feed to the signal indicators in the vehicle, power is looking for a ground.

Joe

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.

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

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?

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

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.

Do you happen to have a wiring diagram for the plug that would plug into the junction box from the front of the truck?

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.

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.

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

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.

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.

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.

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

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?

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

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.

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

Okay, so here's the update: I switched all my lights to LED and replaced the flasher with an LED flasher. When I do this the front corner bulbs aren't lighting up very bright, if you plug them in while the lights are on you do get a quick bright flash before going dim. I removed both corner lights and everything works exactly the way it should, but if I put the bulbs back in I have the same problem as mentioned above. I texted the sockets and I have one side that lights up as a parking light only and the other side will flash while the other side remains on solid. Not sure what the deal is with the front corner lights, but everything works fine without them.

Also during testing I put the normal bulbs and flasher back in and tested everything. The left side still hyper flashed (due to LED rear lights) but the passenger side blinked normal.

Are the side marker lights LEDs? If they are, that could be the cause.

Let me know.

Joe

Yes they are LED. They are the 194 bulb corner lights.

That doesn't even make sense. Did you install the 194s? Are you certain they are LEDs? If they go dim with an LED flasher, that makes sense if they aren't LED's. When you disconnect, are you removing the bulbs or disconnecting the lights from the harness?

Joe

They are definitely LED. I remove them from the socket.

Check the ground for the marker lights to see if there is excessive resistance. Something is causing them to dim.

Where would the ground be, both wires in the socket are positive?

Okay, you will have two wires showing power, but it still grounds through the front park/signal lamp.

Take a look below. I highlighted the things to identify them. Ground goes from those marker lights to the front park/signal lights and then to ground on the left rad support bracket.

I have a very strong feeling either one or two things are happening. Either the ground at the rad support is weak and fails under the extra load, or the splices are bad.

Just for the heck of it, run a good ground for both front signal/park lights. That will indirectly create a new ground for the side markers. If the same problem happens, then it has to be the splices.

Also, if you want to try this. Remove one of the 194 and see if the system does the same thing. Do it on both sides. If it is a splice problem, it should only happen when one of the bulbs is plugged in. In other words, if the left side is the problem, the right side and all lights should work normally when the left bulb is out. That will reduce the load on that splice.

Let me know. I know this may sound crazy, but wiring in and of itself can drive anyone crazy. Hang in there. You are doing a great job.


Joe

I'm pretty decent at figuring stuff out and even wiring, but chasing wiring issues on things I didn't install definitely will drive anyone crazy. Hince, why I'm talking to you, lol. But I appreciate the help and the motivation.
As far as the LED bulbs go, you mention extra load could be an issue, but in reality wouldn't this be a less of a load seeing they are LED and the flasher is considered a no load flasher? I can definitely check that ground. The hardest part about this whole thing is it is a custom truck and technically doesn't have a 2000 Sierra front end on it anymore, it has a 2005ish Silverado front end (cat eye). So anything could have happened when changed out, but I feel this is a newer issue.

Okay, so I went out and checked the ground and it is working good. I went as far as cutting the marker light out and wired it to a different ground that I ran and if I hook up one wire or the other (turn or parking) and ground the other side of the socket I get the bulb to work. But it's only one or the other not both.

Hi,

If you run two new grounds, will they both work?

Let me know.

Joe

They will only work with either the turn signal or the marking light but not both.

Ugh!!! LOL Once again, that makes no sense. Okay, check for power to both of the power supplies when each of the two circuits is actuated. If we have power, it doesn't leave much, a socket or bad LED.

I have to be honest, I love a challenge, but this one just doesn't make sense.

Stupid question. If you have all the LED installed and remove the resister relay and replace it with the original one, what happens? I realize flashers will hyper flash, but do all lights work?

We have power to both. LED is good, checked in other sockets and I can make it work in the current socket if I hook up ground and one of the leads (turn or marker lead). So to me the socket seems good as well.
If I change the flasher to original with all LED bulbs it doesn't do anything... No flash at all. Only way to make the original flasher work is to put the incandescent bulbs in the front and then I get hyper flash and everything works.

Hi,

As you're already aware, this makes no sense. LOL The absolute only thing I can think of is if when everything is connected (even though they are LEDs) there is enough of a power draw to cause failure at a connection. Keep in mind, when you have a

I have one more thing to check before I ask the site owner for his opinion. Connect everything so that the lights go dim when turned on. Check voltage at the marker lights. Realize that LED flasher is going to drop voltage, I want you to check it when everything is connected and then when one of the marker lights is disconnected and all others are working normally. I just want to see if there is a power drop.

Let me know.

Take care,

Joe

Will check this tomorrow. thanks

Sounds good. Let me know what you find. Also, I am going to ask Ken for his thoughts. He is excellent with these types of issues.

Take care,

Joe

I appreciate it! I truly would just leave this light out, but the truck has to pass inspection so unfortunately it has to be there.

Voltage stays the same, no drop with or without bulb. Parking light and turn signal.

Ugh!!! Then what could be causing them to dim? Okay, let's look at this from a different point of view. We have focused on the marker light causing the issue. If voltage doesn't change, there has to be an increase in resistance to a ground. I don't want to ask this because you likely already told me, but have you checked grounds for the signals and parking lights as well?

Also, keep in mind that the ground wire may be in good condition, making good contact, but the ground source may be in poor condition. I don't know where you are or if rust is an issue, so I thought I would mention that idea.

Joe