res94025

I hope someone here might be able to provide some insight to an issue that I'm not seeing anywhere else in this (or other) forums. I have a 2010 GL550 with the factory trailer hitch. I've had the standard issue with trailers with LED lights and have dealt with that through use of an adapter. The issue now is that I have a need to install an electric brake controller. Not wanting to have a permanent controller I purchased the Prodigy RF wireless remote controller. After installation I could not pair the front mount unit to the controller mounted on the trailer. In troubleshooting the issue I discovered that I don't have the required constant 12V to pin #4 in the factory trailer connector. In checking the fuse box I discovered that I have only one of the three expected fuses. Fuse #70 (Trailer Hitch 20A) is installed. Fuse #71 (Electric Brake Controller 30A) and #72 (Trailer Hitch Socket 15A) are completely missing. To clarify, by missing I do not mean the socket is there with no fuse installed - the socket itself is missing in its entirety.

I'm assuming that it is fuse #72 (Trailer Hitch Socket 15A) that should ultimately feed 12V power to Pin #4 in the connector - but why oh why is there no corresponding wire fed up to the fuse block? Does anyone know where that wire might be hiding? Or even what color wire I should be hunting for in the general area of the fuse block? Any and all ideas/suggestions/thoughts/help is GREATLY appreciated!

Update (that increases my confusion): This morning I went back out and double checked the wiring in the connector and fuse panel. Pin 4 in the connector is connected to a red wire in the harness, and tracing that wire up to the fuse block it is connected to terminal #70 with a 20A fuse. This appears to be correct from the research I've done as the fuse connector is color coded brown and matches to brown connector on the fuse panel. Although this should be providing 12V power to feed the 12V power at Pin 4 in the connector - there is no 12V power at the fuse with ignition on or off. Of the three "trailer" fuse locations (70/71/72) only fuse 71 has constant 12V power with ignition on or off. But, according to the fuse chart 71 should be power for a hard wired electric brake controller and I would think it should have power only with the ignition switched "on."

Any thoughts on why there is no power at the fuse block for fuse 70? Does anyone know how power should be supplied to each of these locations (switched/constant/delay)? Since I need 12V power I'm wondering if I can just relocate the wire currently at location 70 to location 71 to tap into constant 12V at that location.

lkchris

This is not a direct answer to your question, but the details (and some speculation) could be enlightening ...

I installed a conventional brake controller on my 2007, and when I connected it as specified to my rear SAM everything worked properly. I have the factory trailer hitch and wiring harness, retrofitted post purchase ... which required a different SAM, installed at the time.

My vehicle is an early build 2007 (first year for GL) and the wiring to the front for the controller was not factory installed, nor was it installed by dealer that retrofitted the hitch. This was of no consequence, as I initially pulled only a European motorcycle trailer which had surge brakes. I installed the electric brake controller only some years later when stepdaughter got into the horse trailer scene. There is a retrofit wiring harness available from Mercedes for this and it did include a fuse which I didn’t currently have. I removed a LOT of the interior to run the wiring, but it was otherwise plug and play and everything immediately worked.

Seems obvious you’ve bypassed the rear SAM, but it is my understanding that for your (later) year, the brake controller wiring harness is factory installed when trailer hitch option is factory installed. Still it seems likely operation of rear SAM functions are required to get you the signals you require. I wouldn’t think that with a conventional controller there would be any need for constant power to the trailer plug, only the variable voltage signal required to activate the brakes. Everything that reaches your factory trailer plug is controlled by and comes through the SAM and I doubt it was ever designed to provide the constant signal you require ... except to speculate that the controller connector features that signal designed to be sent to the front to the controller which then sends it back voltage modified. The OE controller connector at the SAM is green, maybe slightly yellow green or dusty green. SAM is under your subwoofer, of course.

res94025

Thanks for your thoughts and observations. Like you, I’ve been towing all kinds of utility and boat trailers with surge brakes with no issues. It’s a new car hauler trailer that has electric brakes that is now creating the need to add a controller. I opted for the wireless controller because (1) I had read lots of horror stories with GL owners trying to install wired controllers and (2) I wanted the flexibility to move the controller between my GL and my Sprinter.

My hitch is also a factory hitch and the SAM is programmed accordingly. I’m not bypassing the SAM, I’m just trying to figure out why I’m not getting 12V at Pin4 on the 7 pin hitch. According to everything I’ve seen in my research the wire to Pin 4 (which is universally constant 12V) should be attached at Fuse 70 (which it is (or was until today)). But - no power at the fuse.

What I have done today that has fixed the issue but still isn’t “correct,” is to move the wire from Pin 4 to the location for Fuse 71 that does provide constant 12V. Someone on another forum had the same issue and resolved it the same way. So - issue addressed for now but I’m still generally curious as to why I’m not getting power at Fuse 70 when by all accounts I believe I should.

Again - thanks for responding!

theswede

I am really late in this thread, but I have kind of the same problem
Mine suddenly did not want to "give" power to the Main Running Lights
At the socket it is totally dead, my first though was the ground, but nope, that is good
And I can't find the cable that's feeds the Running lights or fuse I have power to fuse 70 and 71
BUT what is the one feeding the Running Lights!?