Quick answer
Australian 7-pin flat and 7-pin large round trailer connectors use the same pin numbers for the same jobs: 1 left indicator (yellow), 2 reversing (black), 3 earth (white), 4 right indicator (green), 5 electric brakes (blue), 6 stop lights (red) and 7 tail lights (brown). Wire each core to the terminal with the matching number moulded beside it, make sure the earth is solid, and test every function before you tow.
Is this a DIY job?
- Difficulty: Medium
- Time: 30 to 60 minutes for one plug
- Tools: wire strippers, small screwdriver, side cutters, trailer circuit tester (or a helper), heat shrink and heat gun (optional)
- Skills: stripping and terminating small wires neatly
Replacing the plug on a trailer is a reasonable DIY job. Wiring a new socket into a modern car is different: many current vehicles need a vehicle-specific towbar harness or module, and cutting into factory wiring can cause faults. That part is often best left to an auto electrician.
Plug or socket?
- The socket is on the tow vehicle (at the towbar).
- The plug is on the end of the trailer cable.
The pin numbers and colours are the same on both. These steps describe wiring a trailer plug; the socket is wired the same way.
Step by step
- Identify the connector. Check whether you have a 7-pin flat, a 7-pin large round, or something else. Don’t confuse the Australian 7-pin large round with the smaller European 7-pin round: they’re wired differently and aren’t interchangeable. If you need extra circuits (battery charging, auxiliary power), a 12-pin flat connector is the usual answer.
- Disconnect from the vehicle. Unplug the trailer before working on it.
- Open the plug. Undo the screws and separate the body. Slide the cover and any rubber boot onto the cable before you strip it, or you won’t be able to fit them later.
- Strip the outer sheath. Remove enough outer sheath to reach every terminal comfortably, without cutting into the inner cores.
- Strip each core. Strip only as much insulation as the terminal needs. Twist the strands tightly. No stray strands should be able to reach a neighbouring terminal.
- Match each colour to its number. Each terminal has its number moulded beside it. Use the pin chart and table below. Go by the number, not by where a terminal sits in a picture: viewed from the wire side, the layout is a mirror image of the face.
- Fasten each core. Insert the twisted strands fully under the terminal screw or into the crimp and tighten firmly. Tug each wire gently to check it’s held.
- Check the earth. Pin 3 (white) is the earth. On the trailer, run the earth wire to a clean, bare-metal point on the chassis with a ring terminal. Don’t rely on the coupling or towball to carry the earth. A poor earth is the most common cause of dim, flickering or “crossed” trailer lights.
- Fit the cable clamp. Tighten the strain-relief clamp on the outer sheath, not on the inner cores, so a pull on the cable doesn’t load the terminals.
- Close the plug. Make sure no wire is pinched, and fit any seals.
- Test every function. Plug into the vehicle or a trailer circuit tester and check: tail lights, left indicator, right indicator, stop lights, reversing lights (if fitted) and electric brakes (if fitted, using the brake controller’s manual output). Check hazard lights work too.
- Protect it. Use the plug cover or a holder when the trailer is unhooked, so water and dust stay out of the pins.
Specs table
Australian 7-pin flat and 7-pin large round (AS 4177.5)
| Pin | Function | Cable colour |
|---|---|---|
| 1 | Left-hand indicator | Yellow |
| 2 | Reversing lights | Black |
| 3 | Earth (return) | White |
| 4 | Right-hand indicator | Green |
| 5 | Electric brakes | Blue |
| 6 | Stop lights | Red |
| 7 | Tail, clearance and side marker lights | Brown |
Notes on these pins
- Pin 2 is for reversing lights. Don’t use it for auxiliary power, a breakaway feed or a pump, even if the trailer has no reversing lights. If you need those circuits, use a 12-pin connector.
- Pin 5 is for electric brakes only. If the trailer has no electric brakes, leave it unused.
- Pin 3 carries the return current for everything else, including the brakes. A good earth matters.
- Colours on older trailers, or on cable made to another code, may differ. Always confirm with a tester before relying on colours.
12-pin flat (for comparison)
Pins 1 to 7 on a 12-pin flat connector follow the same functions and colours as above. Pins 8 to 12 add circuits such as battery charging or auxiliary power, a second earth, rear fog lights and a spare. See (see What’s the difference between 7-pin flat, 7-pin round and 12-pin trailer plugs?).
Trailer brakes (check your state rules)
Braking requirements depend on the trailer’s mass. Under the national guide for small trailers (VSB 1), a single-axle trailer up to 750 kg gross trailer mass (GTM) doesn’t need brakes. Above that, brakes are required. Over-run (mechanical) brakes are only allowed up to 2000 kg GTM. Over 2000 kg GTM, the trailer needs brakes on all wheels, operable from the driver’s seat, that apply automatically and stay on for at least 15 minutes if the trailer breaks away. In practice that means electric brakes, a brake controller and a breakaway system. Check the rules for your trailer with your state road authority.
What you’ll need
- 7 pin flat trailer plug or 7 pin large round trailer plug
- 7 core trailer cable (if replacing the cable)
- trailer plug cover or trailer plug holder
- trailer circuit tester
- ring terminals and adhesive heat shrink
- wire strippers
- towbar wiring harness (vehicle-specific, if fitting a new socket to a modern car)
When to call an auto electrician
- You’re adding a trailer socket to a modern car. Many vehicles use CAN-bus or LED lighting that needs a plug-in harness or isolation module; tapping into tail-light wiring can trigger faults.
- You’re fitting an electric brake controller, breakaway system or 12-pin auxiliary/charging circuit. These carry high current and are safety-critical.
- The lights still don’t work properly after re-wiring, or blow fuses.
- The trailer wiring itself is damaged, corroded or has been repaired many times.
Need an auto electrician? Find local auto electricians and mobile services in your city.
Related questions
Q: Is a 7-pin flat the same wiring as a 7-pin round? A: The Australian 7-pin flat and 7-pin large round use the same pin numbers, functions and colours. The European 7-pin small round is different and shouldn’t be mixed up with them.
Q: Why are my trailer lights flickering or working the wrong way? A: The usual cause is a poor earth, either at the plug or on the trailer. Clean the earth points back to bare metal, check pin 3, and test again. (see Why aren’t my trailer lights working?)
Q: Can I use pin 2 for power to my caravan fridge? A: No. Pin 2 is for reversing lights. Feeding power through it can cause problems with other vehicles and trailers. Use a 12-pin connector or a separate Anderson plug with heavy cable. (see How do I wire an Anderson plug?)
Q: Can I use an adaptor between 7-pin flat and 7-pin round? A: Yes, adaptors are common and the functions line up. Make sure the adaptor carries every circuit you need, especially electric brakes, and that it’s secured so it can’t drop onto the road.
Q: Which way is the plug drawn in wiring diagrams? A: It varies. Some show the face (pin side), some show the wire side. That’s why it’s safest to go by the numbers moulded beside each terminal.
Sources
- Standards Australia, AS 4177.5 – Caravan and light trailer towing components, Part 5: Electrical connectors (the standard itself is paywalled; pin functions confirmed through the secondary sources below). Reference via Australian Business Licence and Information Service: https://ablis.business.gov.au/VIC/pages/ce32f8e1-d088-45fc-8849-115e51df44ee.aspx (checked October 2026)
- ExplorOz, “Trailer wiring diagrams” – pin functions and colours for 7-pin flat, 7-pin large round and 12-pin flat. https://www.exploroz.com/vehicle/electrics/trailerwiringdiagrams.aspx (checked October 2026)
- Wikipedia, “Trailer connectors in Australia” – pin 2 and pin 5 cautions, viewing-direction notes, ISO 1724 differences (cites VSB 1 section 14). https://en.wikipedia.org/wiki/Trailer_connectors_in_Australia (checked October 2026)
- RACQ, “A guide to trailer brakes” – plain-English summary of brake, breakaway and controller requirements by trailer mass. https://www.racq.com.au/articles/driving-tips/a-guide-to-trailer-brakes (checked October 2026)
- Department of Infrastructure, Transport, Regional Development, Communications and the Arts, Vehicle Standards Bulletin 1 (VSB 1) Revision 5 – Trailers with an aggregate trailer mass of 4.5 tonnes or less (June 2009) – connectors must comply with AS 2513 or AS 4177.5 (up to 3.5 t ATM); pin 5 for service brakes only; brake requirements by GTM. https://www.infrastructure.gov.au/sites/default/files/documents/vehicle-standards-bulletin-1-revision5-trailers-with-an-aggregate-trailer-mass-of-4-5-tonnes-or-less-june2009-pdf.pdf (checked October 2026)