R-class hitch install with Factory wiring harness

[radodrill]

We purchased a 2008 R350 about a year ago and wanted to install a hitch primarily for accessories such as a bike rack, but also wanted the ability to tow a trailer if necessary. Locating an aftermarket hitch was no issue (we installed the Hidden Hitch 2" Class III/IV hitch), but the wiring proved to be difficult as most hitch dealers only have units (such as the ModuLite HD) that splice into the vehicle wiring harnesses. That said, we now have a working factory wiring harness and the details are outlined below. When first inquiring about the hitch wiring, the local dealer said that Mercedes told him it wasn't a viable option to install a hitch on the R-class. The Mercedes Consumer support hotline wasn't any more helpful saying they don't have an official solution for a hitch install on our vehicle. Not being satisfied with that answer (particularly since the manual mentioned towing in conjunction with manually selecting the gear range and the fuse table also lists the trailer and trailer brake), I called Mercedes in Stuttgart Germany and the people there were a lot more helpful. I was informed that all R-class vehicles where the VIN/FIN ends in A068595 or higher are indeed hitch compatible from the factory. They even gave me a part number for a US specific hitch package (not listed as being available through MB USA) and the part number for the Wiring harness. Note, The R-class, ML-class, and GL-class are electrically virtually identical so most parts are the same. Another note is that the Rear SAM installed in the 2008 and prior cars (as well as the 2009 model year that wasn't purchased with the hitch) will need to be replaced for the trailer wiring to work. Further, for the 12V line on the trailer plug to work, you'll need to make sure that the relays in positions U and K of the fuse box are present. Also, the R-class wiring harness is not set up for a trailer brake controller, but you can use the wiring harness from the ML or GL as they are ready for a brake controller. Ironically, when I had the part number for the rear SAM, the local dealer called Mercedes in Tennessee and they told him that was was for an ML-class and wouldn't work in the R-class (even so the EPC listed it for the R, ML, and GL) so they refused to order it. As a result, I called the next (much larger) dealer and they had 2 in stock and installed it without any issue and programmed it (and the central gateway) to enable the hitch. Note: the system only does not recognize the presence of LED trailer lighting, so if you need to tow a trailer with LED lighting, you will need a wiring adapter with bulb out bypass circuitry in place. Valley towing makes such adapters, but essentially all they have done is install 100 Ohm 5W power resistors across the loads for the running lights, left stop/tail, and right stop/tail. In case you're wondering about my hitch cover, I used a Curt F-10P chrome finish metal hitch cover and epoxied a Mercedes wheel center to it. On a side note: the hitch package became a factory option starting with the 2009 model year. That year Mercedes added a larger oil cooler primarily as a safety precaution to ensure that the engine wouldn't overheat when towing a heavy trailer in a hot climate, but this would most likely only be an issue on a hot day in the desert. Here is all the important info in list format: Hitch compatible R-class VIN/FIN ending in A068595 or higher Mercedes Hitch Package B6 685 0077 Wiring harness R-class (no electric brake) A 251 440 8510 ML/GL-class A 164 440 64 34 Plug Socket A 020 545 40 26 Relays to enable +12V to trailer socket Position U A 002 542 81 19 Position K A 002 542 26 19 Hitch-compatible Rear SAM A 164 900 54 01 - newest model used since 11-Jan-2011 A 164 440 46 01 A 164 900 72 01 (super-ceded by A164 440 46 01) Aftermarket Receivers 2" Class III - rated at 350 tongue / 3500 tow DrawTite 75470 Hidden Hitch 87413 Curt C13103 1-1/4" Class II - rated at 250 tongue / 2500 tow Curt C12056 Bulb out Bypass wiring adapters 7-way to 7-way cable Valley Towing # 39010 VW # ZVW 808 004 7-way to 6-way adapter plug Valley Towing # 39009 VW # ZVW 808 005 7-way to 4/5-way adapter plug Valley Towing # 39008 VW # ZVW 808 003 Mounting brackets for plug socket DrawTite 18138 - 7-way mounting bracket DrawTite 18140 - Short no-drill mounting bracket

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[Cal]

Sounds like quite a project. I wish I could have ordered one for the car, as it is only a ~$500 option. It may end up costing more to do it after the fact. In any case, thanks for the detailed write-up. I will probably end up doing this myself also.

[radodrill]

Originally Posted by Cal

Sounds like quite a project. I wish I could have ordered one for the car, as it is only a ~$500 option. It may end up costing more to do it after the fact. In any case, thanks for the detailed write-up. I will probably end up doing this myself also.

It certainly is quite a bit more expensive doing it as a retrofit compared to ordering with the option. The main reasons being that ordering from the factory you probably only pay wholesale for the parts (you pay retail when ordering the part itself) as well as the additional labor at the dealer to R&I the bumper, install the hitch wiring, and program the SAM; and for older cars you also have the cost of the replacement SAM, which isn't necessary when they install the hitch from the factory.

We would have preferred buying a car with a pre-installed hitch, but we wanted a sleeker looking vehicle than a traditional SUV (e.g. ML or GL) and we got a good deal on this on from a used sportscar dealer we know.

[stugilmour]

Thanks Radodrill for the excellent write-up.

I will be getting my 2012 in about five or six weeks and will check things out then. Can you remember how many wires were in the ML or GL loom? Is this what you installed to get brake controller, or did you opt for just the wiring w/o electric brakes? Reason I ask is my car will be equipped with the factory hitch and wiring, and if I understand correctly I will just have to run a couple of wires to the front to handle the brake controller as the lights & 12v supply should be already installed.

thx Great write-up.

[radodrill]

Originally Posted by stugilmour

Thanks Radodrill for the excellent write-up.

I will be getting my 2012 in about five or six weeks and will check things out then. Can you remember how many wires were in the ML or GL loom? Is this what you installed to get brake controller, or did you opt for just the wiring w/o electric brakes? Reason I ask is my car will be equipped with the factory hitch and wiring, and if I understand correctly I will just have to run a couple of wires to the front to handle the brake controller as the lights & 12v supply should be already installed.

thx Great write-up.

I just got the R-class wiring harness that has 6 wires at the trailer plug and at the other end is a ground, +12V (to pos 70 on the fuse box), and a plug that goes into the rear SAM. The ML and GL harnesses have an additional plug socket in it to which the brake controller gets connected; I believe this also as another +12V line (30A; to pos 71 on the fuse box) for power to the brake controller.

With the factory hitch on the R-class, you'll be set up in terms of the rear SAM, programming, and bumper plug; but if you want the brake controller you'll most likely need to replace the wiring harness in addition to running a cable to the front.

[stugilmour]

Thx Radodrill. Think I get it now. I was incorrectly thinking the quoted harness p/n's were to run to front of the car rather than SAM to trailer plug.

Sounds like if i go the controller route I will have to run one additional wire to the trailer plug from the after market controller output, and a brake trigger, power & ground to the controller at the front of the car. This is covered pretty well for the ML & GL in other documents; your description of the relays & fuses is very helpful though.

One more question. As I am new to MB, I don't know any info sources yet. Where can I find basic details for the car like wiring schematics, fuse & relay lists, etc.? Used to BMW where one gets the ETM (Electrical troubleshooting Manual) for your model and good to go. The on-line driver's manual doesn't even have the fuse list, but indicates I will get one in the tool kit.

[radodrill]

Originally Posted by stugilmour

Thx Radodrill. Think I get it now. I was incorrectly thinking the quoted harness p/n's were to run to front of the car rather than SAM to trailer plug.

Sounds like if i go the controller route I will have to run one additional wire to the trailer plug from the after market controller output, and a brake trigger, power & ground to the controller at the front of the car. This is covered pretty well for the ML & GL in other documents; your description of the relays & fuses is very helpful though.

One more question. As I am new to MB, I don't know any info sources yet. Where can I find basic details for the car like wiring schematics, fuse & relay lists, etc.? Used to BMW where one gets the ETM (Electrical troubleshooting Manual) for your model and good to go. The on-line driver's manual doesn't even have the fuse list, but indicates I will get one in the tool kit.

Actually, the wiring harnesses for the ML and GL have a plug on them with all 4 lines that go to the brake controller namely +12V, Gnd, Brake in, and electric brake out (to trailer plug); so all you would need is an extension or leave the brake controller in the trunk. It's only the R-class harness that does not have the plug for the brake controller. Note: the bumper plug and rear SAM are the same for the R, ML, and GL so the wiring harnesses are also interchangeable.

The place where I was able to dig up a lot of part numbers was the Mercedes EPC (Electronic Parts Catalog), for which you can register to use it for free (1 yr subscription).

[stugilmour]

Thx again, Will follow up when I get the car.

When searching on the controller wiring topic I had seen that another poster (probably early GL) had opted to position the controller in the rear of the car as you suggest. Might be the easiest thing to do. Understand they don't require a lot of adjustment once set up correctly.

[radodrill]

It all just depends on your towing needs. Some people like the adjust-ability if they are going to tow a lot of different trailers or want to compensate for varying road/weather conditions (e.g. wet vs dry road), but if you're always going to use the same trailer you shouldn't need to adjust it much at all after the initial set-up.