Saturday, July 21, 2018

trailer prep of brunnhilde - part 2

This is a follow on post to "trailer prep of brunnhilde". Installing a trailer brake controller as well as retrofitting the electric brake wiring from the cockpit to the 7-pin trailer turns out to be an involving project. I didn't want to do a hack job so it is like as if I was designing this aspect of the James Cook trailer prep design and execution. Tens of hours has gone into this.

this is one location that crossed my mind

Instead of Curt Triflex 51140 I bought a Tekonsha 90195 P3 brake controller instead. It is more polished while costing just a little more. As the space between the dash and the knees is already less than generous I did not want to install there like most people would. I want a location that is most ergonomic, and hopefully with minimum negatives. I came up with the idea to repurpose the location for the ashtray.

I will never use the ashtray nor the 12V receptacle in it so this is a no brainer choice

While repurposing the ashtray location seems good, I need to check interference with the use of the two cup holders below. Also I want to maximize the utility of the ashtray opening. I set out to make provision for future installation of mobile device power supply.

the way the ashtray is fastened to the center console make this location ideal as I can reuse the two fasteners

a bit of elbow grease to fabricate a panel from a piece of sheet aluminum


rough test fit

more filing and refining as the dash opening has many bevel edges

I wanted to maximize the utility of this real estate so I tried to leave space for future installation of a mobile device power supply. For mounting the brake controller, I want to leave the option to use both type of mounts - a fixed mount (top) and a removable mount (bottom) so I drill two bolt holes that can accommodate either.

the bolt holes can accommodate either mounts

the controller is affixed to the repurpose ashtray opening faceplate

the wiring harness needed to be addressed later

dry-fitting the brake controller to check interference with the use of the cupholders

I tested against the tallest drink containers

The brake controller has 4 electrical connections. B+, ground, brake light, and trailer electric brake power output. The B+ and trailer electric brake power output wiring requires large diameter wire to minimize voltage drop so I chose 12 AWG wires. That should be more than adequate for the 18.5 feet James Cook and tandem trailer of 5000 lb capacity. I managed to find the contact insert for the trailer connector.

only the B+ (black) and Brake (blue) wires in the diagram carry the trailer brake current so I use 12 AWG for both; the ground (white) carries only the current for the controller electronics (no more than a few 100mA)

this is the crimped contact for the trailer connector's missing electric brake power signal; I transition from 12 AWG to 14 AWG for the crimped contact

extract the seal insert - this is a well design connector

I had no certainty the contacts that I bought are correct. The only way to find out is to insert it into the cavity. My first try the contact would not lock into the connector housing. It is either the contacts are wrong, or they are poorly made and does not engage the locking barb. As I try to insert it again after checking the proper orientation, it snapped into place. I just didn't push hard and deep enough. The other thing to check is to verify the contact engage the 7-pin trailer connector properly. All is well.

the convention is to use blue wire but I happen to have 25' of red 12 AWG wire for the job

Next is the miserable job os running the 12 AWG wire all the way under the camper from the trailer hitch to the engine bay. I managed to retrofit it into the flexible plastic cable chase that I installed for the rearview camera.

the 12 AWG wire for the trailer brake power ready to be fed through the firewall into the cockpit

Most installers would wire the power supply for the trailer controller directly to the starting battery with a 30A fuse. I want to do a proper job. I want to tap the power from under the driver seat where most of the Sprinter's fuses and relays are. It also offer a dry location for a auto-resetting mechanical breaker.

finding the starting battery B+ wasn't that hard

I removed the plastic footstep cover to route the wire to the center console

I took care of the little bit of surface rust

trying a better grounding point I removed the cupholder hoping to find body sheet metal to create a ground

there is this huge wasted space under - enough to hide a few kilos if you a smuggler

As I thought through the current path, I realized that the the ground is not a high current path as the return current of the electric brake power traverse back through the chassis from the trailer hitch connector and to the negative post of the battery. What this means is I can just grab the ground from the cigarette lighter to make my already hard live easier.

Getting to the brake light signal is easier said than done, and I was expected it. I thought it would be easier if I just remove the switch from the brake pedal assembly so to make the harness more accessible to tap into. Not so. It turned out the brake light switch or the harness connected to it cannot be removed without a lot of struggle. I knew I would be a sorry ass if I try. I decided to just try to lower the wiring loom as much as possible and tap into the brake light signal.

so it is the BK/RD wire that I need to tap into

good that I check; I stripped the insulation off the RD/BK wire only to realized it is the wrong wire; there is a RD/BK and a BK/RD and only if you understand the convention of the code that you would know which is which

I made the tap connection with soldering so it is 100% secure; I also adequately strain-relief the wire

I tap into the cigarette lighter harness for the ground reference as it is the easiest; with this the wiring for the controller's harness is completed and I can mount the controller

I designed the aluminum faceplate so that the harness may be detached should in the future I want to modify the faceplate to add another accessory

I deliberately offset the brake controller as much to the left as possible to avail some real estate for adding another accessory in the future

this is the ideal location for the brake controller as I can use the right hand to modulate the manual brake control lever should it be necessary

I next dressed up the B+ power supply wiring into the driver seat box where a 30A auto-resettable breaker will be installed; I chose 30A in case I would add a battery onto the trailer; the breaker hasn't arrive so the driver seat cannot be reinstalled

As I don't yet have the trailer I can only power on the controller but not much else

this is a message I don't want to see when you are towing the Porsche GT4 behind me

I think this is as clean of an installation one can do


  1. Hi, looks like I’m going down the same rabbit hole on my trailer break electronics as well. I’d love to know the specifics of the connector/contacts and 30 A auto resettable breaker you chose. -Vance-

  2. They are very commonly used for trailer brake controllers. Just go to eTrailer and search for fuse or breaker and you till find 15A, 20A, and 30A ones.

  3. Thank you. I believe ferreting out the appropriate connector contact might be more elusive.