13-Pin Trailer Plug Wiring: Socket Diagram, Colours and Adapters
13 pin trailer plug wiring: full ISO 11446 socket diagram, pin functions and cable colours, how 13 pin maps to 7 pin 12N and 12S, adapters and testing.
In this guide
A 13-pin trailer plug (ISO 11446) carries the 7 lighting circuits of a 12N plug on pins 1 to 7, with the same functions and colours. Pin 8 is reversing lamps, pin 9 a permanent 12V feed, pin 10 an ignition-switched 12V feed, pins 11 and 13 their earths, and pin 12 is reserved.
That makes the 13-pin plug a single connector that does the job of a 12N and a 12S pair. This guide gives the full 13-pin pin table and cable colours, shows how the pins map to 7-pin 12N and 12S through adapters, explains which vehicles and trailers use 13-pin, and covers wiring and testing a plug or socket.
What is a 13-pin trailer socket?
A 13-pin trailer socket is the 12V towing connector defined by ISO 11446, often called the Euro socket. The plug is round, has a keyed face so it only fits one way, and locks with a quarter-turn bayonet collar rather than the push fit and spring flap of a 7-pin plug. That locking collar, and the seal on the plug face, make it less likely to pull out or let water in.
It is standard on new cars, vans, pickups and 4x4s fitted with towbar electrics, and UK caravan makers moved to 13-pin in the late 2000s. Most tractors and farm trailers still use the 7-pin 12N connector, so a farm that runs both a pickup and a tractor will meet both. For the 7-pin layouts, see 7-pin trailer plug wiring.
13-pin wiring diagram: pin functions and colours
The table shows the ISO 11446 pin allocation with the cable colours most commonly used in UK 13-core cable. Pins 1 to 7 match 12N exactly. The colours for pins 8 to 13 vary more between cable makers than the lighting colours do, so check by pin number.
| Pin | Marking | Function | Typical cable colour |
|---|---|---|---|
| 1 | L | Left-hand direction indicator | Yellow |
| 2 | none | Rear fog lamp | Blue |
| 3 | 31 | Earth for pins 1 to 8 | White |
| 4 | R | Right-hand direction indicator | Green |
| 5 | 58R | Right-hand tail, marker and number plate lamps | Brown |
| 6 | 54 | Stop lamps | Red |
| 7 | 58L | Left-hand tail, marker and number plate lamps | Black |
| 8 | none | Reversing lamps | Pink |
| 9 | 30 | Permanent 12V feed | Orange |
| 10 | 15 | Ignition-switched 12V feed (for example a caravan fridge) | Grey |
| 11 | 31 | Earth for pin 10 | Black/white |
| 12 | none | Reserved (used for trailer detection in the 2002 edition) | Varies |
| 13 | 31 | Earth for pin 9 | Red/white or white/red |
Why there are 3 earth pins
Pin 3 is the return for the lighting circuits. Pins 11 and 13 are separate returns for the 2 power feeds, which can carry several amps each when they charge a caravan battery or run a fridge. Keeping those currents off the lighting earth stops lamps flickering or glowing faintly when the fridge cycles. Don’t link pins 3, 11 and 13 together in the plug; each should run back to the vehicle on its own core.
Pin 12
The 2002 edition of ISO 11446 used pin 12 to tell the towing vehicle a trailer was attached, by linking it to pin 3 inside the trailer plug. The 2004 edition dropped that and made pin 12 reserved, and it has stayed reserved since. Modern towing electrics usually detect the trailer by sensing the current drawn by its lamps instead. You can leave pin 12 unconnected on a new installation.
How 13-pin maps to 7-pin 12N and 12S
A 13-pin connection is a 12N and a 12S combined, so every 13-pin function has a direct equivalent on one of the 7-pin plugs. This is how adapters and twin-socket conversions are wired.
| 13-pin | Function | 7-pin equivalent |
|---|---|---|
| 1 | Left indicator | 12N pin 1 |
| 2 | Rear fog lamp | 12N pin 2 |
| 3 | Lighting earth | 12N pin 3 |
| 4 | Right indicator | 12N pin 4 |
| 5 | Right tail | 12N pin 5 |
| 6 | Stop lamps | 12N pin 6 |
| 7 | Left tail | 12N pin 7 |
| 8 | Reversing lamps | 12S pin 1 |
| 9 | Permanent 12V | 12S pin 4 |
| 10 | Ignition-switched 12V | 12S pin 6 |
| 11 | Earth for pin 10 | 12S pin 7 |
| 13 | Earth for pin 9 | 12S pin 3 |
13-pin to 7-pin adapters: which one you need
Adapters let a vehicle with one type of socket tow a trailer with the other. There are 4 common types:
- 13-pin socket to 7-pin 12N plug. For a car, van or pickup with a 13-pin socket towing a farm or general-purpose trailer with a 7-pin plug. All the road lighting works. Reversing lamps and power feeds are not passed through, and a 7-pin trailer doesn’t use them anyway.
- 7-pin 12N socket to 13-pin plug. For a tractor or older vehicle with a 7-pin socket towing a trailer with a 13-pin plug. The indicators, tail, stop and fog lamps work. The trailer’s reversing lamps and any power feeds will not.
- 13-pin socket to twin 12N and 12S plugs. For a 13-pin vehicle towing an older caravan or horsebox with 2 separate 7-pin plugs. This passes every function through.
- Twin 12N and 12S sockets to 13-pin plug. The reverse of the above, for an older vehicle with twin sockets towing a newer 13-pin caravan.
Short rigid adapters are fine for occasional use. On a farm vehicle that hitches the same trailer every day, it is better to change the trailer’s plug to match the vehicle, because each adapter adds 2 more sets of contacts to corrode. If you use an adapter, keep it in the cab rather than leaving it in the socket, and grease the contacts. Browse our 13-pin plugs and sockets for plugs, sockets and adapters.
Which tractors and trailers use 13-pin?
Use the following as a rough guide, and always check the socket on the actual machine:
- Cars, vans, pickups and 4x4s: most new towbar installations are 13-pin. Older vehicles may have a single 12N or twin 12N and 12S sockets.
- Caravans: UK caravans built from the late 2000s onwards normally have 13-pin plugs. Older caravans have twin 12N and 12S plugs.
- Car trailers, horseboxes and livestock trailers towed by pickups: either type. Many are supplied with a 13-pin plug or a 7-pin plug and an adapter.
- Tractors: most have a 7-pin 12N socket for trailer lighting. A tractor that regularly tows car-type trailers may have a 13-pin socket added.
- Farm trailers and implements: almost all use 7-pin 12N, and 24V machinery may use 24N.
How to wire a 13-pin plug
13-pin plugs are wired the same way as 7-pin plugs, with more cores in a tighter space. Use 13-core trailer cable, a fine stripper, ferrules and a small screwdriver.
- Thread the parts on first. Slide the rear housing, cable gland and seal onto the cable before stripping.
- Strip the outer sheath by about 30 mm to 40 mm, and each core by about 5 mm to 6 mm. Fit ferrules so fine strands don’t splay into the next terminal.
- Connect by moulded number. The terminals are numbered 1 to 13 on the wiring side. Work from the centre outwards so the outer cores don’t block access.
- Keep the earths separate. White to pin 3, the pin 10 earth to pin 11, the pin 9 earth to pin 13.
- Leave unused pins empty. On a trailer with lighting only, you need pins 1 to 7, and pin 8 if it has reversing lamps.
- Clamp the outer sheath in the cable clamp so the terminals never take the pull, then refit the housing and tighten the gland.
Fitting a 13-pin socket to a vehicle is a larger job than fitting the plug, because pins 9 and 10 need fused feeds from the battery and ignition, usually through a relay, and modern vehicles need a towing electrics module that talks to the vehicle’s own wiring. That is normally a vehicle-specific kit.
How to test a 13-pin socket
The quickest test is a plug-in 13-pin tester with a light for each function. A multimeter does the same job.
- Set the meter to DC volts and put the black probe on pin 3.
- Switch on each lighting function and check pins 1, 2, 4, 5, 6 and 7. With reverse gear selected (ignition on), check pin 8. Expect close to battery voltage.
- Move the black probe to pin 13 and check pin 9 for a permanent 12V feed.
- Move the black probe to pin 11 and check pin 10 with the ignition or engine on. Some vehicles only feed pin 10 with the engine running.
- Check each earth pin to chassis on the resistance range. All should read close to zero.
The 13-pin socket is part of the car and van MOT. The MOT inspection manual tells testers to use an approved device to check the socket correctly operates the position lamps, stop lamps and indicators. A 13-pin socket that doesn’t work correctly is a major defect, and one that doesn’t work at all is dangerous. 7-pin sockets aren’t listed in the same way. If a test shows a fault, our guide to trailer lights not working covers the likely causes.
Common 13-pin problems
- Fog lamp or tail lamps on all the time. Usually an adapter or a trailer wired for a different use of pin 2, or a tail feed bridged to pin 2.
- Indicators flash fast with the trailer connected. The towing electrics aren’t detecting the trailer load, often because LED lamps draw too little current. A towing module that suits LED trailers usually fixes it.
- Caravan battery doesn’t charge. Pin 9 not fed, a blown fuse at the vehicle end, or a pre-1999 caravan expecting charge on a different 12S pin through an adapter.
- Corroded contacts. The 13-pin plug seals well when locked, but an open socket left without its cover fills with water. Grease the contacts and keep the cover closed.
- Bent pins. Forcing the plug in at an angle bends the small pins. Line up the key, push straight and turn the collar.
The lighting the trailer has to carry, whatever plug it uses, is covered in our guide to agricultural trailer lighting requirements. For cable, see our wiring cable and harnesses.
Frequently asked questions
What are the 13-pin trailer plug colours?
Pins 1 to 7 follow the 12N colours: yellow, blue, white, green, brown, red, black. Pins 8 to 13 are typically pink (reversing), orange (permanent feed), grey (ignition feed), black/white (earth for pin 10), a reserved pin 12, and red/white (earth for pin 9). Colours vary by cable maker, so always wire by pin number.
Can I tow a 7-pin trailer from a 13-pin socket?
Yes, with a 13-pin to 7-pin 12N adapter. Pins 1 to 7 on both connectors carry the same functions, so all the road lighting works. Only the reversing lamp and power feeds are lost, and a 7-pin trailer doesn’t use them.
Will a 13-pin trailer work from a tractor’s 7-pin socket?
The road lighting will, through a 7-pin to 13-pin adapter. The trailer’s reversing lamps and any equipment that needs the permanent or ignition feeds will not, because a 12N socket doesn’t carry those circuits.
What is pin 12 on a 13-pin socket?
Pin 12 is reserved in the current ISO 11446. The 2002 edition used it to detect that a trailer was attached, by linking it to the earth on pin 3, but that was dropped in 2004. Leave it unconnected unless the vehicle maker’s instructions say otherwise.