Uploaded July 2025 | Updated September 2026, 1 week ago
In this video, I show the development, fabrication, and testing of a small tester for trailer lighting circuits and lamp fixtures, connecting to the trailer using a conventional 4-pin/4-way/"4 Flat"/"Flat Four" connector.
Major components were sourced from Amazon, and the listings are shown in the video.
I made the described tester because I was not able to find a commercially available product to do the same thing, EXCEPT for some that required the use of an external motorcycle & car battery or required AC power. MY tester runs on an internal battery comprised of 4x "readily available AA" lithium cells, which are able to power the tester and the connected trailer lights for brief moments only, to verify correct trailer wiring and lamp functionality.
This tester does NOT test the tow vehicle circuits.
A schematic & wiring diagram may be downloaded here:
serpentwebsite.com/fileshare.htm
CAVEATS:
Caveat:
The tester described in this video was designed to operate the lighting systems of small utility & boat trailers having the common 4-pin/4-way/"flat four"/"four flat" connectors, and having modern LED type lamps. The
prototype was tested successfully on numerous modern trailers at a local marine equipment dealer and also at the local truck & trailer dealership.
Unlike most trailer light testers, it does not require an external motorcycle/car battery or power supply. Using lithium "AA" cells, it can briefly test trailer
lights many times before the "battery" is depleted.
However, the DC-DC Converter used in this tester cannot source enough current (1A maximum) to reliably operate trailer lighting curcuits that
use incandescent light bulbs rather than LEDs.
Plugged into such a trailer, the user might find that the "marker" light circuit does not work (it might consist of several incandescent bulbs
in parallel), while the turn signal/brake lights DO work (they might have only a single bulb).
Replacing the described DC-DC Converter with one of the available higher current versions MIGHT work better for such trailers, at the expense of battery life.
The two LEDs on the tester are for diagnostics:
When the Power button is pressed, the "6V OK" LED will light to show that the "battery" still has some ability to deliver power to the circuit. If this LED is out, or dim, replace the four "AA" lithium cells in the "battery". If this LED dims
when any of the other buttons are pressed (while the tester is plugged into the trailer wiring), replace the battery cells.
If the "12V OK" LED lights when the Power button is pressed, but goes out when any of the other buttons are pressed, this indicates that the DC-DC Converter has gone into self-protect mode due to the trailer circuit pulling too much current. This might indicate a faulty trailer light or short circuit in the wiring.
In this video, I show the development, fabrication, and testing of a small tester for trailer lighting circuits and lamp fixtures, connecting to the trailer using a conventional 4-pin/4-way/"4 Flat"/"Flat Four" connector.
Major components were sourced from Amazon, and the listings are shown in the video.
I made the described tester because I was not able to find a commercially available product to do the same thing, EXCEPT for some that required the use of an external motorcycle & car battery or required AC power. MY tester runs on an internal battery comprised of 4x "readily available AA" lithium cells, which are able to power the tester and the connected trailer lights for brief moments only, to verify correct trailer wiring and lamp functionality.
This tester does NOT test the tow vehicle circuits.
A schematic & wiring diagram may be downloaded here:
serpentwebsite.com/fileshare.htm
CAVEATS:
Caveat:
The tester described in this video was designed to operate the lighting systems of small utility & boat trailers having the common 4-pin/4-way/"flat four"/"four flat" connectors, and having modern LED type lamps. The
prototype was tested successfully on numerous modern trailers at a local marine equipment dealer and also at the local truck & trailer dealership.
Unlike most trailer light testers, it does not require an external motorcycle/car battery or power supply. Using lithium "AA" cells, it can briefly test trailer
lights many times before the "battery" is depleted.
However, the DC-DC Converter used in this tester cannot source enough current (1A maximum) to reliably operate trailer lighting curcuits that
use incandescent light bulbs rather than LEDs.
Plugged into such a trailer, the user might find that the "marker" light circuit does not work (it might consist of several incandescent bulbs
in parallel), while the turn signal/brake lights DO work (they might have only a single bulb).
Replacing the described DC-DC Converter with one of the available higher current versions MIGHT work better for such trailers, at the expense of battery life.
The two LEDs on the tester are for diagnostics:
When the Power button is pressed, the "6V OK" LED will light to show that the "battery" still has some ability to deliver power to the circuit. If this LED is out, or dim, replace the four "AA" lithium cells in the "battery". If this LED dims
when any of the other buttons are pressed (while the tester is plugged into the trailer wiring), replace the battery cells.
If the "12V OK" LED lights when the Power button is pressed, but goes out when any of the other buttons are pressed, this indicates that the DC-DC Converter has gone into self-protect mode due to the trailer circuit pulling too much current. This might indicate a faulty trailer light or short circuit in the wiring.
