Troubleshooting 7-Way on Mercedes ML-350 Where Trailer Lights Dont Work

Published 12/08/2015

Products Featured in This Question

Combination LED Trailer Tail Light - Submersible - 6 Function - 11 Diodes - Passenger Side

(1285)

$20.84

Hook Probe Circuit Tester

(66)

$14.40

Question:

I just had a new hitch and 7pin connector installed on my Mercedes ML350. I went to tow my boat that is on a trailer with incandescent bulbs and when I hooked everything up the trailer lights wouldnt work. Im told the new trailer hitch and connector is set up for a trailer with LED lights and therefore is not putting out enough power to light the incandescent bulbs. My travel trailer has LED lights so Im OK Is there a solution so that I can tow both trailers with the same vehicle?

asked by: Paul J

Helpful Expert Reply:

If your vehicle is set up to only be able to tow a trailer with incandescent lights. Then you would be able to replace all of the trailers lights with LEDs and it would, in theory, work just fine.

Since we have never heard of a vehicle being only set up to tow a trailer with just LED lights I suspect you may have something else going on. Especially since German vehicles typically have the opposite problem where they don't like LED trailer lights.

First thing you should do at this point is test the 7-way of the wiring harness with a circuit tester part # PTW2993 for voltage on all of the right pins when the taillight signals are being applied. Just like in the video I attached.

If the voltage checks out there but the trailer lights still don't function right then the problem is on the trailer wiring. That would mean that problem is either due to a ground issue or a short issue.

Make sure that the trailer side connector are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces. The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

If the problem is on the vehicle side then I would first recommend you double check how the harness is installed on your vehicle, make sure all of the connections are good and tight. You should check all towing related fuses and relays as well.

I also attached a link