I bought a used hitch for my 2012 GL350. It came with no fuses or relay(s).
I also purchased a new wiring harness.
I connected one of the plugs from a harness to the rear sam, ground wire to ground, power wire with fuse to the red slot in fuse box and left the green plug for electric brake alone. And of course the other end to the 7-way trailer plug on the hitch.

Then I took the truck to a dealer to re-code rear sam for hitch presence.

I took the truck home - hooked up to the trailer - no lights at all.

Back to the dealer - they said I am missing a relay - okay ordered it - installed - still nothing.

Back to the dealer again - now they say I need another relay - another $25 relay on order.

Is there something I am missing? I really hope that relay will do it, but doubt it. I see some fuses on the pictures of the new hitch kits. Do I need to put some fuses somewhere for those lights to start working?

They wanna charge me couple hours of labor to diagnose the problem, which of course I am not going to do.

 

I have a 2011 model with factory hitch and wiring. I wonder if a pic of rear fuse box would help? Also when I first plugged a trailer into my truck it did not work, I have to use an in line resistor (which I bought in plug form) for the rear SAM to recognize the trailer as all of the trailer lights are LED.

 

If you could get the pic - I would appreciated. My trl don't have LEDs - so that is not an issue.

 

I have a Witter towbar fitted by a dealer heres my rear fuse box for comparison. I'm in the UK so not sure if theres a difference if you are in the US or somewhere funky.

 

I'm surprised they did not test it before they let it go? That itself seems pretty lazy to me.


These things are pretty touchy when you tap into the electrics. The draw of the lights on the trailer is what triggers the SAM to act up. It senses the draw and thinks there are lights out. Also when you plug in it sets the vehicle in trailer mode. Not sure what all takes place.


Was the wiring harness OEM or aftermarket?


Also not sure what you're going to be pulling either. I've seen so pretty stupid people on this board and others pulling way too much weight. People see the 2" receiver and think they can pull 12K. These trucks are not set up for that kind of pulling or weight. Most vehicle that have tow packages are set up with heavy duty options to offset the added weight. With Benz there is no such thing.


I have a 2" receiver on my Honda CRV. My max towing weight is 1500. Again I've seen people pulling twice that and wonder why they burned up the transmission or overheated.


Also if you have the BlueTec/Diesel that's not the go pedal either. Yes, the torque is more than the gasser but you still have weight restrictions.


Please don't take the negative statements as you not being informed. Just trying to make your life much easier.

 

The wiring is OEM. And I think if I won't be able to make it work will go aftermarket - that for sure will work when connected directly to the lights in a back. Just really wanna make it work the right way.
And no - I don't plan on pulling anything heavy yet - I have a jetski and a foldable flatbed trailer that is rated I believe for 1500 pounds.
So now I am thinking maybe that trailer don't have enough lights to pull enough current to trigger the SAM?

 

I pull skis as well. With the Benz I just tow the single ski. I know it's back there for sure. Like pulling a parachute. When we had the GL350 BlueTec it was great for pulling as you did not have that initial slow steady climb up to speed. We had a VW Touareg TDI too. It was the same as the GL350. The acceleration was great. Both were the biggest disappointment for reliability though. Also had a Ram 3500 with the Cummins diesel. You could load that up and never knew you had a load. the HOA did a total beat down on it. So the VW took it's position.


My wife has the Lexus LX570 and that thing is awesome. It's got a trailering package built into the deal. Just remove the bumper cover and insert the draw bar and away you go. Have never had an once of trouble out of it. Pulls like the Tundra truck does.


When pulling the double ski trailer the Benz does not like it. It pisses and moans the entire time. Just because it has a 2" hitch it's not liking the added weight for some reason. It constantly hunts between the gears for the best position. After a few trips with that I put the double back on the Lexus. The double skis come in about 4500LBS when loaded up with fuel and added fuel tanks.


Have fun with your toy and let us know if you rectify the harness issue. I bought the Honda set up from etrailers.com. U-Haul actually installed the hitch for 20.00 if you bought the hitch online from them. That was a no brainer. etrailer shipped the harness and I installed it. 100% plug and play with no issues. Honda has a wiring loom inside behind the left rear panel. Just plugged it in and ran the wire. U-Haul sold the exact harness. Can't remember what they wanted to install it.


All of my trailers are LED too. After the incandescent bulb failure one after another and short on the double trailer. Load-rite torn down the harness and reinstalled with the LED lights. I did the signal trailer myself. Not a single issue with any of the vehicles.

 

I found some harness installation instructions for 2010 GL350
there is some brown/white (br/ws) wire that goes from one of the connectors in fuse box to the one of the connectors in SAM:





That wire didn't come with my harness and of course I didn't install it.

Can anyone comment on that? - Is it different for my year GL. Or I need to install that wire?
Also there is 2 fuse wires in a harness that goes to the fuse box - My harness came with only one fuse wire.

 

Just got back from the dealer again.

2nd ordered relay arrived so I made the dealer install it this time and check the system again.

Exact words of the service adviser:"With that new relay we installed we had to do a new system update - and after all that we tested the system and everything is working"

Got back home - hooked up the power cord from my jetski - and wholaa everything is working just as it should.

My conclusion is that they didn't reprogram the SAM (even that I asked about it several time the service guy that was working on my vehicle and he said that he did) or they did it wrong some how. But everything is working now - so I am happy.

Still don't know whats up with that brown/white wire from fuse box to the SAM. Anybody know what it is for?