4-, 5- & 6-Way Trailer Connectors

What it is

These connectors join a light truck, SUV or car to a utility, boat or small cargo trailer. The 4-way flat is the simplest and carries only lighting. The 5-way flat adds one circuit, most often a backup lamp feed that locks out surge brakes when reversing. The 6-way round adds two circuits, often electric brakes and a 12 V feed, for trailers with electric brakes or a battery. Larger trailers with brakes and accessories move up to the 7-way RV blade connector.

How to identify

Count the terminals and look at the shape. The 4-way flat is a molded flat plug with four pins and sockets side by side. The 5-way flat looks the same with a fifth terminal. The 6-way round has a metal or plastic round shell with six pins around a center pin. Adapters convert between types for the circuits they share.

4-way flat wiring (common color code)

On a 5-way flat, the fifth wire is usually blue and used for backup lamps or surge brake lockout. On a 6-way round, the added circuits are commonly electric brakes and 12 V auxiliary, but pin assignments vary by maker. Confirm with the trailer and vehicle wiring diagrams.

Key points

• The trailer ground (white) must connect to clean frame metal on both the vehicle and the trailer. Most lamp problems are ground problems.
• Many vehicles with separate stop and turn lamps need a converter to drive a 4-way trailer, where stop and turn share one wire.
• A circuit tester or test light is the quickest way to confirm each function at the plug.
• Seal flat plugs with a cap or dust cover when unhooked. Corroded terminals cause flicker.
• Adapters do not add circuits. A 4-way trailer behind a 7-way vehicle socket still only gets lighting.
• For heavy trucks and commercial trailers see 7-way trailer connectors and the 7-way trailer wiring chart.

Common mistakes

• Grounding the trailer through the hitch ball only.
• Assuming every 6-way round uses the same pin layout.
• Wiring a separate-stop-and-turn vehicle straight to a 4-way plug without a converter.
• Leaving connectors on the ground or in water.

Governing standards: SAE recommended practices for light trailer connectors; FMVSS 108 for required lamp functions. Vehicle and trailer maker wiring diagrams.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.