Adding a trailer hitch

Member Query: I added a trailer hitch to my 2012 ML but have trouble with the wiring for the trailer lights. I’m told the SAM (Signal Acquisition Module) must be changed out and programmed for the trailer light current load. Is this true?

Yes it is – IF you choose to splice the trailer wiring into the car wiring as shown below.

(Picture courtesy of StarTuned magazine)

Such wire taps (blue object) upset the current draw to the SAM and can result in unexpected or failed lighting issues.

If you have to replace a SAM, there are some additional steps you will need to follow to complete the repair. Whichever SAM you replace (Front, Driver’s, Passenger’s, or Rear), you will have to tailor it to the rest of the vehicle. Once again you will have to perform “Initial Startup” of a module. You are then basically given two choices, either pull the information out of the old control unit and install the data into the new SAM, or manually enter the SAM information if you cannot communicate with the offending SAM (or module). This second option will require that you know what options are on the vehicle and how those options are configured. Keep in mind that the purpose of the SAM is to control multiple functions that are close by. When replacing these SAMs, be sure to check all of the functions the SAM is in charge of.

Typical SAM (Picture courtesy of StarTuned magazine)

I too chose to add a trailer hitch to my newly acquired 2014 GLK 350. The hitch was easy – I found a Draw-Tite brand hitch at for a reasonable price. It was easy to install – no drilling was required. See below

Trailer wiring is a different story. MBs starting in 1996 use a CAN bus to transmit signals to the on-board devices, including external lights. (CAN means Controller Area Network, “Bus” is an electronic term for a device or system that links multiple components together). I have received several technical inquiries regarding repair of MBs with botched trailer wiring.

Consequently, add-on electrical devices such as trailer light connections cannot be simply spliced into existing wires in the vehicle. If you try to “fix” a CAN bus by rewiring it, or adding a component that was not originally part of the system, the network won’t work properly. Splices, replaced wires, wires that do not have the correct turns per inch in the braiding, etc., will change the length of the wiring, which will interfere with network operation.

A search of MB literature convinced me to not directly connect any trailer wiring to the existing CAN wiring for the tail, stop and turn signal circuits.

Fortunately I found a trailer wiring system that does not require splicing into the car wiring. has a wiring system from that does not require splicing into the CAN wiring for the lighting on the car. It is the only product in the industry that has ZERO contact with the vehicle’s electrical system. It uses sensors that easily snap around wires for no direct contact. (See picture below). Tekonsha also has electric trailer brake controls available for larger trailers so equipped.

When electrical current is detected, the lights are turned on. No cutting or splicing vehicle wiring is required. The system isolates trailer light operation from the tow vehicle’s electrical system.  It eliminates all Interference and electrical draw on the vehicle’s lighting system.  It is rated for 4.2 amps for turn/stop lights and 7.5 amps for taillight trailer circuits.

These sensors easily snap over wires to detect an electrical current and engage trailer lights – crucial for Mercedes-Benz with complex wiring in which snipping, cutting, or splicing voids the vehicle’s electrical system warranty. The system can be moved to your next vehicle with the same easy installation.

Only two wires are used for direct connection: 12 volts from the battery and a ground wire. The tail, stop and turn signal wires are not spliced into the trailer wiring. Instead the Tekonsha kit includes four inductive pickups that are simply installed on the appropriate turn, stop and backup wires. (See figure below – inductive pickups are covered in gray foam to eliminate rattling).

As installed in 2914 GLK right side tail light access

Available from

Submitted by George Murphy

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