How to Wire a Trailer From 5-Wire to 4-Wire

by Bob White
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A trailer with a 5-wire harness has connections for marker, brake and signal lights as well as an electric braking system. By reducing a 5-wire to a 4-wire you eliminate the connection for the trailer's brakes. If your trailer is significantly heavy and is at the upper limits of the towing capacity of the vehicle, it is recommended that you upgrade the vehicle to a 5-wire harness. If the loss of the braking system is within the allowed limits of the vehicle, making the connection can be done in one of two ways.

Install Wiring Adapter

Step 1

Go to a trailer supply store and choose a wiring adapter that bridges the gap between your two connectors. Both 4-wire and 5-wire connectors are available in flat and round configurations. Understand what shape your connectors are before choosing an adapter.

Step 2

Push the 4-wire side of the adapter into the vehicle's harness.

Step 3

Push the 5-wire side of the adapter into the trailer's harness.

Test that the trailer's lights respond to the vehicle's commands as expected.

Rewire the Trailer

Step 1

Cut the 5-wire harness off the trailer's tongue leaving as much wire as possible. Strip three-eighths of an inch of insulation off the white, green, yellow and brown wires with wire strippers.

Step 2

Attach the 4-wire harness's green wire to the trailer's green wire with a butt connector. These work by placing one wire in each end and compressing with pliers.

Step 3

Connect the yellow, white and brown wires of the harness to the like-colored wires on the trailer with butt connectors.

Wrap each connection in electrical tape as well as the cut end of the blue wire. Wrap all of the wires together with electrical tape to provide a second layer of protection against moisture.

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