Left Side Trailer Brake Light Not Working

06Voy · April 7, 2009

I believe the brake lights are fused together so it could be a ground problem causing one not to light up. I have had problems like this in the past and they are a pain to track down.

bill190 · April 8, 2009

Using the above wiring diagram which is for a 2009 and noting that yours is a 2004, there is a signal called 900_bcm_brk_ped which I assume means brake pedal. You can copy that and search for it in the wiring diagram.

On one page it shows a switch along with this signal. I can't tell if that would be the brake pedal switch or not?

Anyway this signal goes to the BCM. And I assume left turn is a separate signal to the BCM as is hazard.

Then on page D-276, it shows a signal for *both* left stop and turn comming from the BCM and going to a relay. So these signals seem to be together at this point. Seems to me if one works (left turn), then both would work at this point (So long as the BCM is sending the signal that is)???

So I guess the question is if Relay-TRLR STOP/TRN LT on page D-276 is clicking for both left turn and stop?

Then on page B-15 it says...

TRLR STOP LT

PCB Relay

And above that is says: Fuse Block – Underhood, Device Usage – Gas

So if this 2009 wiring diagram works for a 2004, look under your hood and listen for two relays clicking (left and right trailer). Perhaps it says left trailer relay on the cover? No that would be too easy!

Anyway if that relay is not clicking... Well it seems to be controlled by the BCM. See if it is clicking with left turn, but not with stop...

bill190 · April 8, 2009

BTW... Have you owned the truck since brand new, and have there been any non-factory modifications to the BCM programming? (Have you had the BCM "flashed" by anyone?)

c4cruiser · April 8, 2009

Using the above wiring diagram which is for a 2009 and noting that yours is a 2004, there is a signal called 900_bcm_brk_ped which I assume means brake pedal. You can copy that and search for it in the wiring diagram.

On one page it shows a switch along with this signal. I can't tell if that would be the brake pedal switch or not?

Anyway this signal goes to the BCM. And I assume left turn is a separate signal to the BCM as is hazard.

Then on page D-276, it shows a signal for *both* left stop and turn comming from the BCM and going to a relay. So these signals seem to be together at this point. Seems to me if one works (left turn), then both would work at this point (So long as the BCM is sending the signal that is)???

So I guess the question is if Relay-TRLR STOP/TRN LT on page D-276 is clicking for both left turn and stop?

Then on page B-15 it says...

TRLR STOP LT

PCB Relay

And above that is says: Fuse Block – Underhood, Device Usage – Gas

So if this 2009 wiring diagram works for a 2004, look under your hood and listen for two relays clicking (left and right trailer). Perhaps it says left trailer relay on the cover? No that would be too easy!

Anyway if that relay is not clicking... Well it seems to be controlled by the BCM. See if it is clicking with left turn, but not with stop...

The '09 wiring diagram should be different from the GMT800 trucks. The GMT 800 trucks used a separate turn signal lamp where the GMT900 uses the upper and lower bulbs for both turn and stop lamps. If there is a separate relay for trailer turn signals, then the problem could be there.

Also, try the 4-way flashers on the truck with the trailer connected. If the lights on the trailer work with the 4-ways on, then there should be no ground issue.

As far as bulb filaments, a two-filament bulb has one for tail lamp and the other for both turn and brake. If the brake light works, and the tail light works on that side, the turn signal problem is not with the bulb.

Groondogg · April 8, 2009

I checked all the fuses again and they are all working fine. I replaced the trailer lights ones with new ones just in case. Looking at the trailer harness diagram, it says that the trailer left signal and brake light are the yellow line. When doing a circuit test on the yellow wire, I get a positive reading when the left turn signal is activated. When the brakes are activated, I do not get a reading. I'm not sure what to do now... I don't really want to drop a pretty penny at the local dealership/shop to get this fixed.

Thanks again to everyone who has replied...

Groondogg

bill190 · April 8, 2009

Well we tried with that '09 wiring diagram above, but no good!

So I guess this means getting an '04 wiring diagram and tracing the lines for the left trailer wiring in the truck. Troubleshoot with a multimeter.

If you know how to use wiring diagrams, then a factory service manual set is well worth the cost (my set included 4 books, one of which was all the wiring). My books paid for themselves in 1 month.

OmarR · April 9, 2009

After reading this thread, I studied my shop manual for the last hour or so. I have an 03 Tahoe, but your 04 should be wired the same.

After looking at the schematics, I firmly believe, IMHO, that the problem lies with the turn signal/hazard module. This module takes the appropriate signal from the turn signals AND the brakes and applies voltage when it is supposed to. So this module, if malfunctioning, could not apply stop lamp supply voltage to the trailer.

On a side note, the truck's stop lamps do NOT go through this module, so that would explain why those DO function.

I believe this module would be the "equivalent" to the converter box (on other vehicles) mentioned in this thread.

I have never changed it, but it's mounted directly to the underside of the left interior fuse box. You may have to pull the fuse box out or maybe remove the panel beneath the steering wheel so you can gain access to behind the fuse box.

Maybe someone who has actually changed it can post up how, but I don't see how it could be that hard.

I really think that changing this out will fix your problem.

Let us know how it goes!

Groondogg · April 9, 2009

After reading this thread, I studied my shop manual for the last hour or so. I have an 03 Tahoe, but your 04 should be wired the same.

After looking at the schematics, I firmly believe, IMHO, that the problem lies with the turn signal/hazard module. This module takes the appropriate signal from the turn signals AND the brakes and applies voltage when it is supposed to. So this module, if malfunctioning, could not apply stop lamp supply voltage to the trailer.

On a side note, the truck's stop lamps do NOT go through this module, so that would explain why those DO function.

I believe this module would be the "equivalent" to the converter box (on other vehicles) mentioned in this thread.

I have never changed it, but it's mounted directly to the underside of the left interior fuse box. You may have to pull the fuse box out or maybe remove the panel beneath the steering wheel so you can gain access to behind the fuse box.

Maybe someone who has actually changed it can post up how, but I don't see how it could be that hard.

I really think that changing this out will fix your problem.

Let us know how it goes!

The funny part about that is that I pulled the the panel from beneath the steering wheel and found a group of modules and wondered if those were the ones.

I will look into this and hopefully it will solve the problem. Thanks again for everyones help and I will let you know if this solves it.

Griptruk · April 10, 2009

The funny part about that is that I pulled the the panel from beneath the steering wheel and found a group of modules and wondered if those were the ones.

I will look into this and hopefully it will solve the problem. Thanks again for everyones help and I will let you know if this solves it.

I hope that's it! Do the next guy a favor and take a picture of where it is and of the actual unit while you're in there.

I hope this solves it, and that the part is cheap if you have to replace it!

what a frustrating issue.

-Jer

RalphG · April 30, 2009

Wondering if you ever solved your left trailer brake light problem. I realized tonight that I have the same EXACT problem with my 04 Tahoe. I have towed trailers since I bought the Tahoe new and all lights on Tahoe and trailer have worked just fine. Last spring my boat trailer lights worked fine but I didn't tow anything since then until now. I recently purchased a motorcycle trailer and after checking the trailer lights I realized that my left brake light on the trailer doesn't work. ALL lights on Tahoe work fine. Left turn signal on trailer works fine and ALL other lights on trailer work fine. I took a tester to the trailer plugin on back of vehicle and sure enough no voltage to left brake/turn terminal when brake is applied.

I would love to hear if the problem was indeed under the dash and exactly what was replaced.

Thanks in advance.

Ralph

sledr · July 14, 2009

Wondering if you ever solved your left trailer brake light problem. I realized tonight that I have the same EXACT problem with my 04 Tahoe. I have towed trailers since I bought the Tahoe new and all lights on Tahoe and trailer have worked just fine. Last spring my boat trailer lights worked fine but I didn't tow anything since then until now. I recently purchased a motorcycle trailer and after checking the trailer lights I realized that my left brake light on the trailer doesn't work. ALL lights on Tahoe work fine. Left turn signal on trailer works fine and ALL other lights on trailer work fine. I took a tester to the trailer plugin on back of vehicle and sure enough no voltage to left brake/turn terminal when brake is applied.

I would love to hear if the problem was indeed under the dash and exactly what was replaced.

Thanks in advance.

Ralph

I too am wondering if this has been solved. I have an 04 Silverado that has towed since day 1. The past few winters it has been hell getting the left trailer brake light to work. I would have to wire brush the connectors and use dielectric grease and everything would work. I plugged in a camper this past week and no left brake light. I took an emery board to the connector and hit the trailer end with some dielectric grease and both trailer brake lights came on after cycling through turn signals and 4 ways. When we left the campsite no left brake light. All fuses are good.

dudeman618 · July 22, 2009

Wondering if you ever solved your left trailer brake light problem. I realized tonight that I have the same EXACT problem with my 04 Tahoe. I have towed trailers since I bought the Tahoe new and all lights on Tahoe and trailer have worked just fine. Last spring my boat trailer lights worked fine but I didn't tow anything since then until now. I recently purchased a motorcycle trailer and after checking the trailer lights I realized that my left brake light on the trailer doesn't work. ALL lights on Tahoe work fine. Left turn signal on trailer works fine and ALL other lights on trailer work fine. I took a tester to the trailer plugin on back of vehicle and sure enough no voltage to left brake/turn terminal when brake is applied.

I would love to hear if the problem was indeed under the dash and exactly what was replaced.

Thanks in advance.

Ralph

I too am wondering if this has been solved. I have an 04 Silverado that has towed since day 1. The past few winters it has been hell getting the left trailer brake light to work. I would have to wire brush the connectors and use dielectric grease and everything would work. I plugged in a camper this past week and no left brake light. I took an emery board to the connector and hit the trailer end with some dielectric grease and both trailer brake lights came on after cycling through turn signals and 4 ways. When we left the campsite no left brake light. All fuses are good.

I had the same problem a couple months back where one trailer blinker wasn't working. I checked all the fuses on the side of the dash, and they were fine. I had diagnosed it to the truck, I have two trailers and both had the same problem. It took me a long time to figure it out, and even took the truck to the dealer. I didn't know to check the fuse box under the hood. One fuse was blown.

-Robert

2002 Tahoe

riff · August 5, 2009

Wondering if you ever solved your left trailer brake light problem. I realized tonight that I have the same EXACT problem with my 04 Tahoe. I have towed trailers since I bought the Tahoe new and all lights on Tahoe and trailer have worked just fine. Last spring my boat trailer lights worked fine but I didn't tow anything since then until now. I recently purchased a motorcycle trailer and after checking the trailer lights I realized that my left brake light on the trailer doesn't work. ALL lights on Tahoe work fine. Left turn signal on trailer works fine and ALL other lights on trailer work fine. I took a tester to the trailer plugin on back of vehicle and sure enough no voltage to left brake/turn terminal when brake is applied.

I would love to hear if the problem was indeed under the dash and exactly what was replaced.

Thanks in advance.

Ralph

I too am wondering if this has been solved. I have an 04 Silverado that has towed since day 1. The past few winters it has been hell getting the left trailer brake light to work. I would have to wire brush the connectors and use dielectric grease and everything would work. I plugged in a camper this past week and no left brake light. I took an emery board to the connector and hit the trailer end with some dielectric grease and both trailer brake lights came on after cycling through turn signals and 4 ways. When we left the campsite no left brake light. All fuses are good.

I had the same problem a couple months back where one trailer blinker wasn't working. I checked all the fuses on the side of the dash, and they were fine. I had diagnosed it to the truck, I have two trailers and both had the same problem. It took me a long time to figure it out, and even took the truck to the dealer. I didn't know to check the fuse box under the hood. One fuse was blown.

-Robert

2002 Tahoe

riff · August 5, 2009

I'm having the sa,e problem with my 04 siverado hd diesel. One thing I notice, is that when the left brake light on trailer dosn't work, I can turn on the left signal and then the left brake light will work for awhile. Anyone else try this?

Cliffhanger · August 17, 2009

I just had this exact same problem with my '05 Sierra 2500HD myself and it was the LCM (Light Control Module aka Turn Signal/Hazard Flasher Module). To get to it you have to remove the beauty panel below the steering wheel. Once you remove the panel you will see the module directly behind the fuse box at the bottom of the box (the fuse box faces towards the drivers door, and the LCM faces towards the steering column).

To remove it pull it straight out towards the steering column (it is black in color, has 10 blades, slides in and out like a fuse would and the end you can see measures 1" x 2-3/4").

Part #'s:

GM OEM: 15764135 (about $90 from a dealer)

Napa Auto Parts (OEM Replacement): LM-455 ($32.99)

Napa Auto Parts (LED Compatible): LM-487 ($76.99)

Hope this helps!