MBS Trailer Brake controller install

[TowPro]

I need to install a brake controller for one trip where I bring home a 7'x14' cargo trailer. I will be running under my towing capacity, but trailer has brakes so I want to use them.

Please, only answer if you have done this or know where I can find the facts.

I really don't care to hear the stories about your buddies uncles wife's sister who took out an ECM installing the brake controller.
I already know about the wireless controllers: They are pretty expensive to buy when my requirements is to make one 1000 mile run.
Since I am an EE and installing this myself, please don't try to justify the wireless using install prices.
and I know how CANbus works.

I know I can get my brake feed by running a wire to the back of the MH picking up my Brake indicator from that module FR installed to operate the tail lights off the MBS wires.

Questions:

1) On the 2018 MBS chassis, is there any wires supplied by MBS to connect a brake controller (under the seat?)

2) on my 2019 MBS forester, is the brake wire to the 7 pin run up to the front of RV where I can hook up my Brake Controller?

If #2 is No, I will just run 2 wires to back for red (stoplight) and blue (trailer brake supply)

But I figured I would ask first since when I had drivers seat out I did see several wires that were definitely FR supplied, that were just capped off with wire nuts under seat. and I think I saw a MBS wire connector laying there that made me think of brake controller.

[RedLdr1]

FR does install the basic brake controller wiring and it does terminate under the drivers seat. Either your dealer, or FR support, should be able to give you the color code they used.

[TowPro]

Thanks RedLdr.

I just called FR and spoke to the forester guy. he said "Mercedes is picky about what we are allowed to cross connect to the MBS wire harness so your rig does not have any wires pre run to hook up the brake controller."
I replied "so this means that the Brake wire from the 7 pin connector is not run either"?
he said that is correct.

[RedLdr1]

Originally Posted by TowPro

Thanks RedLdr.

I just called FR and spoke to the forester guy. he said "Mercedes is picky about what we are allowed to cross connect to the MBS wire harness so your rig does not have any wires pre run to hook up the brake controller."
I replied "so this means that the Brake wire from the 7 pin connector is not run either"?
he said that is correct.

Interesting. Could he explain why there are unterminated FR installed wires under the drivers seat?

Unfortunately I can't directly compare our chassis to yours since we have a 2019 chassis. But I do remember that on our first Sunseeker MBS the wires were run by FR and left under the drivers seat.

[TowPro]

next weekend I will pull the seat again and look underneath.

I have this device I can hook that induces a dual tone onto a wire so I can find the other end of it. I expect to find the brake blue, red and maybe this gray/brown combo you connect together for the Jump start from House switch.


thanks again for the reply

[Go UK]

I've got a 2018/17 chassis, there was a loose blue wire under the drivers seat for the brake control.

[KatanaPilot]

You alluded to the issue - the MB CANBUS is quite picky, so unlike conventional wiring, you can't just tap into the brake light wiring for a signal to your brake controller. Search this forum for some solutions - such as using a wireless brake controller, or grabbing the signal from the rear of the MH from the brake light wiring that FR installs.

Pin 2 on the 7 pin connector is usually for trailer brakes. I assume FR still runs the blue wire from under the seat to Pin 2 on the connector. Pretty easy to determine if that exists and if it runs to the 7 pin using a continuity tester or DVM. So that would be your trailer brake wire from your controller to your trailer's brakes.


Also on your boost switch wiring - on my 2015 2401R, I had to run wires from the BIM/BCC/BIRD (whatever they call it now), up to the cab instrument panel for the boost switch. There were pigtails at the BCC, but no wires run from there.


Don't know if that helps at all.

[FFred]

If you are going to use the trailer often you might consider simply installing hyd brakes.

The pressure on the ball hitch puts pressure on the brakes , proportion to how hard you are slowing down.

Found on boat trailers a lot.

[Chuck_S]

One trip? Never to be used again? Borrowed trailer?

You'll need a brake controller in addition to the wiring (unless your vehicle has one that isn't wired). For the expense of a brake controller you can fit a controller to the trailer and not hassle with tow vehicle wiring and controller. Here's one:

https://www.amazon.com/Autowbrake-Tr.../dp/B007HGI9OO

Looks as easy to remove as well as install (basic wiring).

-- Chuck

[rvdoctor]

Maybe you could install an economical no thrills after market brake controller. They are pretty inexpensive. Wire it to 12vdc power, ground, and run a single brake wire back to 7 pin plug. Use controller to activate trailer brakes manually with the controller. Don't wire it to the MBS wiring at all. Only problem I see would be that trailer brakes would not activate when using controller manually to activate brakes. Trailer brake lights would still come on when pushing on MBS brake pedal if your rig is currently wired for trailer lights. Just a thought..

[TowPro]

Please close this thread now as it's no longer on track

[chroniekon]

I was hoping you would give an update on what direction you end up going. Hard to do if the thread is closed.

[TowPro]

I will post my findings with pictures, in this or a new thread.

[Xvatorguy]

Check out Etrailer . Com they have a picture of the plug you are looking for as well as a harness for about 19.00

[TowPro]

Originally Posted by TowPro

Thanks RedLdr.

I just called FR and spoke to the forester guy. he said "Mercedes is picky about what we are allowed to cross connect to the MBS wire harness so your rig does not have any wires pre run to hook up the brake controller."
I replied "so this means that the Brake wire from the 7 pin connector is not run either"?
he said that is correct.

I quoted this for a reason. Is Forest River Bull shipping me or are are they really this clueless. again they tell me one thing and I see its not true.


This is under my DS seat. see that blue wire in there? it had a crimp cap over end. Using a toner I find it goes to the 7 pin trailer connector. Than using a jumper wire I make it positive and find correct brake pin on 7 pin connector goes hot.

those 3 wire I am pointing at are L to Right
White with yellow hot is with key on
red always hot
little red is hot with engine running.
I think this little red one goes to J-11 IGN on BIM . not sure where the other 2 go. (but I did not trace them since they are hot)


Also under the seat is this skinny black wire, it is dead, not ground or positive, and brakes don't cause it to go hot or ground. traced it with toner and it did not go to rear anywhere or over to the BIM area. NO idea.

also under there is 2 connectors which "might" be an interface for Trailer brakes? (if not I will have to run a Brake light source up to controller from rear of MH).

[TowPro]

Originally Posted by Xvatorguy

Check out Etrailer . Com they have a picture of the plug you are looking for as well as a harness for about 19.00

thanks, I see that. its got all 4 wires in it that connect to what appears to be the factory wire harness (that appears to be under my drivers seat). But my trailer wire harness is supplied by Forest River with a separate Blue wire under the seat that goes to my 7 pin connector (the one Forest River keeps telling me is not there ) so I would need to cut the blue wire in this harness and splice it to my FR blue wire. (you can actually see this FR supplied blue wire under my RS finger next to red fuse holder)

[TowPro]

the green connector is my Trailer Brake connector.
It has the positive wire in it, that test hot
it has the ground wire in it, that test good
it has a black with red in it that is the brake light signal, but it does not work. .

[AudiDudi]

Originally Posted by TowPro

the green connector is my Trailer Brake connector.
It has the positive wire in it, that test hot
it has the ground wire in it, that test good
it has a black with red in it that is the brake light signal, but it does not work. .

Do you need to have a load to make the brakelight signal work?

[Xvatorguy]

Originally Posted by TowPro

the green connector is my Trailer Brake connector.
It has the positive wire in it, that test hot
it has the ground wire in it, that test good
it has a black with red in it that is the brake light signal, but it does not work. .

Wow that is disappointing, I have a 2020 on order and my first mod is to add a brake controller. I believe there is a switch assembly with a bracket available thru E trailer that is mounted to monitor the brake pedal . Hard to believe Mercedes does not have a better option

[TowPro]

I think if it was pre wired by MB for a 7 pun connector there would be a module installed under seat because MB has some kind of software for trailer ABS or something.
Maybe that is what is Missing? But there is some kind of module in there (there is 2 I think)
But there is .7 volts on this wire even with brake off so maybe truck has to be moving? Wish someone knew for sure