Trailer connections on '04 w/ Factory tow pkge.

BB3500

New member
Hi Dick,

Thank you for your response. Oddly, enough I used the black/white wire as the left turn signal in the 4-post plug. I connected my motorhome to the trailer for the ultimate test and all worked well.

Barry
 

BB3500

New member
Hi Roger,

Thank you for your response. As I mentioned to Dick, I used the black/white wire as the left turn signal. All worked well when I attached my trailer.

Barry
 

alexk243

KulAdventure
Old post, but I am going to asking very similar questions.

I have a 2004 with the factory trailer harness and I need to add a brake controller. I understand the 20amp power supply, the ground, and there is a lot written here about finding the brake signal wire, buy I'm not sure where I should be locating this blue trailer brake signal wire under the driver seat. Could use some advice on that.

Secondly I would like to re-wire the trailer aux power line from the trailer wire harness to feed off of the house battery and be constant on (for interior lights in the trailer). Where is the best place to intercept that line? Is that one also under the driver seat?
 

Nautamaran

2004 140” SHC U.S. T1N
This is the brake controller under the seat:1589942507223.jpeg
It has (should) the +12v brake signal you want to intercept for your controller’s brake trigger, then use (or install) the (light blue) wire leading to the 10-pin socket and trailer socket.

The trailer’s 12v power is labeled “trailer plug socket” on my under-seat fuse list (25amps), so it should be straightforward to re-feed it there from your aux battery.

-dave
 

alexk243

KulAdventure
This is the brake controller under the seat:View attachment 141622
It has (should) the +12v brake signal you want to intercept for your controller’s brake trigger, then use (or install) the (light blue) wire leading to the 10-pin socket and trailer socket.
Is there no existing wire under the seat that runs back to the trailer socket? Or am I misreading this. I was under the impression the BC would feed to an existing line under the driver seat and not have to be ran all the way back to the harness.

Also I am unable to locate that box under my driver seat. So no luck locating the power to the trailer also.

I'm basing my belief that I have a factory tow package on the unique way the wire harness was bundled by the trailer socket that looked factory to me... I could be wrong. Is there an easy way to tell if I do?

If for some reason I do not, is there anywhere up front to connect to "trailer brakes signal wire" and "trailer 12v power wire"?

Thank again!
 
Last edited:

Nautamaran

2004 140” SHC U.S. T1N
You can use your VIN to get your van’s “Data Sheet” showing the build options. It will list “E58 ELECTRICAL COMPONENTS FOR NAFTA TRAILER SOCKET” or similar... but if your driver’s seat fuse panel lists the trailer power fuses then you’ve got a factory harness, and likely the under-seat module pictured above?
My harness has the heavy blue brake wire running back to the 10-pin connector located at the rear of the driver-side frame rail, but I’ve read that some don’t? I just have a Flat-4 connector beyond the 10-pin plug, and have no immediate need of electric brakes, but if I ever do then my plan is to insert a brake controller into the blue wire where it exits the module under the driver’s seat, then replace my 10-pin to Flat-4 wiring adapter with a 7-pin version. The brake controller will receive a +12v braking signal from the van’s towing module and provide a modulated brake signal to the trailer.

-dave
 

alexk243

KulAdventure
I'm guessing that I don't have the module under the seat so I'm thinking the best way may just be to run wires back to the trailer socket. Would that be the best bet for 12v power supply to the trailer also or can that be tapped in somewhere?

Where do I find that VIN date card info?
 

alexk243

KulAdventure
Looks like I do not have the factory trailer package... Alright well Ill run the brake signal cable back to the harness as well as the 12v power supply... once I figured out where the person who installed this harness attached it.
 

alexk243

KulAdventure
So I got kinda lucky... I guess.

IMG_20200527_164152.jpg

IMG_20200527_164204.jpg


Looks like whoever installed the trailer wire harness (7 pin) ran the red, blue, and white wire from the trailer harness to the battery/engine bay already and just bundled it up for that day when someone (me) would install trailer brakes. So I ordered a trailer brake controller (this one, universal brake controller) and hooked it up according to their directions with the signal wire from the brake pedal (as discussed here), Hooked up the blue wire to the trailer brake controller output and the white wire that was not attached to anything (odd... since the trailer lights were working) to ground on the battery and the red line (which was connected with an inline fuse to the positive terminal) to the rear battery fuse box... Trailer brake controller was connected directly to the battery (with 20amp automatic reset circuit breaker) and ground. All voltages check out in the front:

12+v on red line
ground on white line
12+v on brake signal line to controller

but when I check at the harness for the 12v constant power supply I am getting 6 volts... how is that possible? Also the trailer brake controller is not showing a trailer connected when one is (equipped with brakes), nor is the trailer getting any power to accessories, but the brake/turn/running lights function correctly. Trailer brakes are not functioning either both with trailer attached and voltage test at trailer harness (trailer not hooked up)

Observations: I did note that when I traced the red line from the trailer harness it went to the rear passenger side and came back to the center before making its trip to the engine bay... assuming it was needed to power some control module so power was not drawn directly from the vehicles rear lights because (observation #2) the trailer lights did not work when the fuse for the red line was removed...

Leaving on a trip tomorrow cross country with this trailer and hoping to get the brakes working and the auxiliary power. I feel like its something small I am missing. Would appreciate any incite you guys may have.
 

autostaretx

Erratic Member
That's an interesting photo ... compared to the OEM battery (and the proper-sized replacements) your positive and negative posts are on the "wrong side" of the battery.
On the "correct" battery they on the side towards the center of the Sprinter, not the outer.

EngineFront-IntakeLabeled.jpg

quaint.
--dick
 

alexk243

KulAdventure
That's an interesting photo ... compared to the OEM battery (and the proper-sized replacements) your positive and negative posts are on the "wrong side" of the battery.
On the "correct" battery they on the side towards the center of the Sprinter, not the outer.

View attachment 143038

quaint.
--dick
Yeah I am in a constant battle with previous owner adaptations... Everything appears to be connected correctly, just oddly. It is a mess up there and that's a project for another day. Just trying to figure out the trailer wiring right now.
 

alexk243

KulAdventure
Think I solved the issue... Had to replace the 7 pin connector on my van side (too rusted to repair it). I think the previous owner had some confusion with the red wire on the harness side being for reverse light and black being for 12v power, seems that he supplied 12v constant powerv to the red reverse light harness. Cut all wires, tested their output to determine their function and spliced then into a new connector. All seems to work now, trailer brakes are functioning and 12v power is being supplied to trailer.

Only issue:

No trailer lights work when I remove the fuse to 12v constant power line I added from the engine bay and 10v remain in the power line when fuse is removed (must be connected to power somewhere between harness and engine bay). Is there any method of installing trailer lights that requires 12v to some sort of module to power the trailer lights?

Trying to figure out how the previous owner may have wired this. Previously the red line ran to a fuse in the front battery.

(My concern is this bypasses my battery isolater connecting engine battery to rear battery though 12 gauge wire when isolater is open)
 

Nautamaran

2004 140” SHC U.S. T1N

Top Bottom