Test the Wiring for Your Trailer




Introduction: Test the Wiring for Your Trailer

I recently borrowed a trailer to carry some junk. When I connected the lights up, one of the indicators didn't work, which I traced to a loose wire in the socket mounted on the car.

That took far more time than it should have, so I decided to build a little tool to help the next time.

This project is a small circuit board which replicates the lighting wiring on a typical trailer and mounts easily on the tailgate. With this visible through the interior rear-view mirror, I can check that all the signals are arriving at the car's socket from the driver's seat (which is where all the controls are, after all).

EDIT - since publishing this, Instructables has suggested the related 'ible Trailer Light Tester by rjkorn. That is attacking the other part of the trailer light issue (i.e. the trailer) but is much more professional than this one, and has all sorts of good ideas in it. Do go and read it as well :-)

Step 1: Getting the Board

The garage is a fairly hostile environment for a circuit board, so I wanted to protect the gadget while it wasn't being used. I had an old tin for lollies (sweets, candy) sitting on my desk, and since it seemed to be exactly as wide as a chunk of stripboard which I had, I used that.

I cut off a chunk which I figured would be large enough, then got rid of the sharp edges of copper and plastic with a coarse sandpaper.

Step 2: Adding the Components

There wasn't really a great deal of circuit on this board: just lights for each function required by a normal trailer.

I used orange LEDs to indicate the Right and Left turn indicators. Remember that this will be facing forwards towards the driver, so what will be the Left turn indicator is on the right hand end of the board, and vice-versa.

To match the colours of the lights, I used a red LED for the running lights, two red LEDs for the brakes and a white one for the reversing lamp.

I didn't have any while LEDs, so I used one ripped from a small torch which had clipped to a cap brim.

Getting all the LEDs soldered on was easier once I'd held them in position with Blu-Tak. So long as you keep this away from the leads which are being soldered then it doesn't smell.

LEDs need a resistor to limit the current through them. The data sheets for the orange and red LEDs I used both gave 30mA as the maximum safe current. The nominal voltage from my car's alternator is about 14.4.

Ohm's Law gives I = V/R or (restated) R = V/I. Plug the numbers in and you get 14.4/0.03 = 480ohms. I had a load of 560ohm resistors, so that will give 25 milliamps, which is fine. I didn't have data for the white LED, but it didn't blow up, or even get warm at 25mA, so that's what it got too.

I soldered the resistors to take all the cathodes to ground, then added hookup wiring to take the anodes of the LEDs to the connector which would (eventually) go to the car.

Since I have lots of wire colours, I made those connections using the wiring colours given in the book. One link for those is here.

Since I have OCD, I ordered the pins according to the pin number, not the physical layout at the car end of the cable.

Step 3: Wiring the Cable

I measured from the top of the rear windscreen (windshield) to the socket and cut all the wires to the same length. I could have bought a made-up cable and cut off the socket, but they were ten bucks more than just the plug. The packaging for the plug came with a coloured key for the cable, which was nice as I don't have a colour printer. I trimmed that down later to fit into the tin.

Strip the ends of the wires, double them over and give them a blob of solder so that the screw terminals in the plug have something large and soft to grip.

Once all the cores are connected up, add some heat shrink so that the strain relief in the plug has something to grip on to. The grommet and screw connector complete the assembly of the plug.

Add an inch of heat shrink tubing every eight or nine inches along the cable.

Crimp and solder each connector for the to-board socket, and then slip them into the socket housing until they click.

To get the connector for the board right, I laid out the cores in the correct order, and then used a scrap of board to keep them flat, all held together with wider heat shrink tube.

Step 4: Mounting and Use

It would have been possible to wedge the device between the rear-screen wiper and the glass, but not every car has a rear-screen wiper, so I decided to give it a suction cup.

The $2 shop sold me a round dozen of them for two bucks, so I drilled a couple of 2mm holes in the ends of the stripboard, removed the hook from the suction hanger, and attached the two together using cable-ties.

Since I had a selection of ties, I used red and green to indicate port and starboard.

And there it is. It takes a few seconds to install, and another few seconds to sit in the driver's seat and iterate through the various light controls. A lot quicker than running back and forth with a multimeter.

Step 5: Next Time

For a little project, this went OK. I think I've spent longer writing this 'ible than I did designing and making.

Laying out the circuit on paper first would have let me make better use of the area, especially in terms of putting the connector in the middle and the Right and Left indicators at the extreme edges.

It would have been better in a transparent case, which could have been stuck onto the glass unopened, but the only thing I had which would have worked was an old cassette box, and it is a very brittle plastic which would not have survived.

Also, wrangling the individual wires was a bit of a pain. I might either spring for the proper cable, or else make a "six-holes-around-one-hole" guide to keep the wires oriented relative to each other. That could done from a cut-open coke bottle with a paper-punch, or a scrap of thin ply or thick cardboard and a drill. I'll keep the idea in mind in case I have another occasion to make multi-core cable.

Be the First to Share


    • Puzzles Speed Challenge

      Puzzles Speed Challenge
    • CNC Contest 2020

      CNC Contest 2020
    • Secret Compartment Challenge

      Secret Compartment Challenge

    8 Discussions


    3 years ago

    How are the LED's performing on this car?
    Most younger cars have trailer lights controlled through a electronic computer with diagnosis possibilities, a CAN connection and a warning system which detects broken light bulbs.
    If that's the case, your LED's will perform somewhat funny because not enough current is drawn from te electronics.

    Alex in NZ
    Alex in NZ

    Reply 3 years ago

    The LEDs work fine. The trailer hitch is an after-market fit, which might not be well integrated with the car's databus? I've also just tried the rig on a neighbours car (2005 Ford Mondeo) and it worked as expected there. I don't know if that is aftermarket or dealer kit.

    Thanks for pointing me at the CAN (other bus standards are available): I've just spent an interesting half hour on Wikipedia reading about it.

    I'll remember your point if I do have an issue with another car though, and know I've got to get some high-power resistors in parallel to put an acceptable load on the lighting circuit.


    Reply 3 years ago

    Always a pleasure to help.
    I don't think connecting your kit to a multiplexed car will do harm. You maby end up with some fault codes.

    Make sure if you load the wires with high power resistors to use cables that are rated for the load.

    We use a 'dummy trailer' in our workshop. Nothing more than a board with trailerlights corresponding with the 13 pins on the cars socket. Works on all cars. :)

    It's a great thing cars use these multiplex circuits. A lot of luxury features on modern cars are not possible or would requier a lot more wires.
    A great topic indeed!


    3 years ago

    This is actually brilliant!

    I am definately going to build one of these...

    Thank you for the inspiration


    3 years ago

    Great idea!

    I was actually in the middle of a similar project, had already bought all the components but didn't think to include an extension cord so that I could sit in the car and view the lights in the rear view!!! Thank you for saving me from making a foolish mistake!

    Alex in NZ
    Alex in NZ

    Reply 3 years ago

    THANK YOU! I think that's the first time that something I've posted has definitely helped someone. Good luck with your project :-)