Boat Trailer Lights Tester Box

IntroductionA boat trailer light tester is something trailer sailors need. When preparing for a trip, it is a good idea to prepare in advance by testing the trailer lights. Avoid the hassle of fixing wiring at zero dark thirty at the beginning of a road trip. It is not always a good idea to attach an old boat trailer to a new expensive vehicle. Bad trailer wiring could blow fuses or damage the tow vehicle. A tester could help make preliminary testing and fault diagnosis easier. Most testers on the market cost $150 to $400 USD. That does not include a small engine battery. A new battery runs about $30 more. A battery needs charging to keep it in good operating condition.  If you use the tester infrequently, one still needs to charge the battery.  Not a good option if the tester is used once a year. A reliable power supply is the wall socket.  An old printer power supply can provide power to the trailer tester. For LED trailer lights this tool may be the major option for diagnosis in some cases.

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OperationThe tester plugs into 120V AC wall power.  It has an AC to DC adapter.  It goes into a case that has a switch to emulate truck functions of stop and turn signals. It has a four wire plug used to connect to the trailer.  When plugged into an extension cord the box is energized.  Make sure the switch is in the off position, dial location #1. Connect the 4 wire trailer pigtail to the tester. Switch to tail lights stop #2.  Both tail (Dim) lights should be on. Turn dial to position #2,  left  tail light should be on. Turn switch to #3, right turn is on. Any deviation from this behavior should be further diagnosed.  This tester box can be assembled from parts laying around the junk bin.  The case is from an old home router. The power supply is from an old printer. It is an AC adapter that puts out 9VDC at 700 uA.

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Position 1 is off.
Position 2 is tail lights
Position 3 is Left Turn Signal
Position 4 is Right Turn Signal.
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AssemblyParts are easily available.  Assembly is pretty easy. Here is a parts list.  Also is a wiring diagram of the project. Below is a wiring diagram of the project. 

PartsTrailer pigtail - vehicle side
4 terminal strip - Break out
Switch, Single Pole - 5 throw rotary
Fuse -  2 amp fast blow
Fuse holder - inline
AC -> DC Power supply, 9VDC 700 uA
Home Router project box 6” x 4” x 1”
Vehicle tester - 4 wire flat led
Printed switch selector label
Clear packing tape

EquipmentSoldering iron or gun
Solder
Box cutter to strip wires
Electrical tape
Dremel
Multimeter

Get a project box 6” x 4” x 1” will suffice. Attach a 4 terminal strip to be used as a break out. Two small screws will do the job.  Take the vehicle side 4 wire pigtail and strip the ends.  Screw them to the terminal block in order left to right, white, brown, yellow and green. Cut a small slit in the top of the box to run wires from the terminal strip to the inside of the box. Take a 5” length of 4 wire flat trailer wire and strip both ends about 0.5”.  Screw the wires from the short length to terminal block in the same order as the pigtail. Push the wires through the slit. Drill a hole in the center of the box to the size of the rotary switch. Bolt the rotary switch to project box face. Make sure the switch is aligned so the switch is uniform left to right. Drill another hole in the lower right of the face plate. Use a nut and bolt to hold down ground (white) to nut and bolt. Solder short  wires from the terminal block to the switch in order.  SW1 NC SW2 Brown, SW3 Yellow, SW4 Green. Cut a notch in the side of the face plate for the power supply wires. Take a volt meter and check the polarity of the power supply. Negative gets bolted to the small nut in the face plate. Positive from the power supply goes to the center conductor of the rotary switch, solder it in place . Put the case together. Print the selector label and tape to the face plate. Connect the small led vehicle tester to the pigtail.  Plug the AC adapter in.  Move the switch to tail light.  The tail light led should light up.  Rotate the switch to  position #3 the left indicator should light. Rotate to #4 The right tail light indicator needs to light.  If all passes the tester is assembled correctly.
Now test a trailer. Get an extension cord and plug it in the wall.  Plug the test box to the trailer. Rotate the selector knob to off. Plug the ACDC power supply in the extension cord.  Rotate the knob to position #2 (tail light). Walk to the back of the trailer. Both of the tail lights should be lit red. Next check the left turn signal. Turn the switch to position #3, the left turn signal should be a little brighter than the tail light test.  Finally test the right turn signal. Turn switch to position #4. Check to see if the right  turn signal is on.

Trailer problemsBad ground. Ground wires get pulled while coupling the trailer.  They rust out internally in the wire. Bolted grounds can vibrate and fall off.  Grounds to light assembly get ground off on steep driveways. Blown light bulbs.  These can be tested with a multimeter. Corroded light sockets. Shorts from wires vibrating on the frame and cutting insulation. Rusted 4 flat connector. This can be fixed by cleaning and some very fine sandpaper.

Incandescent BulbsThe standard brake light on a trailer is the 1156 stoplight. It rates 26.9 W @ 12.8. It has an average life of 1200 hours.  Seems like a long life, but boat trailers put added stress by being submersed when used. The resistance changes a lot in a light bulb when heated. Whenever it lights it is heated. Use the heated resistance value when calculating load on the tow vehicle.
Some basic electricity W = V x A,  W is watts, V is DC voltage, A is amperage.
26.9 = 12.8 x A =>  A = 2.1
So for the 1156 incandescent stop light we draw 2.1 amps per bulb that is a lot.
V = I x R, V = 12.8, I = 2.1 => The operating resistance of a stoplight is 6.1 ohms.

LED Tail LightsLED tail lights have many advantages over conventional incandescent bulbs.  LEDs are cooler temperature.  They use much less power. They don't have rusty sockets. LEDs will blow out if connected with the wrong polarity current. Miswired they will pop and make some plastic fly.  Light assemblies have a reverse current diode to prevent damage to the light assembly. A multimeter is of less usefulness if diagnosing LED trailer lights. Because of the protection the multimeter will show No Connection for whatever one measures the resistance. The only definitive way is to power LEDS with the tester, set the rotary switch to a function,and observe the light themselves, walking to the back of the trailer.

SummaryA wall powered box to help diagnose trailer light problems is a useful tool to build. It is a good way to protect the towing vehicle from over current. The cost is minimal. Tests can be done in the driveway without use of a tow vehicle.

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