Having trouble with the trailer harness on the truck. Using a brand new light set(not even on the trailer) I can only get parking lights to work. After some testing and wire tracing on the truck I found this wonderful little black box

https://drive.google.com/file/d/0B3Z...ew?usp=sharing

behind the passenger rear taillight. It seems to be an add-on to the stock harness possibly part of a factory tow option.

Anyway can anyone tell me what it is and if there are any common problems with the unit.

It plugs inline to the stock harness using factory connectors.

With the box in place I can only get the parking lights on the trailer to work.

Removing it from the circuit and connecting the factory plugs behind the taillight to one another I can get the parking, and directional s to work on the trailer but no brake lights.

Any advice greatly appreciated.

Thank you in advance

 

That box is necessary for the lights to work. On your vehicle (and most modern vehicles for that matter) the turn signals and the brakes are separate lights however on your trailer the turn signal and the brake are indicated by the same filament in the light bulb. That box takes the current for each separate function on the truck and combines them to illuminate the single filament as needed. What you need to do is confirm if the plug on the truck is functioning correctly. Using a test light will be a better approach since your trailer lighting set is designed to function with all of the grounds attached to the trailer frame.

Connect the clip from the test light to the white lead on the plug then touch the probe to each of the other colors to test function. Green is right stop and turn, yellow is left stop and turn, and brown is tail lights.

 

I ended up just buying something like this a few years ago.


60 inch Taillight Converter with Molded Terminals

I got it at Walmart I think, for maybe $30? The flat-4 plug has the actual adapter module built in. It takes care of the common turn / brake light issue and adds a dedicated power wire so you can throw up to ~2 amps per side I believe. It's essentially identical to LR one but like 5x cheaper and you can get one locally.

You still remove the taillight to gain access to that same harness, but I just used inline snap splice things to hook up the trailer adapter and I was DONE.

The dash trailer indicator lights even work like the factory unit!

This all of course counts on you to install the kit (which is designed for a generic tail light install) into the TRAILER harness of the rover. This gives you even better results and uses the factory wiring for the trailer stuff (fuses etc).

The only disadvantage is that you don't get the RV plug also, like the LR kit included I believe... thus no electric trailer brakes either but I only towed a boat and motorcycle trailer ever so it was fine as they were both flat-4 connectors. In the LR3 I just bought the factory adapter harness to get my plugs and the brake controller plug under the driver footwell was standard ford 4-pin, plug and play.

 

Thank you both for the responses.

I used a meter to determine that the trailer plug on the truck was not functioning properly. This is what lead me to the black box. The trailer light set I'm using is simply two light units with dedicated grounds built into the harness. The fact that everything but stop lights work after removing the box leads me to believe that the 5 prong to 4 prong connector is properly wired as well thus casting my suspicion on the black box as it is not properly combining brake and turn functionality.

I will test the lights on the truck today to confirm but I thought they where all still working with the box removed.

 

Those boxes do go bad. I installed a new one as mine was missing when I got the D1. 4 months later in a snow storm, pulling a trailer with sleds on it, I had no lights at all. Traced to the black box. I wired a spare battery to the trailer lights to get home and prayed nobody hit me with no stop lights. replaced the box and everything has been fine since.