7-pin trailer plug wiring: pins, wire colours and the common faults

Left and right swap

The socket on the car is wired from behind, and the plug on the trailer is wired from behind too, so each is a mirror image of the other when you look at the faces. Pin numbers are moulded beside each terminal: follow the numbers, not the position you see from the front, and most crossed indicators disappear.

When the trailer lights don't work

1. A bad earth (pin 3). The most common cause. A corroded or loose earth makes lights dim, flicker or light up the wrong lamps together.
2. Corroded terminals in the socket or plug. Clean them; a light smear of dielectric grease helps.
3. A blown fuse or the towbar module. Many modern vehicles feed the trailer socket through a separate module or fuse.
4. Crossed indicators from a mirror-image wiring job.
5. A broken wire where the cable flexes at the drawbar.

Wiring a plug, step by step

1. Strip back the outer sheath of the trailer cable by a few centimetres, enough to reach every terminal without strain.
2. Identify each core by colour and match it to the pin number: yellow 1, black 2, white 3, green 4, blue 5, red 6, brown 7.
3. Read the numbers moulded beside each terminal inside the plug. Ignore where the pins look like they sit from the front.
4. Strip each core just enough to sit fully in its terminal, twist the strands, and tighten the screw firmly with no stray strands touching a neighbour.
5. Earth (pin 3) gets extra care. It carries the return current for every lamp at once; a weak earth causes most strange faults.
6. Fit the cable clamp so the outer sheath, not the cores, takes any pull.
7. Close the plug and test every function before the first trip, ideally with a helper watching the lamps.

Testing with a multimeter or test light

With the car's ignition on, a test light or multimeter between pin 3 (earth) and each live pin should show power only when the matching function is on: left indicator on pin 1, reversing on 2 (in reverse), right indicator on 4, stop lamps on 6 with the brake pedal pressed, tail lamps on 7 with the headlights on. Pin 5 carries the electric brake signal only when the brake controller is working and the brakes are applied. If a pin is live on the car but the trailer lamp stays dark, the fault is in the plug, the cable or the lamp; if the pin is dead on the car, look at the vehicle's fuse or towbar module.

LED trailer lights and warning messages

Many modern trailers use LED lamps, which draw far less current than old globes. Some vehicles watch the current on the trailer circuits to detect a failed globe, and can show a "trailer lamp fault" warning or make the indicators flash fast with LED lamps fitted. The cure depends on the vehicle: some need the towbar module coded for LED trailers, some need load resistors fitted in the trailer circuit. Ask the towbar fitter or an auto electrician which applies to your car, rather than guessing.

Keeping it working

Trailer plugs live in water, dust and road grime. Fit the dust cap on the car's socket when not towing, store the trailer plug off the ground, and clean the terminals at the start of each season. A light smear of dielectric grease on the pins keeps moisture out. If the cable is cracked where it flexes at the drawbar, replace that section before it fails on the road.

Small round, large round and flat plugs

Australian trailers use a few plug shapes, and they do not all fit each other. The flat 7-pin plug is the common caravan and trailer standard, and the large round 7-pin is found on many older trailers, trucks and some caravans. Both follow the same pin functions in the AS 4177.5 table above. A small round 7-pin, often on light box trailers, uses a smaller plug body that does not fit the other two, so check its own pin chart before wiring it to match a flat or large round socket. When the car and trailer plugs do not match, an adaptor that converts one shape to the other is the usual fix; check the adaptor carries pin 5 through if your trailer has electric brakes.

Wire size matters for two pins

Most lamp circuits carry only a few amps, but two pins work harder. The earth on pin 3 returns the current for every lamp and, with electric brakes, the brake magnets as well. The brake wire on pin 5 carries the full current for the brake magnets. Thin cores on those two pins cause dim lights, weak brakes and voltage drop on long trailers. Many trailer cables use heavier cores for the earth and brake circuits for that reason; when replacing a cable, match or exceed what was fitted.

Trailer lights not working: a five-minute routine

1. Check the vehicle's own lights work, then the trailer fuse or towbar module fuse in the car.
2. Look into the car's socket and the trailer plug with a torch for green corrosion or bent pins.
3. Wiggle the plug while a helper watches the lamps; flickering points to the plug or socket.
4. Test the car's socket pins with a test light, as above.
5. If the car end is good, follow the trailer cable back, checking the earth at the drawbar and at each lamp.

Fitting a new socket to the car

If the car came without a trailer socket, or the old one is damaged, a towbar fitter or auto electrician usually supplies the feed from the vehicle's own wiring or a dedicated towbar module. Many modern vehicles need that module, because their lighting circuits are monitored electronically and do not like extra lamps simply spliced in. Splicing into the tail lamp wiring of a late-model car can trigger warning lights or damage control units, so let the fitter use the method the vehicle maker specifies.

7-pin or 12-pin?

The 7-pin flat is the common Australian standard for trailers and caravans. A 12-pin flat socket adds pins for things like battery charging and accessories, and a 7-pin flat plug fits it and works. Many European cars come with a 13-pin socket, which needs an adaptor.

A trailer plug is not a charging circuit for a caravan battery; that is usually a separate heavy-cable circuit with an Anderson plug. See dual battery systems.

Common questions