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Discussion Starter #1
I am wondering if any one else has come across this – I have an 2004 GMC3500 when I ordered it I got the trailer towing package, I had only pulled my tag trailer up until recently when I bought a 5th wheel set up. I took it to the local hitch place had them install the rails on the floor of the bed and to put a plug for the trailer lights in the bed, Since then I can only get one brake light to work on the trailer truck works fine and that is on the passenger side, I took it back to the trailer shop they went all thought the fuses couldn’t find any thing. Everything worked ok before they moved the plug and the two brake lights get there signal from the same wire – I also tried a friends trailer same thing and the hitch place also says it’s in the truck and they don’t think they did anything wrong but cant figure out what to do. Any thoughts?
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Broken wire or loose connection.
 

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You said only one brake light works on the trailer. What about turn signals?
 

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remuda;1573057; said:
and the two brake lights get there signal from the same wire –
Cheers
That statement right there has me worried. In a semi, yes that would be the story, but in a pickup and just about every trailer designed to be pulled by one, each brake light gets its power from its own wire....the same wire that runs the blinkers. Thus the reason why there are 4 wires on a simple trailer connection rather than 5.
 

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and the two brake lights get there signal from the same wire
Nope. The left and right turnsignals each have their own wires. When you use the truck turn signals only one of the trailer lights flashes accordingly. When you apply the brakes, BOTH wires are energized to make BOTH lights come on together. The turn signals and brakes use the same filaments within the light bulb. This is the standard arrangement. Your 5er might be different, but I dont know why it would be.
My guess - a broken wire or bad connection or the new plug is mis-wired. Can you temporarily plug the 5er into the factory plug on the back bumper? If it works ok from there then the trailer guys probably goofed somewhere - very likely the wires within the new plug.
 

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Ther are more fuses than you think. In the dash fuse panel, find "lt turn trlr and rt turn trlr." One of them is blown.
 

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Discussion Starter #7
thank you for your thoughts eveyone to follow up on some of the points.The truck comes with a trailer plug from the factory - and it all worked fine when I took it to them, as I mentioned they put the plug in the bed for me - now the rear plug that GM put in does the samething will only light on brake light, same as the one in the bed. They are saying it might be the body control mod - I had never heard of this controler before. I hate to take it back to them and let them hack and cut and I can only imagine what a GM dealer would charge to play with the problem.
 

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Read the owners manual under fuses or the panel itself. It is very easy to pop these fuses (I think they're 10amp). Turn on the four ways and use a $3 test lamp and probe the little hole in every fuse. Now as far as whoever installed the wiring, I would check their wiring. It's simple.

Slide back the boot at the trailer plug and see if they used the light blue wire. It should remain unused. Left turn and brake is yellow, right turn and brake is dark green. The light blue is for third brake light only. There is a light green that is for backup lamps only.

As far as the BCM goes, think simple. I'm not saying it's bulletproof, but you'd have to be wiring somewhere besides the back of the truck to confuse or damage it.
 

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This diagram may help. It is from the 2004 Helms manual. As you can see, the flasher splits the brake/turn function for the trailer and each side is protected by its own fuse. Starting at the lower right corner, count left one column and go up to the 4th and 5th fuses. The 4th fuse is for the RT TRLR ST/TRN and the 5th one is for the LT TRLR ST/TRN. Both are supposed to be 10A fuses. Hope this helps.
 

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Now knowing that the factory bumper plug is also malfunctioning it sounds like Heymccall and BillB are on the right track in thinking one of the trailer turn signal fuses is blown in the left dash fuse panel. There would be no reason for them to disturb the existing wiring to the bumper plug.
I realize you said the trailer guys checked all the fuses - and surely they wouldnt miss these ones since they deal with this stuff all day every day - but sometimes when you want it done right, ya gotta do it yourself.
 

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Did they "tap" into your existing factory plug wires?
 

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On my o5 there are two plugs 1) under the rear bumper 2) bundled to the frame under the bed/cab spacing. It is hidden there specifically for mounting in the bed and is already hot. If yours was like this they should not have needed to "wire" anything, just drill a hole and mount the plug. check you manual.

Chad
 

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Discussion Starter #14
Thank you to every one that took the time to try and help me - I have been very busy on a project but will get at the problem this week, I will post the out come. Thanks again to every one.
Cheers
Mack
 

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there are trailer light fuses under hood in your fuse box,one for left and one for right
 
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