6-Pin Trailer Wiring Diagram

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A complete reference for 6-pin trailer connectors, covering running lights, turn signals, brakes, electric brake output, and 12V auxiliary power.

The 6-pin round trailer connector extends the capability of the 4-pin and 5-pin flat connectors by adding two additional circuits: a dedicated electric brake output and a 12V auxiliary power feed. This makes it the standard choice for trailers carrying heavy loads — horse floats, boat trailers with winches, camper trailers, and car haulers — where electric brakes are legally required above certain gross trailer weights in most jurisdictions.

Pin assignments follow the SAE J560 standard for 7-way round connectors as a reference point, but the 6-pin round (also called the 6-way round) has its own widely accepted pinout. The six circuits are: ground/earth (white), tail/running lights (brown), left turn and brake (yellow), right turn and brake (green), electric brake output (blue), and 12V auxiliary/battery charge (black or red depending on manufacturer convention). Always verify against the specific vehicle and trailer manufacturer documentation before wiring.

The electric brake output (blue wire) carries a proportional signal from the brake controller mounted in the tow vehicle cab. When the driver applies the brakes, the controller sends a voltage — typically 0 to 12V DC — to the trailer's electric brake assemblies. The voltage level corresponds to brake application intensity, modulated by inertia sensors or a time-based ramp depending on the controller type.

The 12V auxiliary circuit (black) allows the trailer battery to be charged while under tow, powers trailer-mounted accessories such as electric jockey wheels, hydraulic tipping systems, or fridge units, and keeps trailer battery-backed brake controllers topped up.

Wiring gauge matters here. The brake output and auxiliary circuits should be run in 4 mm² (12 AWG) minimum due to potential current draw from multiple brake magnets — a tandem-axle trailer with four electric brake assemblies draws approximately 16–20 A under hard braking. All circuits require proper fusing at the tow vehicle, and the trailer chassis must carry a continuous low-resistance earth path back to the vehicle chassis. A bad earth causes brake fade, dim lights, and erratic turn signal behaviour.

How to wire 6 pin trailer wiring diagram

Identify the tow vehicle socket pinout Locate the vehicle's trailer wiring harness socket and use the vehicle service manual or a verified pinout reference to confirm which terminal corresponds to each circuit. Do not assume pin positions — orientation and numbering vary by vehicle manufacturer. Use a multimeter to verify live voltage on the tail light and brake circuits with the vehicle running and lights on.
Select and route the trailer harness Choose a pre-made 6-core trailer wiring harness rated for outdoor and UV exposure. Route the harness along the trailer chassis using UV-stabilised cable ties or wiring clips, keeping it clear of exhaust heat, moving suspension components, and areas likely to collect standing water. Leave a drip loop before the connector to prevent water ingress.
Connect the trailer chassis earth first Bond the white earth wire directly to the trailer chassis at a clean, paint-free metal surface using a ring terminal and stainless or zinc-plated fastener. This is the single most critical connection — a high-resistance earth causes almost every trailer wiring fault. Measure resistance from the earth pin to the trailer chassis: it must read below 0.1 Ω.
Wire the lighting circuits Connect brown to tail/running light supply, yellow to left stop-and-turn, and green to right stop-and-turn at each lamp. Use waterproof butt connectors or solder-and-heatshrink joins at each junction. Test each circuit at the connector before proceeding — confirm tail, left turn, right turn, and brake lights all operate correctly.
Wire the electric brake output Run the blue wire from the 6-pin connector along the chassis to each brake assembly junction box. Connect in parallel across all brake magnets — not in series. Measure total circuit resistance: four brake magnets in parallel should read approximately 3–4 Ω. A reading significantly higher indicates a broken magnet wire or bad connection.
Wire the 12V auxiliary circuit Connect the black (or red, confirm with your connector spec) auxiliary wire to the trailer battery positive terminal via an in-line fuse of no more than 30 A, sized to the load. Include a diode or isolator relay if bidirectional battery protection is required. Connect the auxiliary earth to the main chassis earth point, not to the trailer battery negative alone.
Test all circuits and seal the connector With the trailer connected to the tow vehicle, verify all six circuits using a circuit tester or multimeter. Apply tow vehicle brakes and confirm brake output voltage at the blue pin reaches 12V. Test the auxiliary circuit with a voltmeter under load. Apply dielectric grease to all connector pins and mate the plug. Test again after the first 50 km of towing — vibration can reveal loose connections.

Specifications

Operating voltage	12V DC (nominal automotive)
Brake output current (4 magnets)	16–20 A maximum under full brake application
Auxiliary circuit maximum current	30 A (fuse protected)
Minimum wire gauge — brake and auxiliary	4 mm² (12 AWG)
Minimum wire gauge — lighting circuits	1.5 mm² (14 AWG)
Earth path resistance (maximum allowable)	0.1 Ω from earth pin to trailer chassis
Electric brake magnet resistance (each)	Approximately 3.5 Ω (verify with manufacturer datasheet)

Safety warnings

Electric trailer brakes are a safety-critical system. Incorrect wiring can result in complete brake loss or brake lockup under load. Have the installation inspected and tested by a licensed auto-electrician before towing on public roads.
Always disconnect the tow vehicle battery negative terminal before working on the vehicle's trailer socket wiring to avoid short circuits against chassis.
Trailer wiring must comply with applicable road vehicle standards in your jurisdiction — including ADR (Australia), FMVSS (USA), or equivalent — as well as general electrical wiring codes such as NEC/NFPA 70, BS 7671, or IEC 60364 where applicable.
Never use undersized wire for the brake output or auxiliary circuits. Sustained overcurrent without proper fusing causes insulation failure, fire risk, and brake malfunction.
Test the breakaway battery and breakaway switch function before each long-distance journey. A failed breakaway system means the trailer has no brakes if it separates from the tow vehicle at speed.

Tools needed

Digital multimeter (voltage, resistance, and continuity functions)
Crimping tool for automotive terminals
Wire stripper
Heat gun (for heatshrink connectors)
Drill and stainless fasteners (for socket and earth bonding)
Circuit tester or test lamp
Cable tie gun and UV-stabilised cable ties

Common mistakes

Earthing through the trailer ball coupling rather than a dedicated wire — the ball mount is a mechanical connection, not a reliable electrical earth, and will cause dim lights and brake faults under load.
Connecting all brake magnets in series rather than parallel, which increases circuit resistance and reduces braking force significantly.
Omitting an in-line fuse on the auxiliary 12V circuit, leaving the circuit unprotected against shorts that could cause a vehicle electrical fire.
Using undersized wire (e.g., 1.5 mm² for brake output) that overheats under the brake current draw of multiple magnets.
Failing to apply dielectric grease to the connector, causing corrosion of pins within weeks of road use — particularly in wet or coastal environments.
Not verifying brake controller operation after installation. Many controllers require calibration of the gain and sensitivity settings for the specific trailer weight.

Troubleshooting

No brake output voltage at the blue pin: Cause: Brake controller not powered, controller wiring fault, or the controller is not detecting brake pedal input Fix: Verify the brake controller has 12V supply and earth at its input terminals. Check that the brake signal wire from the vehicle's brake light circuit is connected to the controller. Test the controller's output directly with a multimeter while pressing the brake pedal.
Trailer lights work but electric brakes do not apply: Cause: Blue brake output wire is open-circuit between the 6-pin connector and the brake assemblies, or a broken magnet coil Fix: Measure resistance across the brake circuit at the connector with the trailer disconnected: expect approximately 3–4 Ω for four magnets in parallel. An open circuit (infinite resistance) indicates a broken wire or disconnected magnet. A reading of 0 Ω indicates a short circuit.
Auxiliary battery not charging while under tow: Cause: Blown fuse in the auxiliary circuit, corroded connector pin on the black/red wire, or low voltage on the tow vehicle's auxiliary output Fix: Check the in-line fuse first. Then measure voltage at the 6-pin auxiliary pin with the tow vehicle running — it should be 13.5–14.4V. If voltage is correct at the plug but absent at the trailer battery, trace the black wire for breaks or corrosion.
Left and right indicators swapped on the trailer: Cause: Yellow and green wires connected to the wrong lamps or reversed at the plug Fix: With the vehicle connected, activate each turn signal individually and observe which trailer lamp flashes. Swap the yellow and green connections at the lamp or at the plug to correct the orientation.

Frequently asked questions

What is the blue wire on a 6-pin trailer connector used for?

The blue wire carries the electric brake output signal from the in-cab brake controller to the trailer's electric brake assemblies. The voltage on this wire (0–12V DC) is proportional to braking effort. It must be connected for electric trailer brakes to operate and is not present on 4-pin or 5-pin flat connectors.

Can I use a 6-pin connector if my trailer does not have electric brakes?

Yes. You can wire only the four or five circuits you need and leave the brake output and auxiliary pins unused, provided the unused terminals are protected from moisture. However, if your trailer is over the weight threshold requiring brakes in your jurisdiction, fitting electric brakes is a legal requirement, not optional.

What wire gauge should I use for the brake output circuit?

A minimum of 4 mm² (12 AWG) is recommended for the brake output wire, running from the brake controller to the connector. Each electric brake magnet draws approximately 3–4 A; four magnets on a tandem-axle trailer can draw up to 16 A under full braking load.

Why do my trailer brakes work intermittently?

Intermittent electric brakes are almost always a wiring or earth fault. Check the white earth wire at the connector and at each brake assembly. A resistance reading above 0.3 Ω on the earth path will cause voltage drop sufficient to reduce brake force. Also check the brake controller gain setting and breakaway switch function.

Is a 6-pin the same as a 7-pin trailer connector?

No. A 7-pin connector (SAE J560) adds a seventh circuit, most commonly a second earth or a reverse light output, and uses a larger round housing. The 6-pin round is physically smaller. The two are not interchangeable without an adapter, though adapters are widely available.

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