Trailer Lights Problem
 

The towing vehicle is a 2008 ML320 CDI with a factory installed towing package. There have been no problems with trailer lights with our past trailers. In the past two months we have bought two new small trailers. When the first trailer lights were first connected, by the trailer technician, the trailer lights would only flash. I had the vehicle running when the connection was made. I shut the 320 off and restarted it, with the lights connected. The lights then worked and still do.

Two days ago we picked up the second newly built and delivered trailer. It is an enclosed 5' x 8' with an interior light and LED brake, turn signal, fender clearance lights and tail lights. When we hooked up the light harness none of the lights would function except for a couple of tail lights at a low intensity. I went through the same routine of shutting off the engine, re-connecting the light and restarting the 320. No combination of engine off or on, lights off or on, made a difference. The dimly lit bulbs are part of a 5 bulb light bar on each side of the rear of the trailer. These dim lights come on when the lights are connected whether the 320 is running or not and whether the lights are on or not. None of the other light functions work; brake, hazard, turn signal or tail lights.

The technician tested the wiring on both the vehicle and trailer and all checked out as working. He tried a different adapter plug and wiring from this adapter to the trailer and still no lights. The trailer wiring was hooked up to the dealers' portable tester and all the trailer lights functioned as they should. The technician said he had not encountered this issue before, and he has worked at this trailer dealer many years, and said he was not familiar with Mercedes-Benz vehicles. He was the same tech that checked out our first trailer.

We towed the covered trailer home and as a test I hooked up our other trailer. At first the trailer lights flashed then began working as they should. This trailer is also equipped with LED lights although not as many as the new covered trailer. I re-hooked the newest trailer and nothing worked.

Any advice or sharing of similar experience is much appreciated.

I'd need to see a wiring diagram of the car side to determine what is going on. Some trailer light hookups ( car side ) have separate wires for turn signal and brake lights. If these are bridged I'd expect problems.

Try unhitching the trailer but leaving the wire attached and note results. I wonder if grounding the trailer through the ball if the harness ground is open will cause the problems you have.

I've read that some cars use PWM ( Pulse Width Modulation ) to control brightness of car side lights , I wonder it this extends to trailer wiring.

The LED lights on the trailer do not pull enough current so the ML doesn't know that a trailer is even connected, therefore never turning the respective lights on. There are available in the after market an LED inline trailer module to simulate an incandescent load on the circuit. (U-Haul: Moving supplies: LED Light Trailer Module) Thats just one example. Almost all european vehicles require some electrical load to "turn-on" the trailer lighting. I know that a lot of American and Asian manufactures are starting to do this as well for the trailer sway programming in the ESP. If the lighting and trailer stability program was just set to on all the time, the ESP system would just assume a trailer is always connected and your traction control would not operate correctly.

Hope that helps!

Thanks for the response. After digging around I found the solution and you authenticate it. In my case there is a replacement adapter plug from Tow Ready, the TR20141, that has resistors built into it.
When I called the dealer the parts guy knew right away what was needed and gave me the TR20141 number as what I need. It is the plug the M-B dealer stocks.