I had a problem with my 2008 Volvo electronic trailer control box. Two of these blew out in a year. They are $30 each and made by Kurt company. I suspect I have an intermittant short in the wall of my aluminum trailer as there is a broken side marker and the wire has fallen inside the wall.
I called them up and asked why their short circuit protection was not working. He had no good answer, just said it only works for a limited time.
So I decided to build my own using relays and protected by fuses which are easily replaceable. It was easy to design, build and install. Parts were inexpensive too. It worked perfectly on the bench so today I installed it in the car. To test it I have a small test plug with 3 LEDs (Tail, left and right). Both left and right come on when the brake is depressed. I was working in the back with the hatch open and it worked perfectly. I closed the hatch, put tools away and decided to try it one more time. Well of course now it would not work. The tail lights come on, the left and right blinkers work but when I depress the brake pedal I can hear the relay click but the test LEDs do not come on! When I open the rear hatch (XC70 wagon) it works perfectly again. Close it and no brake LEDs.The cars brake lights are working fine.
The 12V power must be getting into the box because the tail lights LEDs come on and the turn signal LEDs work. And the brake light input is working because I can hear the relay when I step on the pedal. Anyone see something I can measure or try?

 

Is there a switch that is actuated when the hatch is fully closed to switch off interior lights or some other function ? If so does your relay unit stop working it the switch is pressed to simulate the hatch being closed ? These questions are establish if there is a faulty cable that shorts or becomes open circuit due to flexing when the hatch is closed.

Les.

 

You mention brake power, left and right turn signal power. What about ground? Are you depending on the ball and socket to complete the ground? If so - I've seen that be the problem of many a trailer lighting systems. When I wire a trailer I include an independent ground wire so as to not have to rely on parts that wear and rust, and cause intermittent problems.

It could be that when the hatch is open you redistribute enough weight to move the ball hitch ever so slightly and that's when things make the connection. Try adding a jumper cable (like what you use to jump start the car) and connect it between a good clean and solid ground point on the trailer and the other end on a good solid clean ground point on the car. See if that solves the problem. If so then you have a grounding issue only. Even if you've never had a ground issue before, you may have one now.

Funny how intermittent problems are the hardest to solve.

 

You mention brake power, left and right turn signal power. What about ground? Are you depending on the ball and socket to complete the ground? If so - I've seen that be the problem of many a trailer lighting systems. When I wire a trailer I include an independent ground wire so as to not have to rely on parts that wear and rust, and cause intermittent problems.

It could be that when the hatch is open you redistribute enough weight to move the ball hitch ever so slightly and that's when things make the connection. Try adding a jumper cable (like what you use to jump start the car) and connect it between a good clean and solid ground point on the trailer and the other end on a good solid clean ground point on the car. See if that solves the problem. If so then you have a grounding issue only. Even if you've never had a ground issue before, you may have one now.

Funny how intermittent problems are the hardest to solve.

A missing ground will certainly cause the problem, and there is another possibility, which is that you are connecting to the feed for the hatch light, and with the hatch closed that feed switches off. I have a similar problem with a trailer connector installed by U-HAUL, because they thought that they were so much smarter than me. They chose the easy way instead of doing it the correct way. A REAL DUMMY AT U-HAUL!!!

 

U-HAUL, ••• thought that they were so much smarter

Yeah, I've seen that too. But remember, these are usually kids just out of school (or still in school) who know all they know but don't know what they don't know.

The hatch: May very well be that the lamp is powered through a switch and some technician may have mistaken that for a continuous hot line (or a line that's on when the key is on).

 

Thanks for the suggestions and inputs!
First, I told the story about my trailer with the intermittant short only as an introduction to why I made my own box with relays and fuses instead of electronics. When I am testing I am ONLY using the test plug with 3 LEDs. I have not hooked this to the trailer yet so all that conjecture about trailer grounds and weights does not apply. Also the 4 output wires (tail, left, right, ground) are never outside the hatch when testing so no pinching occurs.
The 12V power that comes into the box seems like it must be present if the hatch is open or not because the tail lights work and the signal lights work in both modes. It is only the brake relay output that is missing when the hatch is closed. Yet I can hear the relay click. That makes no sense to me at all.
When the snow is over tomorrow I may measure the voltages before and after the brake light relay with hatch open and closed. That is the only test I can think of now. This is a bit difficult to troubleshoot because I have to recruit someone to step on the brake while I take measurements.

 

Thanks for the suggestions and inputs!
First, I told the story about my trailer with the intermittant short only as an introduction to why I made my own box with relays and fuses instead of electronics. When I am testing I am ONLY using the test plug with 3 LEDs. I have not hooked this to the trailer yet so all that conjecture about trailer grounds and weights does not apply. Also the 4 output wires (tail, left, right, ground) are never outside the hatch when testing so no pinching occurs.
The 12V power that comes into the box seems like it must be present if the hatch is open or not because the tail lights work and the signal lights work in both modes. It is only the brake relay output that is missing when the hatch is closed. Yet I can hear the relay click. That makes no sense to me at all.
When the snow is over tomorrow I may measure the voltages before and after the brake light relay with hatch open and closed. That is the only test I can think of now. This is a bit difficult to troubleshoot because I have to recruit someone to step on the brake while I take measurements.

I have used a piece of wood wedged between the steering wheel and the brake pedal on some cars, and between the seat and the brake pedal on others. In addition, the hazard flasher may flash the brake lights, making it a handy helper.

 

Thanks Bill, good idea about using the flashers.

 

I just wanted to report that I have found the problem and it all works now. There was more than one cause, as is common. First of all, I failed to recognize that my brake lights are located on the edge of the rear hatch AND for some reason Volvo decided that when the rear hatch is raised the brake lights should not function.
Secondly, the Curt company apparently sold me an incorrect harness for my vehicle. I spoke with a tech guy on the phone who did not know why the brake light would not work with the rear hatch open but he gave me the number of the PDF file that shows the correct harness. It is part number 56386-INS-RA. Looking at this document I found the brake light control wire is shown. There are two connectors just to the left of the rear dome light behind the rear-most ceiling panel. I found the wire by looking at their photo of the T- connector and counting the wires. It is a black wire 2nd from the right, looking at the connector with the clasp on the bottom. I scraped away some insulation and carefully soldered a wire onto it. Then ran the wire down the left corner post under the plastic edge to my relay box. It all works as it should now.

 

I just wanted to report that I have found the problem and it all works now. There was more than one cause, as is common. First of all, I failed to recognize that my brake lights are located on the edge of the rear hatch AND for some reason Volvo decided that when the rear hatch is raised the brake lights should not function.
Secondly, the Curt company apparently sold me an incorrect harness for my vehicle. I spoke with a tech guy on the phone who did not know why the brake light would not work with the rear hatch open but he gave me the number of the PDF file that shows the correct harness. It is part number 56386-INS-RA. Looking at this document I found the brake light control wire is shown. There are two connectors just to the left of the rear dome light behind the rear-most ceiling panel. I found the wire by looking at their photo of the T- connector and counting the wires. It is a black wire 2nd from the right, looking at the connector with the clasp on the bottom. I scraped away some insulation and carefully soldered a wire onto it. Then ran the wire down the left corner post under the plastic edge to my relay box. It all works as it should now.

I wonder why they thought that brake lights should not function with the hatch open. Perhaps they never thought that somebody might haul something that kept the hatch from closing completely. Some folks have strange ideas like that. But thanks for letting us know what the problem ultimately was. The result is that many of us are now more educated.