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Old 05-08-2019, 01:54 PM   #1
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Trailer lights problem

I'm rewiring my boat trailer for lights and I'm having an issue that has got me completely perplexed. I have wired many trailers without any issues so this is driving me crazy. I have tail lights on driver and passenger side but no brake light or turn signal on the passenger side. If I turn the tail lights out then I have brake light and turn signal on the passenger side. What in the world would cause the brake light and turn signal to stop working on the passenger side when I turn the tail lights on? I'm using a brand new pre-wired harness so I'm sure it's not the harness . Please help me keep my sanity and help me figure this out
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Old 05-08-2019, 01:59 PM   #2
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Poor ground.
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Old 05-08-2019, 02:02 PM   #3
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Poor ground.
That's what I initially thought but I ran a dedicated ground from the plug back to both lights with no difference and I'm grounding through the trailer frame with no difference.
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Old 05-08-2019, 02:14 PM   #4
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Make sure your plug on the TV matches the harness. I verified what each connector on the TV plug was and made sure I matched the color on each side. Son and I pooched his on his boat trailer....by not paying attention...and maybe should've waited on the beer. It was a quick easy fix when we figured out what we did.
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Old 05-08-2019, 02:28 PM   #5
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Make sure your plug on the TV matches the harness. I verified what each connector on the TV plug was and made sure I matched the color on each side. Son and I pooched his on his boat trailer....by not paying attention...and maybe should've waited on the beer. It was a quick easy fix when we figured out what we did.
What doesn't make sense is the passenger side brake and tail light will work as long as I don't turn the tail lights on so I know they're hooked up right
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Old 05-08-2019, 03:09 PM   #6
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What doesn't make sense is the passenger side brake and tail light will work as long as I don't turn the tail lights on so I know they're hooked up right
That's because its using the taillights for a sneak path ground. Once you turn the tail lights on, you removed its ground path. You still have a bad ground somewhere.
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Old 05-08-2019, 03:34 PM   #7
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Are you connected to the correct side of the bulb? I have seen the same thing when the turn/brake is hooked to the run side of the bulb. During the day, sometimes you don't notice the difference. Just a thought. Been there, done that, didn't get the shirt![emoji16]
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Old 05-08-2019, 03:46 PM   #8
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That's because its using the taillights for a sneak path ground. Once you turn the tail lights on, you removed its ground path. You still have a bad ground somewhere.
How can I go about finding where the ground is bad I have hook the ground through the trailer I'm grounded the lights with it additional wire back at the lights to the frame and I have also hooked a dedicated ground wire from the plug back to the lights neither one makes a difference
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Old 05-08-2019, 03:51 PM   #9
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You are connected to the correct side of the bulb! I have seen the same thing when the turn/brake is hooked to the run side of the bulb. During the day, sometimes you don't notice the difference. Just a thought. Been there, done that, didn't get the shirt![emoji16]
And I've also seen this when someone jimmied the bulb into the socket incorrectly. Make sure the bulbs are installed with the proper pin orientation.
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Old 05-08-2019, 04:18 PM   #10
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If "bulbs" work at all, the largest single issue with trailer lights not working correctly is the ground.

Second most common is what 5picker stated if the lights are old-school using removable bulbs. Not uncommon for the two filament bulbs to be inserted with the pins in the wrong slots. Sockets are so flimsy today the bulb can be often turned no matter which way they are installed and sometimes the contacts on the bulb sit between the contacts in the socket, making both filaments light up no matter which one is supposed to.


If LED's I'd disconnect the lights and using a separate power source (wires from an uninstalled 12 v battery) and check to see if the lights work properly when disconnected from the trailer wiring.

FWIW, if it's a painted trailer frame and the ground wire for the lights is sandwiched between light housing and frame, with nut and lock washer on other side of frame, take a grinder to the frame where the ground contacts and add a "star washer" to the assembly.
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Old 05-08-2019, 04:20 PM   #11
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How can I go about finding where the ground is bad I have hook the ground through the trailer I'm grounded the lights with it additional wire back at the lights to the frame and I have also hooked a dedicated ground wire from the plug back to the lights neither one makes a difference
You have a picture of what you are installing including the color of the wires you are connecting on the fixture?
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Old 05-08-2019, 04:20 PM   #12
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Trailer lights problem

I agree with the wiring/bulb base issue. I also assume it is incandescent...as led would be an entirely different issue. Remember Kim Young Un’s revenge. Could be a quality issue in manufacturing that has you chasing your tail. One other thing. Sometimes the tungsten wire can break and reconnect inside and do strange things. Did you try a new bulb?

Another story. I have seen more than one bulb burned at the same time....head and tail. Usually ask if they have boosted anyone? The standard answer is yes and here in lies the problem.
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Old 05-08-2019, 04:22 PM   #13
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Scratch the paint off of the ground wire connection. Then try it.
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Old 05-08-2019, 04:25 PM   #14
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You have a picture of what you are installing including the color of the wires you are connecting on the fixture?
I'm going yellow brown on the driver side green Brown on the passenger side
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Old 05-08-2019, 04:27 PM   #15
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I'm going yellow brown on the driver side green Brown on the passenger side
Let's see a picture.
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Old 05-08-2019, 05:01 PM   #16
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Let's see a picture.
I want to put this out there before I can get pictures I'm wiring this to a Honda Civic which is 5 wire and running a module on the car 2 convert to a 4-wire set up could it be possible that the module is bad
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Old 05-08-2019, 05:15 PM   #17
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I want to put this out there before I can get pictures I'm wiring this to a Honda Civic which is 5 wire and running a module on the car 2 convert to a 4-wire set up could it be possible that the module is bad
Well that certainly throws a monkey wrench into the situation!

Heck yes it could be bad.

Swap the working side wires to the non-working side and it will eliminate the bulb, grounds, feed wiring and the housing.

If that works, I'd say you have a bad module.
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Old 05-08-2019, 05:45 PM   #18
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Well that certainly throws a monkey wrench into the situation!

Heck yes it could be bad.

Swap the working side wires to the non-working side and it will eliminate the bulb, grounds, feed wiring and the housing.

If that works, I'd say you have a bad module.
Okay I'll try that and see where that gets me
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Old 05-12-2019, 06:15 PM   #19
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Okay I'll try that and see where that gets me
Okay I tried a little experiment I took the lights completely off of the trailer and hook them up right directly to the harness coming out of the car and one light is burning both filaments when on tail lights the other light is burning just the low filament. One turn signal the one is turning the high side of the ball the other is burning the low side of the bulb. Here is a picture of what I am seeing would this indicate that I've got a bad moduleClick image for larger version

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Old 05-16-2019, 04:31 AM   #20
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Okay I tried a little experiment I took the lights completely off of the trailer and hook them up right directly to the harness coming out of the car and one light is burning both filaments when on tail lights the other light is burning just the low filament. One turn signal the one is turning the high side of the ball the other is burning the low side of the bulb. Here is a picture of what I am seeing would this indicate that I've got a bad moduleAttachment 204418Attachment 204419
Well I got it working with a new module and new LED lights everything is working now
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