Trailer converter

One of the electrical considerations when building an RV hauler is converting the taillight signals. Semi trailers have separate brake and turn signal lights. RV trailers traditionally have combined brake and turn signals. Some sort of trailer converter needs to be implemented to combine the turn and brake lights.

You might be thinking..."The truck has combined turn and brake lights. Can I just tap into them?" I can't speak for all brands, but you can't for Volvos. Volvo trucks use a lighting computer to output and control the lights. The lighting computer doesn't have enough capacity to run trailer lights. Also, when the computer 'sees' the additional load of extra lights it will report an error to the computer.

The seven pin RV connnector has the following signals:

Left turn/brake
Right turn/brake
Running lights
Reverse lights
Brake controller output
12V - to charge the trailer batteries, etc.

The Volvo trailer connnector has the following signals:

Left turn
Right turn
Brake lights
Running lights via a 20-amp relay
Marker lights via a 20-amp relay
Key-on 12V via a 40-amp relay

Note: Marker lights are the same as running lights but are on a separate circuit so the driver can flash them.

The brake and turn signals can be combined using three relays like this... all the signals needed, except reverse, are available from the semi trailer connector. The reverse signal is normally snagged from the harness going to the semi taillights. The trailer connector also has a 'brake only' signal used to trigger a brake controller.

A lot of people converting their RV hauler use a Jackalopee designed and sold by Henry Szmyt. He has made it easy to do the conversion and it works great for most people. However, I had a few other things I wanted to integrate into my trailer converter so I decided to create my own. I'll be adding a relay for work lights, a circuit for drom lighting, etc. My drom box also has turn, brake, running, and reverse lights.

In addition to the semi trailer connector there is also the aforementioned lighting harness going to the rear of the tractor. It has these signals:

Left turn/brake
Right turn/brake
Running lights
Reverse lights
The turn and brake signals are already merged and all the signals we need are there except the 'brake only' signal for the brake controller. There is also no 'key-on' power to trigger the trailer 12V relay, but that can be found in the sleeper climate control area. There are lots of ways to design the trailer converter but any way will involve using at least one signal from both harnesses. I was very tempted to use the truck lighting harness and powering everything from the battery using relays. This would keep everything independent of the factory wiring.

I am also running a harness to the instrument panel for brake controller wiring and for the switches that will control the drom and deck lighting. It has a spare wire in it. Then a thought popped into my head. The reverse lights are powered by the light controller which is located in the dash. If I could tap into the reverse wire at the light control module I could send it back via the spare wire. I wouldn't have to molest the harness externally.

The reverse wire (circuit 410) wasn't that hard to find once the fuse panel was moved aside. I don't like to use the ScotchLock connectors but space dictated it. However, moving the fuse panel aside revealed another project that took most of the day...

Not long after I got the truck I removed the remnants of a Qualcomm installation. One of things I found during this removal was that they had cut the accessory connection under the steering wheel to trigger the Qualcomm. This led to me finding that all the 'accessory' items (radio, windows, etc.) were now powered all the time. A quick look at the fuse/relay block hidden under the cupholders led me to believe this was some sort of factory thing as the accessory relay appeared to be bypassed by a factory jumper. Fast forward to today...

When I moved the fuse panel to access the lighting control module I saw something that gave me one of those 'oh hell no' moments. Someone had cut the accessory power to fuses F1-F7 and the constant power wire to fuses F8-F13. They had put ring terminals on the wires, put a large bolt through the rings, then wrapped the whole mess in electrical tape! What a half-assed job!! Not only was this a mechanically crappy way to do it but by piggybacking the accessory circuit onto the constant circuit now two 60A circuits were running through one 60A fuse. Bad! It looked like this...

I want this truck to be reliable. I don't want the 60A fuse to be overloaded or the bolt to touch metal and be blowing fuses with big sparks. I have an accessory-powered wire directly above this area from my voltage gauge project. I decided to use this connection to make the accessory relay circuit functional again. It took some time to finally find the accessory relay trigger wire (circuit 196) behind the fuse/relay block (I mapped it here). I hooked the trigger wire up, turn the key to accessory, and probed the accessory wire that had been cut. Nothing!

Well &*%$#^. There was power to the relay and the relay checked good. Apparently there is an issue inside the fuse/relay block or possibly the accessory power wire. This might explain the snipped accessory wire and the rewiring of the power leads. I wasn't going to leave it cobbled up like I found it. I reconnected the constant power to fuses F8-F13. I ran a separate wire from the accessory relay slot to the wire for fuses F1-F7. The accessory wires are still powered all the time but now they are on their own circuit again. I'm glad to have remedied this previously unknown problem.

With the reverse wire sourced I decided to use the Volvo trailer harness to power the trailer converter. It would be a little simpler this way and I wouldn't have to hack into the taillight harness outside on the frame. (Schematic)

The implemented design.

The converter will go in the driver's side jockey box. There is a little ledge in the sheet metal that I used to position the converter.


I installed a 1/4" bolt to use as a ground lug. All the large ground wires will connect here. I also used some strapping to connect the ground points on the terminal strip.

Here I have connected the Volvo trailer cable and the cable I ran to the dash. I drilled three holes in the each for the factory trailer cable, drom box cable, and the cable to the rear trailer connection. I used electrical box cable clamps to protect the cables and to hold them in place.

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