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O/T - Question about towing a trailer

Discussion in 'Off Topic Threads' started by GBatch_25, May 20, 2011.

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  1. GBatch_25

    GBatch_25 Active Member

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    Have a 2005 GMC Denali Yukon XL with 5.3 liter v8 plus the factory trailer hitch and plug for the lights.
    Occasionally, I may have to tow a 10-12' enclosed trailer and when I hook up the light cable, the brake lights on the trailer do not come on when the brake pedal is depressed. The lights on the trailer do work, but not brake lights. What's possibly the problem. I admit almost complete ignorance on this.

    So, how do I troubleshoot this?

    Gene Batchelar
    Wheaton, IL
     
  2. Shooting Jack

    Shooting Jack Active Member

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    Gene, first let's get some basic's. Is this a 7 pin plug, an adapter to 4 pin Flat? Does the Yukon have the factory plug connected under the dash for trailer brakes? Whichever plug is used pull it up off the internet and it will tell you which pin supplies power to what. For example on a 4 pin color code is yel is left turn, green is right, brown is runing and white is ground. It uses the same colors on the 7 pin I belive but adds several others that are used for trailer brake, power for lights, outlets etc and one other that I don't remember but you get the idea. Once you check the plug from your vehicle you can determine whether it's the Yukon or the trailer wiring. Jackie B.
     
  3. grntitan

    grntitan Well-Known Member

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    Gene--Its hard to pinpoint via the internet but as a truck driver i can tell you 99% of lighting problems on trailers are ground issues. Make sure everything is grounded good.--Matt
     
  4. Pull & Mark

    Pull & Mark Well-Known Member

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    Matt is l00% correct. Its more than likely a ground issue. Take both the truck and trailer to a repair shop that deals with trailers and have them fix it. You need heat shrink adhesive lined connectors for each connection. This seals out the water and stops the rust as well as possible. If your handy do all the grounds before you take it in yourself. Good Luck and Break-em all. Jeff
     
  5. trim tab

    trim tab Active Member

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    My 35 years as a new car dealer and the vehicle wiring has not been altered, it is on the trailer side and like the above posts, it is probably a gorund issue.
     
  6. 221

    221 Banned User Banned TS Supporters

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    It could just be the bulbs, thats the first place to look. No turn signals either?
     
  7. ken a

    ken a Member

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    Shooting Jack, Taking your problem to a shop sure will work and maybe the best for you. However I would recommend you get a GOOD standard test light and use it to determine if you are getting power to the light bulb in question. You obviously have power on the wires feeding the tail light. Next with the test light check the wires for the brake and turn signal. If so you have bulb or connection problem. Everyone with a trailer should have a test light and voltage tester. My .02
     
  8. GBatch_25

    GBatch_25 Active Member

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    Thx guys. I have some investigative work to do.

    Gene Batchelar
    Wheaton, IL
     
  9. halfmile

    halfmile Well-Known Member

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    He says the trailer lights work. This tells me it is not a ground issue.

    If the turn signals work, the problem is in whatever brake switch is in the truck. If not there may be wiring probs in the trailer.

    Your truck probably has a round connector. (my 05 Trailblazer does) I notice that sucker does not go all the way in very easy. Will have to get some dieelectric grease for it.

    I had to buy an adaptor to flat so I could use my old 4 pin trailer plugs. At last reckoning, they work.

    Harbor Freight has a 2.99 voltmeter that is peachy for these little chores.

    They also have a set of lights with magnets that are for towing cars, with a flat connector. These can be had for 10.99 with a coupon from the paper or their site. I bought them cause the wires alone were worth that.

    A 12V running light bulb with a pigtail socket makes a very nice simple tester.

    HM
     
  10. grntitan

    grntitan Well-Known Member

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    HM--You'd be suprised what a loose or broken ground can do. It can still allow most all lights to work but no properly. The problem is, neither i nor you or anyone else here can reliably diagnose the problem unseen via internet. That said, your guess is as good as mine or anyone elses for that matter. Trailer lights can be a pain in the ass sometimes. The best method is to take all the advice given especially the test light requirement and start hunting and back tracking. I've spent more than my share of time searching and scratching my head only to find a simple loose or corroded ground wire. Just say'n....................
     
  11. psfive

    psfive Member

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    I always run a single separate wire for ground-----always. Use a simple pull apart connection in the center and fastened permanently on both ends. More than once the trailer plug goes in and some work and some do not, connect the ground and everything now works. Paint, rust. dirt and corrosion. All can cause problems with grounding. Just my .02 worth. Paul in Nebraska.
     
  12. Duck Head

    Duck Head TS Member

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    Have a helper watch when you press on the brakes. If you try to do it yourself, the lights will go out by the time you run to the back of the trailer to see if they're on. No, check the plug connections for juice first. If you have tail lights and turn signals(yes,no,yes,no) you have a ground.The turn signal uses the same element as brake lights in an 1157 bulb if you have them on your trailer.
     
  13. Brian in Oregon

    Brian in Oregon Well-Known Member

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    <i>"He says the trailer lights work. This tells me it is not a ground issue."</i>

    Uh, no. Logically you'd think so, but a bad ground throws logic out the window.

    The problem is that it is possible for the brake lights to backfeed through another circuit.

    I've seen this happen more times than I care to think about.

    Always make sure the grounds are 100% first. Then proceed to the other circuits.
     
  14. rooferbob

    rooferbob Active Member

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    Check the basics first, The fuses probably blown fuse in the trailer towing box under the hood. These are seperate for truck brake lights and trailer brake lights.
     
  15. letts

    letts TS Member

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    You can buy a plug in device that will tell you that all is well at the plug on your tow rig. If the tester indicates that your rig is not lighting up the brake lights, check the fuses, Trailer light have their own fuses which are separate from the tail lights on your tow truck. After that checks out Ok it must be the trailer wiring. As the problem seems to be in the brake lights, Put the emergency flashers on and start looking for the problem with the trailer wiring. As someone said, a bad ground connection will cause all sorts of odd thing to happen such as when the left turn signal is on, All lights will flash, not brightly but will still be flashing as the electricity is looking for a path back to ground through the hot wire circuitry. Good luck.

    Letts
     
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