R-class hitch install with Factory wiring harness - Mercedes-Benz Forum
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post #1 of 112 (permalink) Old 02-26-2012, 11:22 AM Thread Starter
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R-class hitch install with Factory wiring harness

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 164 440 15 39
    • GL-class/ML-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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post #2 of 112 (permalink) Old 02-26-2012, 03:31 PM
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Great write up! thanks!

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post #3 of 112 (permalink) Old 02-26-2012, 04:38 PM
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radodill,

Great job.:

How much did you pay for all parts? (D.I.Y?)

Any pictures of your hitch installation, wiring harnesses and the hitch mounting?

Thanks,

2008 R350 4 Matic w/P1 package/7 seater/Ipod/Front heated seats/ Retrofitted OEM GPS Navigation / 10.2" LCD Overhead Mobile Video system w/ 30 Sega Games, Wireless HeadPhone, hard wired to OEM HK sound system, B/U Camera integrated W/ 6 Disc DVD Changer , hard wired Passort Escort X8500, Mtec HID, 17 SMD LEDs city lights.

2007 Acura RL w/ tech package, A-spec body kit, Deck lid spoiler,TVandNAV2go,USA spec Audio, 18 " Wheels, Wooden steering wheel, Wooden shift knob

Last edited by AsianBlackR350; 02-26-2012 at 04:50 PM.
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post #4 of 112 (permalink) Old 02-26-2012, 09:57 PM Thread Starter
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Originally Posted by AsianBlackR350 View Post
radodill,

Great job.:

How much did you pay for all parts? (D.I.Y?)

Any pictures of your hitch installation, wiring harnesses and the hitch mounting?

Thanks,
The Hidden Hitch receiver was ~$200, wiring harness was ~$70+tax, plug was ~$40+tax, and the rear SAM was ~$400 ($600 incl tax and installation), and the relays were ~$27 and ~$32; so about $1000 total.

I installed the receiver and wiring harness myself, but had to have a dealer install the rear SAM as that also had to be programmed.

I don't have any pictures of the install process, but I can take a few of the underside and electrical connections.
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post #5 of 112 (permalink) Old 02-27-2012, 06:25 AM
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Great info;

FYI, I was playing with my DAS-WIS interface and found that the rear SAM could be programmed for LED trailer lights and trailer hitch etc, the options where all in my rear SAM.
Mine is a late 2008 build.
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post #6 of 112 (permalink) Old 02-27-2012, 08:23 AM
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Frankly, there is no need to pay a thousand dollars for trailer light wiring from Mercedes.

I installed this kit for just $75 as many others here have done.
Works perfectly

I have installed the same kit on a 1998 Mercedes e320 Wagon around 5-6 years back. That too was working perfectly when we sold it last year.

Oh by the way, I installed it myself... very easy to install as a DIY job.........took me around an hour to do the whole thing. Just 3-4 wires to connect. Easy splice connectors are included in the kit. Very reliable!

Last edited by rajdude; 02-27-2012 at 08:25 AM.
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post #7 of 112 (permalink) Old 02-27-2012, 10:22 AM
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rajdude,

This is a really good bargain versus OEM wiring harness and new rear SAM.
It might interfere with rear parking but you can disable by puing the switch to mute it.

2008 R350 4 Matic w/P1 package/7 seater/Ipod/Front heated seats/ Retrofitted OEM GPS Navigation / 10.2" LCD Overhead Mobile Video system w/ 30 Sega Games, Wireless HeadPhone, hard wired to OEM HK sound system, B/U Camera integrated W/ 6 Disc DVD Changer , hard wired Passort Escort X8500, Mtec HID, 17 SMD LEDs city lights.

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post #8 of 112 (permalink) Old 02-27-2012, 11:00 AM
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Quote:
Originally Posted by AsianBlackR350 View Post
rajdude,

This is a really good bargain versus OEM wiring harness and new rear SAM.
It might interfere with rear parking but you can disable by puing the switch to mute it.
Actually, there is no need to disable the rear parking beepers on mine. With just the tow bar or the whole trailer attached.....when first started...... the car beeps and then quiets itself automatically. The front sensors keep on working fine.

I took a lot of pictures while installing my wiring....let me see if I can find them somewhere.
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post #9 of 112 (permalink) Old 02-27-2012, 11:40 AM Thread Starter
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Great info;

FYI, I was playing with my DAS-WIS interface and found that the rear SAM could be programmed for LED trailer lights and trailer hitch etc, the options where all in my rear SAM.
Mine is a late 2008 build.
Ours is a 2008 model year built around September 2007. In October 2008 they started using another rear SAM; the one that was in our car (not hitch ready) was used on some deliveries until July 2009. Apparently all the Central Gateways are hitch ready, it just seems to be that some of the older rear SAMs that are not.

Do you by any chance have a screenshot of that so that I can see if our dealer can enable LED trailer lights for us without needing to the bulb out bypass cable.


Quote:
Originally Posted by rajdude View Post
Frankly, there is no need to pay a thousand dollars for trailer light wiring from Mercedes.

I installed this kit for just $75 as many others here have done.
Works perfectly

I have installed the same kit on a 1998 Mercedes e320 Wagon around 5-6 years back. That too was working perfectly when we sold it last year.

Oh by the way, I installed it myself... very easy to install as a DIY job.........took me around an hour to do the whole thing. Just 3-4 wires to connect. Easy splice connectors are included in the kit. Very reliable!
I am well aware of the Modulite kits and the fact that they are a lot less expensive than a Mercedes solution integrated into the vehicle. However, there are some of us who don't like the idea of splicing into the taillight wiring as that can potentially cause problems with the onboard computers. Further, having the integrated solution means that the vehicle disable the parktronic when a trailer is present and also detect non-working bulbs on the trailer.

So for some people it's well worth it to pay more for the OEM solution; particularly if their rear SAM already supports the trailer wiring and they just need to have it programmed and install the factory wiring harness (in that case essentially the only price difference is having the SAM programmed).

Last edited by radodrill; 02-27-2012 at 11:43 AM. Reason: adding reply to another post
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post #10 of 112 (permalink) Old 02-27-2012, 12:09 PM
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Quote:
Originally Posted by radodrill View Post
Ours is a 2008 model year built around September 2007. In October 2008 they started using another rear SAM; the one that was in our car (not hitch ready) was used on some deliveries until July 2009. Apparently all the Central Gateways are hitch ready, it just seems to be that some of the older rear SAMs that are not.

Do you by any chance have a screenshot of that so that I can see if our dealer can enable LED trailer lights for us without needing to the bulb out bypass cable.




I am well aware of the Modulite kits and the fact that they are a lot less expensive than a Mercedes solution integrated into the vehicle. However, there are some of us who don't like the idea of splicing into the taillight wiring as that can potentially cause problems with the onboard computers. Further, having the integrated solution means that the vehicle disable the parktronic when a trailer is present and also detect non-working bulbs on the trailer.

So for some people it's well worth it to pay more for the OEM solution; particularly if their rear SAM already supports the trailer wiring and they just need to have it programmed and install the factory wiring harness (in that case essentially the only price difference is having the SAM programmed).
I'll take screen shot next time in the weekend, I'm traveling all of this week so I can't do it any ealier, sorry

Agree with teh OEM solution, I would prefer that way to ensure no hick ups...
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