5-Pin Trailer Wiring Diagram: Flat 5-Way Connector Pin Functions and Wiring
This is a free printable 5 pin trailer wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 5-pin flat trailer connector provides ground, tail/running lights, left turn/brake, right turn/brake, and a dedicated 12 V auxiliary power circuit, used on light utility trailers, boat trailers, and caravans.
The 5-pin flat connector (SAE 5-way flat, also called 5-flat or 5-pin flat) is the most common light-duty trailer connection in North America and is also widely used in Australia and parts of Europe for small to medium trailers. It is an extension of the universally adopted 4-flat connector, adding a fifth circuit for auxiliary power or electric brakes on lightweight trailers.
The five standard circuits and their colour coding (SAE/North American convention) are:
1. Ground (White wire): Common ground return for all trailer circuits. This must be a low-resistance connection to both the tow vehicle chassis and to every light and device on the trailer. A high-resistance or corroded ground connection is the single most common cause of trailer lighting faults.
2. Tail/Running lights (Brown wire): Illuminates the trailer's rear marker lights, clearance lights, and side marker lights whenever the tow vehicle's parking or running lights are on.
3. Left turn and brake (Yellow wire): Powers the trailer's left turn signal and brake light. The tow vehicle combines the turn and brake functions on this pin via the same light circuit.
4. Right turn and brake (Green wire): Powers the trailer's right turn signal and brake light, same function as yellow but for the right side.
5. Auxiliary power (Blue wire): Provides 12 V DC, typically key-switched or always-on depending on wiring. Used for electric trailer brakes (brake controller output), reverse lights, or auxiliary power devices. On brake-controller equipped tow vehicles, the blue wire carries the brake controller's modulated 12 V output.
In Australia, the 7-pin flat connector (ISO 11446 / AS 4735 7-flat) is more common for caravans, adding a 12 V battery charging circuit, reversing lights, and a continuous auxiliary supply. Always verify which standard your region and application require before purchasing a connector.
Use weatherproof connectors and apply dielectric grease to all pins to prevent corrosion — especially important in salt-air coastal and marine environments.
How to wire 5 pin trailer wiring diagram
- Verify connector type and regional standard Confirm you need a 5-pin flat connector (SAE North American standard) versus a 7-pin flat (Australian/NZ standard) or 7-pin round (European DIN/ISO standard). Match the connector at both the tow vehicle and trailer ends. Use the same standard at both ends.
- Identify tow vehicle connector wiring Locate the tow vehicle's trailer connector. If installing a new socket, identify the vehicle's trailer wiring harness or tap into the rear light circuits using a T-harness adapter specific to your vehicle (preferred over direct splicing into factory harness). Verify pin functions with a multimeter by activating each light circuit individually.
- Run trailer wiring from front to rear Route a 5-conductor trailer cable (or 4 individual wires plus ground) from the trailer's coupler end to the tail lights along the trailer frame. Secure the cable to the frame every 300–450 mm using cable clips, keeping it away from sharp edges and areas where it can be pinched by moving components.
- Connect the ground wire Connect the white ground wire to a dedicated clean metal ground point on both the tow vehicle (frame or battery negative) and on the trailer frame. Use ring terminals with star washers to bite through paint. Run a separate ground wire from the trailer frame to each light fixture — do not rely on the trailer frame alone as the sole ground path, as corrosion often makes this unreliable.
- Connect the tail/running lights Connect the brown wire to the tail/running light circuit. On the trailer, connect brown to the tail light element of each rear lamp and to any side marker or clearance lamps. Verify that all running lights illuminate when the tow vehicle's parking lights are switched on.
- Connect turn and brake lights Connect yellow to the trailer's left-side lamp (combined turn/brake element). Connect green to the right-side lamp. Test each circuit individually using the tow vehicle's turn signals and brake pedal. Each side should flash with turn and illuminate steadily when brakes are applied.
- Connect the auxiliary (blue) wire and test If using electric brakes, connect the blue wire to the brake controller's trailer brake output on the tow vehicle side, and to the trailer brake actuator terminals on the trailer side. Run a brake controller test. If using the auxiliary pin for another purpose (reverse lights, 12 V accessories), verify the source circuit is fused and rated for the load.
Specifications
| Connector standard | SAE J2863 5-way flat (North America); verify local standard for AU/NZ/EU |
|---|---|
| Pin 1 — Ground | White wire; common chassis ground return |
| Pin 2 — Tail/Running lights | Brown wire; illuminates with tow vehicle parking/running lights |
| Pin 3 — Left turn/brake | Yellow wire; left combined turn signal and brake light |
| Pin 4 — Right turn/brake | Green wire; right combined turn signal and brake light |
| Pin 5 — Auxiliary | Blue wire; 12 V auxiliary or brake controller output |
| Typical wire gauge | 18 AWG (0.75 mm²) per conductor for lighting; upsize blue wire for brake loads |
| Maximum recommended current per pin | Typically 20–30 A for the connector body; derate to cable ampacity |
Safety warnings
- Ensure all trailer lighting circuits comply with the road traffic regulations of your country and state/province before towing on public roads. Trailer lights are a legal safety requirement. Inadequate or malfunctioning brake lights and turn signals create a risk of rear-end collision.
- Fuse the auxiliary (blue) circuit at the tow vehicle. An unfused circuit connected to the battery or a high-current supply can start a fire if the trailer wiring is damaged or shorts to the trailer frame during towing.
- Never exceed the connector and cable current ratings. Overloaded wiring in a trailer harness can overheat, melt insulation, and cause a fire. If adding high-current loads to the trailer, use a relay to switch the load from the trailer connector signal wire rather than running load current through the trailer plug pins.
- Inspect the trailer connector and cable before each tow for corrosion, cracked insulation, and damaged pins. A corroded connector is the leading cause of trailer lighting failure, which may result in fines and road safety hazards.
- Ensure the trailer connector cable is long enough and routed so it does not become taut on full lock turns or when the tow vehicle and trailer are at maximum articulation angles. A cable pulled tight under cornering will damage both the connector and the wiring.
Tools needed
- Digital multimeter (continuity and voltage)
- Wire strippers
- Crimping tool (for ring and spade terminals)
- Flathead screwdriver (connector terminal screws)
- Drill and bits (for socket mounting if drilling into tow bar)
- Heat-shrink tubing and heat gun
- Dielectric grease applicator
- Cable ties and P-clips
Common mistakes
- Poor ground connection: failing to provide a low-resistance ground at every light fixture as well as at the connector, causing cross-illumination and intermittent light faults.
- Reversing the turn signal wires (yellow and green): left turn signal flashes on the right side and vice versa. Always verify which side is left and right with the trailer in the normal towing orientation.
- Using the trailer frame as the sole ground return without verifying continuity: many trailer frames have high-resistance joints due to paint or corrosion. Always run a dedicated ground wire.
- Omitting dielectric grease on connector pins: corrosion on the flat pin contacts is the most common cause of intermittent or failed trailer lighting.
- Running the trailer wiring cable without securing it, allowing it to hang and chafe on the trailer frame or road surface during towing.
Troubleshooting
- Trailer tail lights work but turn signals do not flash — they glow dimly
- Cause: Ground fault: the turn/brake circuit is back-feeding through the tail light circuit due to a poor or missing ground at the trailer Fix: Check the white ground wire at the connector and at every light fixture on the trailer. Measure resistance from the trailer frame to the tow vehicle frame — should be less than 0.5 Ω. Clean and retighten all ground connections, and run a dedicated ground wire to each lamp.
- No lights at all on the trailer
- Cause: Bad connection at the 5-flat plug or socket, corroded pins, or blown fuse on the tow vehicle trailer wiring circuit Fix: Inspect connector pins for corrosion or bent contacts. Apply dielectric grease. Check the tow vehicle's trailer fuse (typically in the main fuse box). Measure 12 V at the tail and turn pins on the vehicle socket with running lights on.
- Electric brakes on trailer do not engage
- Cause: Blue auxiliary wire disconnected or not wired at connector; brake controller not calibrated or not outputting voltage; blown brake fuse Fix: Measure voltage on the blue wire at the trailer plug while applying the brake pedal — should show 0–12 V depending on brake controller output level. Verify brake controller wiring and calibration. Check fuse on the brake controller circuit.
- One light on the trailer does not work
- Cause: Burned-out bulb; corroded or loose ground at that specific lamp; broken wire to that lamp Fix: Replace the bulb first (most common). Check ground at the lamp housing. Trace and test continuity of the wire to that lamp from the connector.
- Tow vehicle turn signal flashes too fast or too slow after trailer is connected
- Cause: Trailer adds or reduces the total bulb load on the tow vehicle's turn circuit. Fast flash = open circuit (no trailer bulbs loading the circuit); slow or no flash = additional load lowering circuit resistance Fix: Verify all trailer turn/brake bulbs are functional. If vehicle uses an electronic flasher, install a trailer-rated electronic flasher or a load equalizer relay that handles the combined vehicle and trailer bulb load without hyper-flashing.
Frequently asked questions
What is the difference between a 4-pin and 5-pin flat trailer connector?
A 4-pin flat connector provides ground, tail lights, left turn/brake, and right turn/brake — the minimum for trailer lighting. The 5-pin flat adds a fifth circuit: an auxiliary 12 V supply, commonly used for electric trailer brakes (brake controller output) or auxiliary power. The 5-pin connector is physically identical to the 4-pin but with one additional contact.
Why does my trailer have turn signals but no tail lights?
This is the classic symptom of a bad ground connection. The turn/brake circuits find an alternative ground path through the tail light circuit, causing cross-illumination. Check the white ground wire at both the tow vehicle connector and every trailer light fixture. Clean and tighten all ground connections.
Can I use a 5-flat connector for electric trailer brakes?
Yes — the blue auxiliary wire (pin 5) is the standard circuit for the brake controller output on a 5-flat connector. The brake controller in the tow vehicle modulates voltage on this wire proportional to deceleration. The trailer's electric brake actuators connect to this wire. Ensure the wire and connectors are rated for the brake current draw (typically 2–5 A per axle).
What colour is the brake controller wire on a 5-flat connector?
The brake controller output wire is blue on the standard SAE 5-flat colour code. It connects from the tow vehicle's brake controller output to pin 5 (the top centre position on most 5-flat connectors). On the trailer side, the blue wire connects to the electric brake actuator terminals.
Do I need a 7-pin connector for a caravan or camper trailer?
For a caravan or camper with a 12 V battery that needs charging from the tow vehicle, reverse lights, and separate indicator/brake circuits (European standard), a 7-pin connector (ISO 11446 or equivalent) is typically required. The 5-flat does not provide a separate battery charging circuit or reverse light pin.
Related diagrams
- 4 pin 5 wire trailer wiring diagram
- 5 pin trailer plug wiring diagram
- 5 pin trailer wiring diagram with brakes
- 5 wire trailer wiring to 7 pin diagram
- 12 pin trailer plug wiring
- 12v 5 pin relay wiring diagram