Trailer Hitch Harness for R320CDI?? - Benzworld.org

jonsager · 05-09-2007, 12:39 PM

I have obtained a Draw-Tite hitch for my R320CDI, but dealer does not know anything about electrical harness that would/could work....

Does anyone actually know what can work with the R-Class and its rear computer???

bjablonka · 05-11-2007, 01:00 AM

I did the same thing - installed a hitch but no electrical connector. It does not exist, MB or any other supplier does not have one. The only choice you have is to import an entire system (hitch and electrical) from Germany, but that is very expensive.

jonsager · 07-13-2007, 04:16 PM

I finally did find a harness rig from HitchKing.com in Queens, NY. Talk to Adam. Installation is a bit of a pain. $100. Adam is quite helpful on the phone and knows his stuff.

Advise that you tie the power into the rear fuse box in the Spare Tire Well.

Running lights use the same bulb filament as the brake lights, just the apllied voltage changes, Very Funky, but kind of neat.

DouglasDSmith · 07-14-2007, 03:48 AM

Is the Harness from HitchKing the factory harness or an adaptor?

Diesel_Burn · 07-19-2007, 10:13 AM

Was the hitch you used on the R320CDI a Draw-Tite Model #75470? Any problems with the installation? Thank you.

sleepyhead · 07-25-2007, 12:41 AM

I would also love to know more info about a hitch for the R320 -- anyone who's done it, please post your experiences, sources for parts, cost$, etc.

rob13572468 · 07-28-2007, 01:58 AM

here is some info on the trailer hitch issue for those interested (i have designed/installed them fro R class specifically).

the R class is not deisgned (electronically) to have a trailer hitch. the reason for this has to do with the networked design of all newer MB models. all newer mercedes have a unit called the rear SAM (signal aquisition module) which is essentially the rearward breakout point for discrete wiring. this means that signals run to the rear of the vehicle over the CAN network and are then converted out to individual wires that do things like run the various tail light bulbs. the SAM unti has just this: a single plug that connects to a split harness, with one half running to each light cluster. The SAM unit (in addition to lighting the bulbs) monitors the bulb resistance and watches for either no resistance (burned out bulb) or a very low resistance (shorted wiring) and will trip an error message to the cluster.

on a normal vehicle adding a trailer harness is a simple as tapping into the wires that run to the bulbs. on mercedes its not that easy. first the wiring is very small (20 gauge) wiring which cannot support lots of bulbs. secondly, the more buls you add the lower the resistance gets until an error is tripped. what this means is that very minimal trailers with basic lighting (left/right turn and stop) can be added and only then with small 13 W bulbs. any larger bulbs start to set warnings off. even then this is really not recommended at all by mercedes. forget about a larger trailer or things like electric braking....

the proper way to install trailer wiring is to replace the SAM with one that supports a trailer harness. the GL SAM is a good example of this in that the part numbers look the same as the R (ML/R/GL share lots of the same components) so a GL SAM can be switched in and programmed to accomodate a trailer harness. you can then purchase the trailer harness from parts as well and plug it in and go. i havent checked the price on a new SAM but im sure that its outrageously expensive.

finally, if the demand is there it would be a straightforward process to design a module to attach to the CAN network and break out those same signals (including the electric brake controller) essentially allowing trailers of any size or configuration to work with the R class. this sort of add-on module would end up costing far less than the prospect of buying a new SAM.

DouglasDSmith · 07-30-2007, 10:18 AM

One question though- The european model of the R-350 does have a trailer hitch and wiring harness availble as an option

Does this mean the euro-R has a differen SAM than the US-R?

rob13572468 · 08-03-2007, 01:04 AM

Quote:

Originally Posted by DouglasDSmith

One question though- The european model of the R-350 does have a trailer hitch and wiring harness availble as an option

Does this mean the euro-R has a differen SAM than the US-R?

to be honest i am pretty sure that the rear SAM for *all* suv variants (meaning ML/GL/R) are identical as well as the overseas variants. the big difference appears to be the programming of the SAM firmware as well as version coding... ill have a better asessment in a week or so when i get a good look at a GL with the towing package and can compare the connections .

at any rate the worst case scenario is a SAM replacement plus special programming to get the SAM coded for the particular model/features. the best case is simply programming the existing SAM.

lkchris · 08-13-2007, 12:40 PM

Quote:

Originally Posted by DouglasDSmith

One question though- The european model of the R-350 does have a trailer hitch and wiring harness availble as an option

Does this mean the euro-R has a differen SAM than the US-R?

The Euro wiring will accomodate a 13-pin connector, providing separate turn signals and brake lights, rear fog light, etc.

The USA hitch is for a standard USA 7-blace connector and combines turn signal and brake light functions for the trailer and of course does not do rear fog light.