Left Side Trailer Brake Light Not Working

Groondogg · April 7, 2009

I have tried everything to try and fix this problem, but nothing has worked. All of the fuses are working properly. The actual wiring to the trailer lighting plug in is not receiving the left brake light power... The left trailer turn signal works, but not the brake light. My father-in-law said something about the trailer lighting converter box may have had a fuse go bad, but I have not been able to find any information about where that is.

Any help would be greatly appreciated!!

Vehicle is a 2004 GMC Sierra

Thanks in advance.

Groondogg

Bowler807 · April 7, 2009

This is from memory because it was over 4 years ago. Just after I got my 06 Sierra I took my camper for its State Inspection. If memory serves, one of the brake lights wouldn't work. I remember we checked the wiring for an hour or so. What it turned out to be was a fuse in a compartment I didn't know the truck had. On the side of the dash just inside the drivers door is a fuse panel. I found a blown fuse. After replacement everything worked.

Groondogg · April 7, 2009

This is from memory because it was over 4 years ago. Just after I got my 06 Sierra I took my camper for its State Inspection. If memory serves, one of the brake lights wouldn't work. I remember we checked the wiring for an hour or so. What it turned out to be was a fuse in a compartment I didn't know the truck had. On the side of the dash just inside the drivers door is a fuse panel. I found a blown fuse. After replacement everything worked.

I checked every fuse in that compartment, and each one was functional... Thanks for the reply though!

Griptruk · April 7, 2009

The fact that the turn signal works and the brake light doesn't is very interesting. To me that would indeed point to some kind of converter box like you mentioned, although I do not know of it's location if such a thing exists.

according to

http://www.gmupfitter.com/publicat/2007_BB...00_Elec_D3b.pdf

on page A47

there is a wiring diagram.

It mentions trailer stop/ turn relay controls located in the BCM (body control module)

This is from the 2009 electrical manual, the 2004 version on that site does not mention such a thing, but I also didn't find a wiring diagram so it's hard to say what it has.

I'd love to see how this issue is resolved.

-Jer

cujo8 · April 7, 2009

My B-I-L just had a similar problem with his Durango and he tracked the problem down to converter box that was part of his towing harness. He ordered a new harness and the problem was fixed.

bill190 · April 7, 2009

Does the left stop light and left turn lights on the truck work, but left turn for trailer works and not left stop?

So you have the left stop light working on the truck, but not on the trailer?

(If working by yourself, use a small mirror with an adjustable stand so you can angle it to see the trailer/truck lights or place a brick on the brake pedal.)

Do the hazard lights work on both lights on the truck and trailer?

Is there any kind of electronic control box on the trailer?

Groondogg · April 7, 2009

Does the left stop light and left turn lights on the truck work, but left turn for trailer works and not left stop?

So you have the left stop light working on the truck, but not on the trailer?

(If working by yourself, use a small mirror with an adjustable stand so you can angle it to see the trailer/truck lights or place a brick on the brake pedal.)

Do the hazard lights work on both lights on the truck and trailer?

Is there any kind of electronic control box on the trailer?

Every light works on the truck. Hazards, Brakes, Running Lights, Turn Signals... Everything works on the trailer except the left side brake light. The left turn signal and brake light are on the same wire, but only the left turn signal is working.

I have tried several different trailers and they all have the same problem.

It has to be something with a converter box, I just have no idea where that is...

Thanks for the reply

cujo8 · April 7, 2009

Every light works on the truck. Hazards, Brakes, Running Lights, Turn Signals... Everything works on the trailer except the left side brake light. The left turn signal and brake light are on the same wire, but only the left turn signal is working.

I have tried several different trailers and they all have the same problem.

It has to be something with a converter box, I just have no idea where that is...

Thanks for the reply

The converter box on mt B-I-L's truck was on the towing harness near the year of the truck. This was for a Dodge vehicle and was an intergral part of the harness and could not be purchased separately. I know you do not have a Dodge truck, but it would not hurt to follow your wiring harness back to see if you find the converter box. Good luck.

ScottyBoy · April 7, 2009

Is it thw factory 7-pin harness or is it a 4 pin trailer harness? IF its a 4 pin harness on your truck, most likely its an aftermarket harness, so you need to trace it back and find out where its spliced in or plugged in. Every factory hitch I've seen has a 7 pin harness/plug mounted to the side of the hitch. On mine, (2001) all the fuses for it are in the fuse panel under the hood.

Groondogg · April 7, 2009

Is it thw factory 7-pin harness or is it a 4 pin trailer harness? IF its a 4 pin harness on your truck, most likely its an aftermarket harness, so you need to trace it back and find out where its spliced in or plugged in. Every factory hitch I've seen has a 7 pin harness/plug mounted to the side of the hitch. On mine, (2001) all the fuses for it are in the fuse panel under the hood.

Its the 7-pin factory one. I will try and see if I can follow it, but yesterday when I tried, it went to the front of the truck and not sure where it went from there.

ScottyBoy · April 7, 2009

I know you said all the fuses checked out to be good, but double check the fuse diagram and make sure none for the trailer harness are blank or missing a fuse. Somebody could have blown one of your other fuses and stole one of the trailer light fuses to replace it. I've had a few electrical issues like that where I finally realized the fuse was completely missing.

ScottyBoy · April 7, 2009

the 7 pin should have fuses for it under the hood. I'd double check eveything in that panel then

Draenor · April 7, 2009

the 7 pin should have fuses for it under the hood. I'd double check eveything in that panel then

+1 check these too. Also did you add the 7pin or was this factory?

kal el · April 7, 2009

there are fuses for the trailer under the hood and in the fuse compartment on the driverside dash, I would check both, is there another truck to plug the trailer into to see if it works correctly. Also just as a shot check the bulb in the trailer the bulb should have 2 filaments, one for turn the other for stop, one filament could be burnt out and the other still good, worth a shot.

Groondogg · April 7, 2009

there are fuses for the trailer under the hood and in the fuse compartment on the driverside dash, I would check both, is there another truck to plug the trailer into to see if it works correctly. Also just as a shot check the bulb in the trailer the bulb should have 2 filaments, one for turn the other for stop, one filament could be burnt out and the other still good, worth a shot.

7 pin was factory installed. I will check both fuse compartments again, but I'm almost 100% sure they were all good. I will let you know what I come up with.