OEM Tow Package (Lights on Travel Trailer vs Utility Trailer) - Mercedes-Benz Forum
 
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post #1 of 4 (permalink) Old 12-09-2015, 05:53 AM Thread Starter
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Date registered: Sep 2013
Vehicle: 2011 GL450
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Question OEM Tow Package (Lights on Travel Trailer vs Utility Trailer)

I'm not very tech savvy so hopefully my question isn't too confusing. We have a 2011 GL450 with the factory installed tow package. We pull a 25 foot camping travel trailer and have never had any issues with the lights, turn signals hazards or brake lights on the trailer once plugged into the vehicle.

This past weekend, I attempted to tow our enclosed utility trailer with the GL (usually use my other vehicle) but had a problem with the lights once I attached it to the trailer hitch and plugged it into the GL electrical outlet. Here's the problem I experienced.

When I attached the camping travel trailer to the hitch and plugged in the power cord to the power outlet on our GL, all the lights, brake lights and turn signals worked fine on both the travel trailer and the GL. I disconnected the camping travel trailer and hooked up the utility trailer and plugged it in to the power outlet on the GL. Once plugged into the GL, the brake lights on the utility trailer came on without the brake pedal even being pressed and remained on. The turn signals, hazard lights and trailer running lights (the lights over the wheel fenders and along the top of the utility trailer) did not come on. However, the brake lights, hazard lights and turn signals worked fine on the GL

I took it to our local RV dealer and they checked (tested) the power outlet on the GL and found nothing wrong with it. The brake lights, turn signals and hazard lights all worked when he tested it on his tester. So, I went home and hooked up the utility trailer to my vehicle (my vehicle is used to tow the utility trailer) as well as the camping travel trailer and everything worked fine on the vehicle and both the camping travel trailer and utility trailer.

Puzzled at this point, I proceeded to hook the camping travel trailer back up to the GL once more and everything worked fine. I then hooked up the utility trailer one more time to the GL and the problem still exists (only the brake lights come on without the brake pedal being pressed and stay on). The running lights, turn signals and hazard lights don't come on, on the utility trailer but the work fine on the GL.

Sorry for the long winded post but I am stumped as to why the lights won't work on the utility trailer when connected to the GL and only the brake lights on the utility trailer come on and remain on, even when the brake pedal in the GL is not being pressed. Does anyone have any idea what might be happening? Could it be a fuse? I have know idea. Any assistance would be most appreciated.
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post #2 of 4 (permalink) Old 12-09-2015, 07:13 AM
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Does one of the trailers have normal bulbs and the other has LED's? The GL can be programmed for one or the other, but not both. At least with my '08, I can not change the programming myself. I have to have it done at MB (Grrrr....!!! - I should be able to do that myself!). I have mine programmed for normal bulbs, and then an adapter for when I'm towing a trailer with LED's. It plugs (first) into the 7-pin receptacle on the truck, then the trailer plugs into that. It tricks the computer into thinking that normal lights are plugged in.

2008 GL320 CDI
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post #3 of 4 (permalink) Old 12-09-2015, 12:24 PM Thread Starter
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Originally Posted by DennisG01 View Post
Does one of the trailers have normal bulbs and the other has LED's? The GL can be programmed for one or the other, but not both. At least with my '08, I can not change the programming myself. I have to have it done at MB (Grrrr....!!! - I should be able to do that myself!). I have mine programmed for normal bulbs, and then an adapter for when I'm towing a trailer with LED's. It plugs (first) into the 7-pin receptacle on the truck, then the trailer plugs into that. It tricks the computer into thinking that normal lights are plugged in.
Thank you so much Dennis! Yes, the camping travel trailer has bulbs and the utility trailer has LED's and as I mentioned in my post, my vehicle usually pulls the utility trailer. Do you mind me asking what adapter you use (name, model, etc.)? Thank you again for the response. Great forum and great people.
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post #4 of 4 (permalink) Old 12-09-2015, 03:01 PM
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Check out the first two products at this link: 7-Way Electrical Adapters Curt is a great company - USA made, too! You can get these through Amazon (still sold through Curt) for half that price). I started out with the 7-5 adapter and it worked flawlessly. Later, I found out my friend's boat trailer (when we take his boat, we use my truck... and vice-versa) uses a 7-pin connector (I don't know why - it doesn't have electric brakes, or anything). I never really took notice before because my other truck is a 3/4 Suburban (also diesel) and that truck doesn't care if the trailer's lights are regular or LED.

Anyways, I had to do something different because a trailer-side 7-pin can't plug into the Curt adapter's flat-5. I ended up getting another vehicle-side 7-pin with a pigtail and splicing that to a trailer-side flat-5. So with his trailer, I use both things. Curt sells something that would work, as well (the second product), but with the way I did my "splice thing", it's just a little more convenient to use than the Curt product with how I hook his trailer up to my GL. I can also get that 7-pin and flat-5 very cheap because I work part-time at a boat dealership... so that was part of it, too.

2008 GL320 CDI
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