Trailer Plug Pinout: 7-Pin, 4-Pin, and 13-Pin Connector Wiring Reference
This is a free printable trailer plug pinout: download the diagram as SVG or open it and print to paper or PDF.
A trailer plug pinout defines which wire connects to which pin of the trailer connector — get it wrong and you leave a caravan park with brake lights and indicators doing the opposite of what the driver intends.
A trailer plug pinout is the standardised assignment of electrical functions to the individual pins of a trailer connector. The pinout determines which terminal carries the running lights, left turn/brake signal, right turn/brake signal, reverse lights, electric brakes, auxiliary 12 V supply, and earth return. Without a correct pinout, a tow vehicle and trailer will have cross-wired lights or non-functional electric trailer brakes — both safety hazards and, in most jurisdictions, legal violations.
The most common connector types worldwide are:
7-pin round (SAE J560, North America): A 7-pin circular connector used on most North American trucks and heavy trailers. Pin functions: 1 = Ground, 2 = Electric brakes, 3 = Running lights (tail/clearance/side markers), 4 = Left brake/turn signal, 5 = Right brake/turn signal, 6 = Battery charge (accessory), 7 = Reverse lights. The connector keys are numbered moulded into the plastic body.
7-pin flat (Australian Standard AS 4735): Extremely common in Australia and South Africa. The flat 7-pin uses a flat blade arrangement: pin 1 = Left turn/brake (yellow), pin 2 = Reverse (white or black), pin 3 = Earth (white), pin 4 = Right turn/brake (green), pin 5 = Running lights (brown), pin 6 = Brake controller (blue), pin 7 = Battery/auxiliary (red). Colour conventions follow the trailer wiring harness, not universal standards.
4-pin flat (North America): Used on light trailers. Left turn/brake = yellow, Ground = white, Running lights = brown, Right turn/brake = green.
13-pin round (ISO 11446): The European standard, replacing the older 7-pin DIN (ISO 1724) on most cars and caravans. Pins 1–7 largely mirror the older 7-pin DIN; pins 8–13 add reverse light, battery charge, and a spare. Pin 3 is left turn, pin 4 is right turn, pin 6 is stop lamp feed, pin 13 is permanent battery supply.
Earth return quality is the single most common cause of trailer light faults. A poor earth connection causes back-feeding through signal wires, making indicators flash in wrong sequences and tail lights glow faintly when brakes are applied.
How to wire trailer plug pinout
- Identify the connector type on the tow vehicle and trailer Count the pins and note whether the connector is round or flat. Check whether the tow vehicle uses a North American 7-pin round (SAE J560), an Australian/SA 7-pin flat (AS 4735), a European 13-pin (ISO 11446), or a simple 4-pin flat. Adapters exist but add resistance and potential failure points.
- Locate the pin number markings Most connectors have pin numbers moulded or stamped into the plastic body. On a 7-pin flat, the flat edge of the connector body is usually the reference orientation. On round connectors, pin 1 is typically at the top or has a moulded indicator arrow. Photograph the connector before wiring.
- Map each pin to its function using the applicable standard Use the pinout for your specific connector type. Do not assume colour codes from one country apply to another — Australian yellow is left brake/turn, while US yellow is also left turn/brake but green is right, and the earth is white not black. Always verify the function at the tow vehicle socket with a multimeter while an assistant operates the vehicle lights and brakes.
- Verify tow vehicle socket pin voltages With the vehicle's ignition on and lights cycling, use a multimeter to probe each socket pin. Confirm: running lights pin goes to approximately 12 V with parking lights on; left turn pin flashes in sync with left indicator; right turn pin flashes with right indicator; brake pin goes to 12 V with brake pedal depressed. Note which pin remains at 0 V — that is the ground return.
- Connect trailer harness wires to trailer connector pins Strip wire ends 8–10 mm. Use weatherproof crimp terminals or solder and heat-shrink tubing. Do not use insulation-displacement connectors (scotch locks) for trailer wiring — vibration causes them to loosen and corrode. Secure each wire to its designated pin and ensure no bare conductor is exposed.
- Test all functions before road use Connect the trailer to the tow vehicle. With an assistant at the vehicle, cycle through: parking lights (tail and running lights should illuminate), left indicator (left trailer indicator should flash in sync), right indicator, brake pedal (brake lights should illuminate), reverse (if wired). Test electric brakes by manually triggering the brake controller.
Specifications
| Australian 7-pin flat (AS 4735) — Pin 1 | Left turn and brake signal (yellow wire) |
|---|---|
| Australian 7-pin flat (AS 4735) — Pin 3 | Earth/ground return (white wire) |
| Australian 7-pin flat (AS 4735) — Pin 5 | Running/tail lights (brown wire) |
| Australian 7-pin flat (AS 4735) — Pin 6 | Electric brake controller signal (blue wire) |
| North American 4-pin flat — White | Ground return |
| North American 4-pin flat — Brown | Running/tail lights |
| North American 4-pin flat — Yellow | Left brake and turn signal |
| North American 4-pin flat — Green | Right brake and turn signal |
Safety warnings
- Trailer wiring faults are a significant road safety risk. Non-functional brake lights or indicators on a trailer can cause rear-end collisions, especially at night. Always verify all trailer light functions before every towing trip, not just after initial wiring.
- The auxiliary battery supply wire (red, pin 6 on Australian flat or pin 6 on SAE J560) must be individually fused as close to the tow vehicle battery as practical — typically with a 15 A or 20 A fuse. An unfused connection is a fire risk if the wire chafes or shorts.
- For trailers with electric brakes, the brake controller wiring must be installed by a competent person familiar with electric braking systems. Incorrectly wired or adjusted electric brakes can lock trailer wheels during braking or fail to apply at all — both are dangerous on roads and result in legal liability.
- In most jurisdictions, trailers above a minimum gross vehicle mass (GVM) — typically 750 kg to 2 000 kg depending on country — are legally required to have functional brake systems. Check the trailer and tow vehicle regulations in your jurisdiction before towing.
Tools needed
- Multimeter (voltage and continuity)
- Wire strippers
- Ratcheting crimp tool with weatherproof terminal dies
- Heat gun (for heat-shrink tubing)
- Screwdrivers (flathead and Phillips)
- Self-tapping screws or drill for chassis ground connection
Common mistakes
- Assuming wire colour codes are universal — an Australian trailer loom brought to North America will have conflicting colour conventions. Always verify function with a multimeter rather than relying solely on colour.
- Using scotch-lock or insulation-displacement connectors in the trailer harness — vibration and moisture cause these to corrode and fail. Use proper crimp or soldered, heat-shrunk connections.
- Grounding the trailer through the connector earth pin only, without a supplementary chassis earth — on a long trailer with corroded frame connections, light circuits back-feed through signal wires. Add a direct earth strap from trailer frame to tow ball coupling.
- Leaving the auxiliary supply wire unfused — this wire runs to the tow vehicle battery and can carry significant current if the trailer's auxiliary equipment shorts. An unfused direct battery connection is a fire hazard.
- Not securing the connector cable with a strain-relief clamp — the plug body cracks or the wiring pulls loose from pin terminals when the cable is not clamped at the connector entry point.
Troubleshooting
- Trailer tail lights glow faintly when brakes are applied
- Cause: Poor trailer earth return — brake signal current back-feeds through the tail light wire to return via another circuit Fix: Measure resistance between trailer frame and tow vehicle chassis with trailer disconnected from the socket — should be below 0.5 Ω. Clean or replace the ground connection at the trailer frame and add a supplementary earth strap if resistance is high.
- All trailer lights work except the left or right indicator
- Cause: Open circuit in the indicator wire — broken conductor, failed crimp terminal, or corroded pin inside the trailer plug Fix: Probe the relevant socket pin on the tow vehicle with a multimeter while an assistant operates the indicator — verify voltage is present at the socket. Then probe the same pin inside the trailer plug with the trailer connected. If voltage drops at the trailer plug, the connector or wiring has a fault.
- Electric trailer brakes do not apply or apply only weakly
- Cause: No voltage on the brake controller pin (blue wire/pin 6), open circuit in the brake controller wire, incorrectly adjusted or faulty brake controller, or low battery voltage on trailer Fix: Measure voltage on the brake controller pin with the tow vehicle brake pedal depressed and the brake controller powered. Verify continuity from the tow vehicle socket brake pin through to the trailer brake assemblies. Adjust the brake controller gain per the controller manufacturer's instructions.
Frequently asked questions
What is the standard pin-out for a 4-pin flat trailer connector (North America)?
On a North American 4-pin flat connector: white = ground, brown = running/tail lights, yellow = left brake and turn signal, green = right brake and turn signal. These colour conventions are widely followed but not universally consistent on older trailers — always verify with a multimeter before connecting.
Why do my trailer indicators flash when I press the brakes instead of activating the brake lights?
This is almost always a crossed or shorted earth return. Back-feeding occurs when the trailer's ground connection is faulty, causing signal current to return through adjacent signal wires instead of the earth wire. Check earth continuity from trailer frame to tow vehicle chassis — resistance should be below 0.5 Ω.
What is the blue wire on an Australian 7-pin flat trailer plug?
On the Australian AS 4735 7-pin flat connector, the blue wire (pin 6) is the electric brake controller output. It carries the proportional braking signal from a brake controller fitted in the tow vehicle to the trailer's electric brake assemblies. It carries no current when electric brakes are not fitted.
Can I use a 4-pin connector for a trailer with electric brakes?
No. A standard 4-pin flat connector does not include a pin for the electric brake controller signal or a dedicated battery charge wire. A trailer with electric brakes requires at least a 7-pin connector (North American round or Australian flat) that includes the brake controller wire.
What is the difference between the ISO 1724 7-pin (DIN) and ISO 11446 13-pin European connectors?
ISO 1724 is an older European 7-pin connector (round, 12 V) common on caravans and horse trailers. ISO 11446 is the 13-pin successor that adds reverse lights, 12 V battery charging, and spares. Many modern European tow vehicles fit 13-pin sockets, with 7-to-13 adapters available for older trailers.
Related diagrams
- 12 pin trailer plug wiring
- 13 pin trailer plug wiring diagram
- 4 prong trailer plug wiring diagram
- 4 way trailer plug
- 5 pin trailer plug wiring diagram
- 5 wire trailer plug