Trailer Harness Wiring Diagram: 4-Pin Flat and 7-Pin Round Connector Colour Codes

Trailer Harness Wiring Diagram: 4-Pin Flat and 7-Pin Round Connector Colour Codes — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable trailer harness wiring diagram: download the diagram as SVG or open it and print to paper or PDF.

A trailer harness wiring diagram shows how the tow vehicle's electrical system connects to the trailer lights, brakes, and auxiliary circuits through a standardised multi-pin connector. Get the correct colour code and pin assignment before you crimp a single wire.

Trailer wiring harnesses serve one fundamental purpose: extending the tow vehicle's lighting and auxiliary signals to the trailer so that following drivers receive accurate braking, turning, and position information. Getting this wrong is not just an inconvenience — a trailer with non-functional brake lights or turn signals is a road safety hazard and will fail a roadworthy inspection in every jurisdiction.

The two most common connector formats are the 4-pin flat (4-flat) and the 7-pin round (also called 7-way or RV connector). The 4-flat covers the minimum legal lighting requirement: running/tail lights, left turn/brake, right turn/brake, and ground. The 7-pin round adds reverse lights, electric brake output, and a 12 V auxiliary power supply for trailer appliances or charging a caravan battery.

Colour coding is a notorious source of confusion because it varies by region. In North America, the SAE (Society of Automotive Engineers) convention is widely followed: white = ground, brown = tail/running lights, yellow = left turn and brake, green = right turn and brake. The 7-pin adds blue = electric brake, red = 12 V auxiliary (battery charge), and black = reverse lights. In Australia and Europe, the colour conventions differ — always verify against a diagram for your specific region and connector standard.

Connector pin numbering also varies. The ASABE/SAE 7-way round connector and the ISO 11446/ISO 3731 (formerly DIN 9680) 7-pin connectors used in Europe are different physically and electrically. The European trailer plug has a different pin arrangement and the auxiliary power function is handled differently. Never assume a 7-pin connector from one region is interchangeable with one from another without checking the pinout.

A trailer harness must include a ground return that is robust and low-resistance. A poor ground connection causes phantom faults — tail lights working but turn signals dim, brake controller errors, or reverse lights bleeding into running lights — that can be maddening to diagnose without a proper wiring diagram to trace from.

Fusing is frequently overlooked in aftermarket trailer harness installations. The auxiliary 12 V circuit in a 7-pin harness must be fused at the vehicle end to protect the vehicle's wiring from a short on the trailer.

How to wire trailer harness wiring diagram

Identify the connector type on your tow vehicle Locate the trailer socket at the rear of the vehicle. Determine whether it is a 4-pin flat, 5-pin flat, 6-pin round, 7-pin round (SAE), or 7-pin round (ISO 11446 European). The number of pins and the connector shape are your starting point for selecting the correct wiring diagram.
Identify the connector type on your trailer Check the trailer's plug at the draw-bar end. Verify it matches the vehicle socket, or identify what adapter is required. For a new harness installation, choose the connector type to match both the vehicle and all the trailer's functional requirements (lights only = 4-flat; electric brakes + auxiliary = 7-pin round).
Obtain the correct region-specific wiring diagram Download or reference a wiring diagram for your exact connector type and region. For North America, use the SAE J560 (7-way) or SAE J1239 convention. For Australia, refer to the relevant ADR (Australian Design Rule) standard. For Europe, use ISO 11446 or ISO 1724 as applicable.
Connect the trailer harness wires to the vehicle For most passenger vehicles with a modern CAN-bus lighting system, use a dedicated bypass relay harness rather than tapping directly into the vehicle's tail light wiring. Direct tapping can cause instrument cluster warning lights and damage the vehicle's body control module. Connect each function wire to the correct socket pin per the diagram.
Run and secure the harness to the trailer Route the harness along the trailer frame, securing it with cable ties at regular intervals to prevent chafing and road debris damage. Leave a service loop at the trailer nose — enough slack that the harness does not pull taut at full articulation of the coupling. Keep the harness away from the exhaust system.
Connect to the trailer light fixtures At each light cluster, connect the correct function wire per the diagram: tail, stop, left turn, right turn, reverse as applicable. Ensure all ground return wires connect to a common ground point that is also bonded to the trailer frame. A poor ground at one light can cause current to backfeed through other lamps.
Test every function before road use With the trailer coupled and the vehicle ignition on, test each circuit: running lights, left turn, right turn, brake lights (have a helper or use a brick on the pedal), reverse, and electric brake output if applicable. Walk around the trailer to verify each light responds correctly. Test the coupling articulated hard left and hard right.

Specifications

North American connector standard (7-way)	SAE J560
European connector standard (7-pin)	ISO 11446 / ISO 1724
Ground wire colour (SAE/North America)	White
Left turn/brake wire colour (SAE/North America)	Yellow
Right turn/brake wire colour (SAE/North America)	Green
Tail/running wire colour (SAE/North America)	Brown
Electric brake wire colour (SAE/North America)	Blue
12 V auxiliary wire colour (SAE/North America)	Red

Safety warnings

Always disconnect the vehicle battery before working on the vehicle's trailer socket wiring to prevent short circuits and accidental activation of airbag systems.
Fuse the auxiliary 12 V circuit (pin 4 on SAE 7-way) at the vehicle end within 450 mm of the battery or source. An unfused auxiliary circuit can cause a fire if the trailer wiring shorts.
Never tap directly into a vehicle's tail light wiring on a modern vehicle with a CAN-bus body control module without using an approved bypass relay — direct tap can damage the BCM and void the vehicle warranty.
Verify all trailer lighting functions comply with local road traffic legislation before towing on a public road. Operating a trailer with non-functional brake lights or turn signals is a traffic offence in all jurisdictions.
Secure the harness cable well above the ground clearance of the trailer to prevent road damage, and ensure it cannot contact the exhaust system — heat will melt the insulation and cause a short or fire.

Tools needed

Digital multimeter (continuity and voltage test)
Wire stripper
Crimp tool and waterproof crimp terminals
Cable ties (UV-resistant, suitable for automotive use)
Drill and drill bits (for mounting socket bracket if required)
Electrical tape or self-amalgamating tape
Test light or trailer wiring tester (plug-in type)

Common mistakes

Using the wrong wiring diagram for the region — North American SAE and European ISO 7-pin connectors look similar but have completely different pin assignments.
Connecting the ground wire (white/SAE) to a painted or corroded surface on the trailer frame rather than to bare metal, causing a high-resistance ground and erratic lighting faults.
Not leaving a service loop at the trailer nose, causing the harness to pull taut during tight turns and eventually tearing wires from the connector pins.
Omitting fusing on the auxiliary 12 V circuit, creating a fire risk if the trailer's charge circuit develops a short.
Ignoring electric brake circuit continuity — if the blue wire has no continuity from vehicle to trailer brake magnet, the brake controller will not function, which is illegal when towing above the threshold weight.

Troubleshooting

Running lights work but turn signals are inoperative: Cause: Damaged ground return, open circuit in the left (yellow/SAE) or right (green/SAE) function wire, or a blown fuse in the vehicle trailer circuit Fix: Check continuity from the vehicle socket pin to the trailer lamp with the harness connected. Test the ground (white/SAE) resistance — it must be below 0.5 ohms from socket to trailer frame. Replace any blown fuse and inspect the harness for crush damage near the hitch.
Electric brakes are not engaging: Cause: No voltage on the blue wire at the trailer plug, brake controller not calibrated, or open circuit in the brake magnet wiring on the trailer Fix: With the brake controller active, measure voltage on the blue pin at the trailer plug with a multimeter — you should see a modulated voltage when the brake controller activates. If absent, trace back to the brake controller output. If present at the socket but brakes still do not engage, check the trailer brake magnet wiring and magnet resistance.
Tail lights illuminate when turn signals are activated (backfeed): Cause: A high-resistance or absent ground, causing current to return through other lamp filaments instead of the ground wire Fix: Inspect and improve the ground connection at both the vehicle socket and every trailer ground bonding point. Clean corroded frame connections back to bare metal and use a crimped ring terminal with a serrated washer for a positive ground bond.

Frequently asked questions

What is the standard wire colour for ground in a North American trailer harness?

In the North American SAE convention, white is the ground (earth return) conductor in a trailer harness. This is the opposite of many mains electrical conventions. A poor or missing white ground wire is the single most common cause of erratic trailer lighting faults, including dim lights, backfeed, and turn signal cross-talk.

What is the difference between a 4-pin flat and a 7-pin round trailer connector?

A 4-pin flat connector carries the minimum lighting functions: ground, tail/running, left turn/brake, and right turn/brake. A 7-pin round adds electric brake output, a 12 V auxiliary power supply for battery charging or trailer accessories, and a reverse light circuit. Use 7-pin for caravans, horse floats, boat trailers with winches, and any trailer with electric brakes.

Are trailer harness colour codes the same in Australia and Europe as in North America?

No — colour conventions vary significantly by region and connector standard. North American SAE 7-way and European ISO 11446 / ISO 3731 connectors have different pin assignments and wire colours. Always use a wiring diagram specific to your region and connector type. Never assume colour codes are universal when working on an imported vehicle or trailer.

Why do my trailer lights work but my turn signals do not?

The most common causes are a faulty ground (white wire), a blown fuse in the vehicle's trailer circuit, or a failed turn signal function at the trailer socket pin. Check all ground connections first — a high-resistance ground can cause tail lights to work (always on, low current) while intermittent functions like turns fail under load.

Does my tow vehicle need a trailer wiring harness adapter?

Many modern vehicles output a 7-pin round connector as standard. Older vehicles, or those fitted with a 4-pin flat, may require a plug adapter or a complete harness upgrade if you are towing a trailer that needs electric brakes or auxiliary power. Some vehicles also require a bypass relay to protect the vehicle's CAN-bus lighting circuits from the trailer load.

Related diagrams

Free electrical calculators

Edit this diagram free in the online editor