2009 ML350 Factory 7 Pin Trailer Plug Dim Tailights

Discussion in 'M-Class' started by mb fan, Jul 12, 2017.

  1. mb fan

    mb fan New Member

    Anyone found and fixed this problem before? Trailer plug pin for running lights has low voltage. About 4 volts while pins for power, left and right turn signals have proper 12 v or 11.4v. Traced wiring harness to passenger side rear compartment and some of the wires go to F70 fused power source, ground on body and the rest go to a plug connected to some kind of box (not sure what it is). No fuse charts are found on any of the fuse boxes (engine, passenger side next to glove box, passenger side rear compartment).
     
  2. Kajtek1

    Kajtek1 Awesome Member

    Do you have MB trailer plug? How did you get it in USA?
    I was using the lights converter for my E class and yes, the W211 tail lights operate at lower voltage in normal setting.
    They get full car voltage when you switch to fog lights.
     
  3. mb fan

    mb fan New Member

    My 2009 ML350 came with factory installed trailer hitch and plug. So not imported part by me. I've used a 7 pin to 4 pin flat adaptor for several years and had not had any problems. Recently when connected my trailer, tailights were dim. When plugged into other vehicle, trailer lights all work normal. So started troubleshooting plug on ML350. Low voltage is present with ML350 lights on auto or on position.
     
  4. Kajtek1

    Kajtek1 Awesome Member

    Is it still working OK with old trailer?
    I would get voltmeter and check all the prongs on plug while having lights on.
     
  5. mb fan

    mb fan New Member

    I have one trailer and two vehicles. Trailer lights work perfect on second vehicle but tail lights are dim when plugged into ML350's 7 pin RV plug via converter to 4 pin flat. This had worked perfect for several years. Voltage on taillights pin is 4.07v and 11.57v on pins for signal lights.
     
  6. Kajtek1

    Kajtek1 Awesome Member

    Now try with fog lights on?
    You have weird problem that not likely many US owners have.
    So at some point you will have to consider drive to a stealer.
     
  7. mb fan

    mb fan New Member

    No change in voltage readings with fog lights on with engine running or fog lights on with engine off. Have not found a schematic that includes the trailer plug wiring, which would help determine if one is used to control voltage to the plug and module location. Not looking forward to taking to dealer.
     

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