How to Wire a 4 Pin Trailer Connector to a 7 Pin Connector

by Jack Hathcoat

There is a lot of competition and confusion in the trailer world when it comes to connectors. In addition to several models of connectors on the market, there are numerous adapters that connect one brand to another. So it makes sense to hard-wire a common connector to a high-use trailer. Adapters have extra wires. They can easily snag or get frayed. This will inevitably lead to blown fuses and major wiring repairs.

Cut the four-pin plug off of the trailer harness. Verify the harness color codes at each tail light. There should be a common color for both the tail lights. The left and right turn signal lights should each have a different color. This accounts for three of the colors in the wiring harness. The remaining color is the ground wire. Note the colors and their function.

Turn on the tow vehicle tail lights. Probe the seven-pin connector with a test light. Note the location of the pin. Repeat this process for both turn signals. Trace the ground wire location from the back of the plug and follow the wire to the body ground location.

Splice the seven-pin trailer plug leads to the trailer harness. Crimp each connector then plug the connector into the tow vehicle. Test each circuit and make any needed corrections.

Wrap the spliced wires in electrical tape and make a wiring harness. Cinch each end of the harness with a small zip-tie to prevent the tape from unraveling. Strap the harness to the trailer tongue.

Tip

  • check Seven-pin connectors are rarely used on trailers for local use. The three additional pins are: trailer brakes, back-up lights and trailer battery charging. They are commonly used when towing travel or camping trailers.

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About the Author

Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.

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