Utility trailers come in various widths, sizes and load capacities. How a trailer is wired depends on what is required. Small trailers have side marker lights, tail lights, brake lights and turn signals. Trailers wider than 80 inches are required to have rear markers. If your state requires a license plate for a trailer, you will need to wire that also. Utility trailers hauling heavy loads may have electric trailer brakes. Utility trailer wiring connectors come in configurations with four to seven pins and corresponding wires dependent on the lights and accessories required.
Select the correct wire sizes and crimp connectors for your trailer wiring. Use 16-gauge white wire for the ground on four- and five-wire trailer connectors, and 12-gauge on six- and seven-wire trailer connectors. Use 18-gauge wire for all colored wires on four-wire trailer connectors and 16-gauge wire on six- and seven-wire trailer connectors. Blue wires should be 12-gauge on six- and seven-wire trailer connectors and 18-gauge on four- and five-wire connectors.
Wire the tail, marker and license plate lights with brown wire. Route brown wire along the left side of the trailer frame to the left tail light, and then over to the license plate light. Route brown wire on the right side to the tail light and over to the rear marker lights. Insert the rear marker and right side brown wire into a quick connect, and then crimp it closed. Repeat with the license plate wire on the left side. Make additional connections for the tail lights and side markers, using mid wire t-taps. Place the taps on the main wire, insert the light wire, close the plastic case and crimp it closed.
Route green wire to the tail light, via the right side of the trailer. Connect the green wire to the brake/turn-signal wire with a quick connect. Route a yellow wire along the left side of the trailer to the tail light. Connect the yellow wire to the brake/turn-signal wire with a quick connect.
Route blue wires along the left and right sides of the trailer frame to the brakes. Connect the blue wires to the electric trailer brake wires at each wheel using quick connects and mid wire t-taps. Connect the brake ground wire at each wheel to the trailer frame.
Route a red wire to the interior trailer lights, break away kit, battery charger or other accessories. Make connections as appropriate to the accessory--some may need crimp-on ring connectors, others may have screw terminals or connect with quick connects. Use a white wire to connect the ground wire of each accessory to the trailer frame.
Route purple wire to the backup lights and make connections using quick connects and mid wire t-taps.
Connect the right side brown wire to the left side brown wire using a mid wire t-tap. Connect all the colored wires to the corresponding wires on the trailer wiring connector with quick connects. Crimp a stud ring to the white ground wire on the trailer wiring connector. Attach the ring stud to the trailer frame at an existing bolt (remove the nut, place the stud ring over the bolt as if it were a washer, and replace the nut).
Connect the trailer wiring connector to the vehicle wiring connector. Turn on the parking lights. The license plate light, tail lights, rear markers and side markers should turn on. Step on the brakes--both tail lights should brighten. Release the brakes and test the left and right turn signals. Put the vehicle in reverse and the backup lights should go on. The interior lights and accessories should also function.
Items you will need
- Trailer wiring connector
- 12- or 16-gauge white stranded wire
- 16- or 18-gauge green, yellow and brown stranded wire
- 16- or 18-gauge red, and purple stranded wire
- 12- or 18-gauge blue wire
- 18-22 or 14-16 gauge quick splice connectors
- 18-22 or 14-16 gauge mid wire T-taps
- 10-12 gauge quick splice connectors
- 10-12 gauge mid wire-T-taps
- Wire crimper/stripper for stranded 12-18 gauge wires
- Wire ties
- Electrical tape
- If you only have a 4-, 5- or 6-wire connector, follow the steps and stop with the last wire color you have on the connector. Leave out lights such as rear markers and license plate lights if you don't have them.
- Use electrical tape to bundle wires, and wire ties to fasten wires to the frame.
- An electrical system on a trailer uses the frame of the trailer as the ground wire. If your lights attach to something besides the frame, you may need to make a ground wire to run from the light kit bolt to the trailer frame.
- The most common wiring problem is a lack of ground. If one or more of the lights do not work, look for a poor or missing ground.
- Always visually test the lights and trailer brakes on your trailer to ensure proper operation before driving the vehicle with the trailer attached.
- Some manufacturers specify different wiring colors than those shown. Check your trailer wiring connector's manufacturer for wire colors.
- Ryan McVay/Digital Vision/Getty Images