Trailer Light Tester

We often use our boat, Big Duck, as an RV. Sometimes we'll hook the boat up to the van and head down the road with no intention of putting it in the water, but most of the time we 'camp' in Walmart parking lots on our way to boating adventures away from the home waters. We live in South Carolina but have splashed at over 100 different ramps from Key West to Homer Alaska. We also have a 16' Wolf Pup travel trailer.

Getting the boat ready for the road requires yearly service of the trailer bearings and brakes. Both the Duck and Pup have electric trailer brakes which in my opinion are much easier to maintain - yes, even the 12,000 pound boat/trailer uses electric brakes. In addition to the yearly maintenance, I check the trailer lights before each outing. I like to check the trailer lights before I hook the trailer(s) up to the van, because it's somewhat of a hassle to get them together due to a tree being right in front of their parking place. I snake them into the carport going around the house, past the tree and park them side by side using a front mounted hitch on the van. I'd rather not hook trailer to the van until I'm sure the trailer lights are as they should be. So I made this little box to test the trailer lights and brakes.

The trailer plug plugs into the trailer socket shown in the front of the box - the one on the box is the same one you'd have on your tow vehicle. In the hinged black area there's a 3 digit read out that alternates between volts and amps every few seconds. Above that there's a row with a switch, pigtail and another switch. This row is for the electric brakes.

The next row, of four switches, is for running/tail and brake/turn lights. The outer switches are running lights, the inner two are for brake/turn. There's really no need for two running light switches as they are both on the same circuit - I just did it for symmetry. Each brake/turn light is of course on a different circuit. The single switch in the top row is wired for the backup light(s).

The box gets it's power from the breakaway battery on the trailer. This battery is charged by the van on the road. So...we plug the trailer cord into the box. Right away we see the voltage of the breakaway battery, then, in a few seconds we'll see the digital readout displaying amps. There will be no amps until we switch on some of the lights or brakes.

The switches have a 'power on' light so it's easy to tell which circuits are live. I have included a blinker in each turn/brake circuit with different flash rates. This way, if I turn on the running lights AND the brake/turn lights I can easily tell the difference between the two when the flash is off.

Truth be told, you don't need all this stuff if you want to keep it simple and just test the lights. Get the vehicle socket and connect the running lights and each turn/brake light post to the Aux 12+ charging post. Now when you plug your trailer into the socket all the lights should go on. However, this way you can't test the electric brakes.

 

I test the brakes two ways. The pigtail is energized by the switch next to it. I connect a pair of long wires to the pigtail. These wires have a momentary switch at the far end. With the trailer plugged into the box, and the switch next to the pigtail on, I plug the wires into the pig tail and take the momentary switch back to a wheel that has been jacked up off the ground. I spin the wheel by hand, then push the momentary switch. If the wheel instantly locks up, I'm good to go. I usually do this during my yearly bearing inspection.

Box with brake test wires attached.

The second brake test is done before each outing during the lights test. When the lone switch on the brake row is turned on, full power is fed to electric brakes. A chart shows how much amperage to expect, depending on your electric brake configuration. This way I can tell if all the brakes are energizing.

What happens if I want to check the lights on a trailer with surge brakes - that doesn't have a breakaway battery? Then I use my 'jumper pack' to power the box.

My jumper pack needs charging, but it will still get the job done at 11.9 volts.

And if I need to check the lights on a trailer that doesn't have a 7 pin connector I use an adapter and the jumper pack.

Adapter in place - I need the jumper pack with the adapter.

So, what's the deal with the hinges? I thought you'd never ask.
The gauge reads in volts or amps or cycles between volts and amps. There's a little button on the back of the gauge to select the reading you want to see. I wanted the option to easily change what the display shows.

The 11 volt reading means my breakaway battery needs attention.

It only took me one dog beer to complete the project. Well, maybe it was 8 or 10?

Pic on the way to Lake Powell