Homemade Trailer Light Tester

by K.K. Lowell
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Testing trailer lights after replacement of a lamp or a complete re-wiring job generally requires connecting the trailer to the tow vehicle. This can be a pain if the trailer is in the shop, or you've loaned your truck to your buddy for a short period of time. In just a few minutes and with very little effort, you can make a homemade trailer light tester with its own battery and trailer connector and check those lights anywhere.

What you'll need

You'll need a 12-volt battery, an inline fuse holder and 20 amp fuse, a tow vehicle-side trailer connector, some 14 or 16 gauge automotive wire, a two-prong turn signal flasher and a center off double throw toggle switch. You'll also need a container such as a milk crate to hold everything. You can use any 12-volt battery, but for portability's sake a lawn mower or motorcycle battery is best.

Wiring your tester

Start by connecting the inline fuse holder to the positive battery terminal. Connect the green wire from the trailer connector and a short length of automotive wire to the opposite wire from the fuse holder. Connect this short piece of wire to one of the prongs of the turn signal flasher. Using solderless connectors makes this job easy as a female spade-type connector fits the flasher prongs perfectly. Connect the other flasher prong to the center terminal of the toggle switch using a short length of wire. Then connect the yellow trailer connector wire to one side of the switch, and the brown trailer connector wire to the other. The final step in the construction of your trailer light tester is connecting the white wire from the trailer connector to the negative battery terminal.

Using the tester

Testing trailer lights with your new tester is easy. As soon as the trailer is plugged into the tester's trailer connector, the trailer tail lights and any side marker lights will illuminate if they are functioning properly. Flip the toggle switch lever to one side to test the turn signal bulb. Flip the switch in the opposite direction to test the light on the other side of the trailer. This also tests the brake light because the turn signal and brake light uses the same filament of the bulb.

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