Trailer Plug Wiring Diagram
Use this trailer plug wiring diagram as a general Australian reference for matching trailer plugs, car sockets and adapters. If you are wiring brakes, auxiliary power or caravan circuits, use a qualified auto-electrician.
Wiring diagram for trailer plugs and car sockets
Wiring diagram for wiring in trailer plugs and sockets. Details for small round 6 pin and 7 pin, flat 12 pin and flat 7 pin; and 7 pin large round.
The image below covers the common trailer plug wiring formats listed on this page, including 7 pin trailer plug wiring and 12 pin flat trailer plug wiring diagram checks.
We stock a full range of trailer plugs and sockets, as well as trailer plug adapters ideal for when you borrow a trailer that has a different plug to your car socket.
How to read the trailer plug wiring diagram
Start by identifying the plug or socket style on the trailer and tow vehicle. Do not assume a flat plug, round plug or adapter has been wired correctly by a previous owner.
Check the diagram against the physical plug shape first, then confirm each circuit by function. The practical checks are tail lights, indicators, brake lights, earth, service brakes and any auxiliary feed fitted to the trailer.
If a light or brake circuit does not match the diagram, stop and test the wiring before towing. A poor earth, damaged plug, corroded pin or incorrect adapter can make a trailer fault look like a tow vehicle fault.
|
Check |
What to confirm |
|---|---|
|
Plug type |
Small round, large round, flat 7 pin or flat 12 pin. |
|
Socket type |
The tow vehicle socket must match the trailer plug or use the correct adapter. |
|
Circuit function |
Confirm lights, earth and brake-related circuits by function, not colour alone. |
|
Condition |
Look for bent pins, corrosion, loose screws, cracked housings and strained cable. |
Choosing trailer plugs, sockets and adapters
Replace damaged plugs and sockets rather than trying to tow with loose pins or intermittent lights. A trailer plug is exposed to weather, vibration and cable strain, so the housing and terminals need to be sound.
Install permanent trailer plugs and sockets to create a permanent, matching connection between the tow vehicle and trailer. Use trailer plug adapters when the trailer and vehicle are already wired correctly but use different plug styles.
Adapters are useful when borrowing a trailer, but they should not be used to hide an unknown wiring fault. If the adapter works on one trailer and fails on another, stop and inspect the faulty trailer's plug, socket and earth connection.
7 pin vs 12 pin checks
A 7 pin plug is common for basic trailer lighting and service brake wiring where fitted. A 12 pin flat plug provides additional positions for extra circuits, depending on the trailer and tow vehicle setup.
Before changing from a 7 pin to a 12 pin setup, confirm what each extra circuit is intended to do. Do not add auxiliary power, charging feeds or brake wiring without checking cable size, circuit protection and the equipment manufacturer's instructions.
For a 7 pin small round trailer plug wiring diagram, match the diagram to the small round plug style before testing. For a 12 pin trailer plug wiring diagram, confirm you are reading the flat 12 pin section and not a 7 pin flat layout.
Electric brake wiring context
Trailer plug wiring for electric brakes needs more care than basic lighting. The brake controller output, service brake circuit, earth path and cable gauge all affect brake performance.
For brake controller installation, read our Brake Controller wiring diagram and use the correct wiring kit for the brake controller and trailer. We recommend an auto-electrician for brake controller wiring.
For electric over hydraulic trailer brakes, check the actuator manufacturer's wiring instructions as well as the trailer plug wiring. HydraStar wiring has specific gauge requirements that should not be guessed from a generic plug diagram.
When to use an auto-electrician
Use an auto-electrician if the trailer has electric brakes, electric over hydraulic brakes, breakaway wiring, battery charging, caravan circuits or unexplained wiring faults. These systems need correct cable size, circuit protection and testing under load.
Also use an auto-electrician if wire colours do not match the expected trailer wiring diagram Australia reference. Wire colour can vary after repairs, adapter changes or previous owner modifications, so testing the circuit is safer than assuming.
Still unsure which trailer plug, socket or adapter suits your trailer? Please call our friendly sales team on 07 3205 8846, email us or visit our store at 4 Leanne Crescent Lawnton.
Disclaimer: The information provided in this article is for general guidance only and does not constitute professional mechanical advice. Trailer components should be installed and maintained in accordance with manufacturer specifications and Australian Design Rules (ADR). Always consult a qualified mechanic or trailer specialist for specific applications. Trailer Spares Direct accepts no liability for work carried out based on this information. Ensure your trailer is roadworthy and compliant with your state or territory's regulations before towing.