Troubleshooting Trailer Wiring 2011 Mercedes GL350 Converted to LED No Brake Lights

Updated 10/08/2018 | Published 10/07/2018

Products Featured in This Question

7-Way to 4-Way/5-Way Trailer Adapter for Vehicles with Lamp-Out Sensors Towing Trailers with LEDs

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Question:

My 2011 Mercedes GL350 came with the tow package installed at the factory and has a 7-pin circular trailer connector. My 2005 boat trailer currently utilizes incandescent tail and side marker lights. The wiring harness is a two-wire system and has a 4-pin flat connector at the hitch. I purchased a 7-way to flat four-pin adapter when I bought the boat. All lights works well with incandescent lights. I am tired of replacing the rear brake/running/turn indicator light bulbs because they burn out when submerged in water so I bought sealed rear LED lights - which seem to be the most readily available lights these days. I did not change the wiring and simply replaced the old lights with LEDs. Because the LED lights have three wires, I drilled a hole and ran the white ground wire to the metal trailer frame when installing the lights. After wiring the LED lights, the running lights work but the brake lights and turn indicators do not. I do not receive any fault warning on the dash console but I didnt drive the trailer around to test for faults. You have great resources on your web site but it is still unclear to me whether the problem is a. the rear turn indicators are separate from the brake lights and require am electronic turn signal converter or b. the current draw on the SUV is less using LEDs than incandescent bulbs which prevent them from working. Also, do I benefit in any way to rewiring the trailer to a three-wire system? I am assuming the extra wire is simply the ground... If the solution involves installation of a electronic turn signal converter, how hard is that to install? When I get a new Merican made SUV next year, do I need to undo everything I am doing now to get the trailer lights to work with this German one? I appreciate your help!

asked by: Martin

Expert Reply:

Since we aren't totally sure if the issue you have is due to your vehicle not wanting to work with LED lights or if it's due to the trailer wiring having a problem let's first determine which issue is at fault. Using a 12 volt jumper wire from a source like a vehicle battery's positive post try jumping the voltage to each of the stop/turn circuits to see if you can get the brighter illumination to come on. If so then the problem is that your Mercedes doesn't like the LED Lights and you'd need an adapter like the part # C35FR.

If not, then the issue is wiring related and you need to find out why the voltage on both of those circuits isn't making it to the assemblies. Check the circuits for cut or exposed wiring and fix as needed. Converting to a separate wiring system on both your vehicle and trailer would not make figuring this issue out any easier. If anything it would add more complications.

expert reply by: Jameson C