View Full Version : Factory plug for brake controller?

04-04-2009, 04:19 AM
Does anyone know where the factory connector for the brake controller would be?I have a 03 Sprinter 3500.I was trying to move a trailer that had electric brakes and when I would hit my brakes it would activate the trailer brakes at full force.It would be my understanding that it is wired from the factory and is wired "hot" .I got a wire harness kit from the dealer,it hasa blue plug to mate to another blue plug on vehicle.It is suppose to be under the dash and to left of steering column according to the directions in kit.I was unable to locate it as was the service tech.Any Ideas?

04-04-2009, 06:15 AM
Hi Todd,
Was your '03 wired at the factory for trailer towinf, or did you add the towing wire harness later?
The blue wire in the factory harness does hook to an aftermarket trailer brake controller, but I do not think
I've ever heard of Dodge or Mercedes supplying the brake controller as part of the towing package.
If you are towing a multi axle trailer with brakes on more than one axle, the # 14 AWG wire in the factory harness
is way undersized.
Recommended wire size is AWG 10 for many brake controllers.
I pulled out the brake switch (on the top end of the brake pedal) and found the wire that goes "hot"
when you depress the brake pedal. It had another wire port beside the required signal wire so I put my
signal wire to the brake controller in the other port.
Works really well on a 4500 lb. tandem axle cargo trailer.
The higher current required by the trailer brakes comes from the battery via an auto reset 20 A circuit breaker.
Hope this helps

04-04-2009, 02:28 PM
The wiring looks factory by the hitch.I bought it used so I can't say it is 100%,they did a super job of tying into the factory wiring harness if it isn't.I Everything is already wired including the break,which is wired hot. I know how to wire for a brake but I would have to cut the factory wiring at the plug in order to run new stuff. Dealer said the part I got was a Sprinter factory part.

04-04-2009, 03:53 PM
Hi Todd,
I used the factory blue #14 wire on a trip from Hatteras, NC to Dallas, TX.
I had very limited brakes on the trailer.
I had some time so I cut the entire factory rear recepatacle off the end and rewired to a more heavy duty aftermarket
receptacle that allowed me to run 2 additional #12 wires from the brake controller to the recptacle at the back of the
Now I have brakes that I can lock up if necessary. I used a Valley Odessey II controller (inertia switch type) and I
can set the controller to give excellent brakes on the trailer, adjustable for load and road conditions.
Can't help with where to plug in the blue wire. Did you look on the brake module under the drivers seat.
Some Sprinter folks have connected their signal wire to the brake controller at the brake module vs the
brake switch at the top of the brake pedal.
I've heard that some of the factory harnesses do not have any provision for trailer brakes, but if you have the blue
wire, your's was set up for trailer brakes like mine.

04-07-2009, 03:33 AM
where is the blue wir2 #14? Would it be normal to have the brakes come on with no controller?

04-08-2009, 01:01 PM
Hi Todd,
Someone could have hooked the blue wire into the brake switch module or the brake switch, and if the blue wire gets charged with current when you
apply the van brakes, yes, your trailer brakes would come on.
With no brake controller, depending on how much current the brake magnets take (one magnet per wheel on the trailer) you could get anything
from full brake lockup/skidding wheels on the trailer to the brakes simply dragging a little.
The blue wire that runs from the front of the van to the factory trailer connector at the back carries the current to the trailer.
With a brake controller, the variable/adjustable output current from the controller goes through the blue wire also, and if the blue wire is the std.
#14 AWG, it won't carry enough current for more than perhaps 2 brake magnets.
On my van, as soon as I increased the capacity of that wire (by adding 2 #12 AWG wires that run from/to exactly the same places) I had enough
current available to power all 4 magnets on my trailer and the brakes can be locked up if I set the controller to push sufficient current to the magnets.
The controller has an inertia switch in it so the harder I need to stop, the more current goes to the brake magnets, including full lockup of all the wheels
on the trailer if I set the controller output current too high.
Are you sure there's not a brake controller hidden away somewhere in the footwell or under the drivers seat in your van?
Having no brake controller and simply powering the blue wire to work the brakes without any control of the current makes no sense to me
as then you have no way to regulate how much trailer braking action you get when you apply the van brakes.

04-09-2009, 12:02 AM
I am positive there is no controller hidden.My trailer has 4 brakes and it locks them all and I can't even move.I do know how the controller works,I just can't find the blue wire or factory plug.

08-27-2009, 07:19 PM
I am positive there is no controller hidden.My trailer has 4 brakes and it locks them all and I can't even move.I do know how the controller works,I just can't find the blue wire or factory plug.

Todd, did you find a fix for your problem?