7-Pin Trailer Plug Wiring: 12N and 12S Diagrams and Colours

A 7-pin 12N trailer plug (ISO 1724) is wired: pin 1 yellow, left indicator; pin 2 blue, rear fog lamp; pin 3 white, earth; pin 4 green, right indicator; pin 5 brown, right tail lamp; pin 6 red, stop lamps; pin 7 black, left tail lamp. The pin numbers are moulded on the insert.

That is the standard 12V lighting connection on UK tractors, farm trailers, cars and vans with a 7-pin socket. This guide gives the full pin tables for the 12N socket, the 12S auxiliary socket and the 24N socket used on 24V vehicles, explains which side of the plug a diagram shows, and walks through wiring a plug and a socket, testing them and the mistakes that cause most trailer lighting faults.

12N, 12S and 24N: which 7-pin connector do you have?

There are 3 common 7-pin trailer connectors in the UK, and they are not interchangeable:

• 12N (ISO 1724): the “N” stands for normal. It carries the road lighting: indicators, tail lamps, stop lamps, rear fog and earth. The plug and socket are usually black. This is the connector on almost every tractor and farm trailer.
• 12S (ISO 3732): the “S” stands for supplementary. It carries reversing lamps and power feeds for caravans and horseboxes. The plug and socket are usually white or grey, and it is fitted alongside a 12N socket, never instead of it.
• 24N (ISO 1185): the 24V equivalent of 12N, used on lorries, some plant and some 24V agricultural machines. The pins are allocated differently, so a 24N plug wired to a 12N layout will not work.

The 12N and 12S connectors look alike, but the arrangement of pins and sleeves inside them differs so a 12N plug cannot go into a 12S socket. Most cars, vans and pickups built in recent years have moved to the 13-pin connector, which combines 12N and 12S in one plug. See 13-pin trailer plug wiring if that is what you have.

12N wiring diagram: pin numbers, markings and colours

The 12N connector uses terminal markings from the DIN 72552 system, and the cable colours below are the standard ones used in UK 7-core trailer cable. The markings are often moulded next to the pin numbers on the back of the insert.

Pin	Marking	Function	Standard cable colour
1	L	Left-hand direction indicator	Yellow
2	54G	Rear fog lamp (UK use); auxiliary on some vehicles	Blue
3	31	Earth (return for pins 1, 2 and 4 to 7)	White
4	R	Right-hand direction indicator	Green
5	58R	Right-hand tail and marker lamps (number plate lamp on 5 or 7)	Brown
6	54	Stop lamps	Red
7	58L	Left-hand tail and marker lamps	Black

Pin 2: the one to check

Pin 2 is the pin most often wired differently. Its marking, 54G, comes from the German DIN system, where it was an auxiliary circuit. UK vehicles use it for the rear fog lamp, but on some imported vehicles and older machines it carries a permanent live feed, a reversing lamp or nothing at all. If a trailer’s fog lamp comes on as soon as you plug in, the towing vehicle and the trailer are using pin 2 for different jobs. Always meter pin 2 on an unfamiliar tractor before relying on it.

Pins 5 and 7: tail lamps and the number plate lamp

The 2 tail circuits are split left and right so that one blown fuse does not leave the trailer with no tail lamps at all. ISO 1724 allows the tail, marker and number plate lamps to be shared between pins 5 and 7. The number plate lamp can be wired to pin 5, pin 7 or both, so check how the trailer’s lamp board is wired before chasing a “missing” plate lamp.

Colours are a guide, not a rule

The yellow, blue, white, green, brown, red, black sequence is standard, but cable from other countries, older looms and repaired harnesses don’t always follow it. Some cable has a thicker white core because the earth carries the return current for every lamp. Always connect by pin number and function, and treat the colours as confirmation.

12S wiring diagram: the supplementary socket

The 12S socket was added for caravans and is rare on farm trailers, but you will meet it on horseboxes, catering trailers and vehicles that tow both. The layout changed around 1998 to 1999, so the age of the caravan matters.

Pin	Function (post-1998 to 1999 wiring)	Older caravans	Standard cable colour
1	Reversing lamps	Reversing lamps	Yellow
2	Spare (some installers use it for an ignition-switched charge feed)	Battery charging	Blue
3	Earth for pins 1, 2 and 4	Earth	White
4	Permanent 12V feed (habitation battery charging)	Permanent feed for interior equipment	Green
5	Spare (formerly caravan sensing)	Sensing or spare	Brown
6	Ignition-switched 12V for the fridge	Fridge feed	Red
7	Earth for pin 6 (fridge return)	Spare	Black

Two rules matter with 12S. Never join the 12N earth and the 12S earth together; each must return to the towing vehicle through its own plug. And pins 4 and 6 carry enough current to need properly sized cable and their own fuses or relays at the vehicle end.

24N wiring diagram for 24V vehicles

The 24N connector (ISO 1185) is the 7-pin socket used on 24V lorries and some 24V tractors and plant. It has the same 7 lighting functions as 12N but the pins are in a different order, starting with earth on pin 1.

Pin	Marking	Function	Standard cable colour
1	31	Earth	White
2	58L	Left-hand tail and marker lamps	Black
3	L	Left-hand direction indicator	Yellow
4	54	Stop lamps	Red
5	R	Right-hand direction indicator	Green
6	58R	Right-hand tail and marker lamps	Brown
7	none	Auxiliary (trailer brake control on some vehicles; often used for the rear fog lamp)	Blue

A 24V system needs 24V bulbs and lamps on the trailer. A 12V bulb on 24V blows almost at once, and a 24V bulb on 12V glows dimly. Most LED lamps sold as 12V/24V or 10V to 30V work on either. Our guide to 12V vs 24V tractor systems explains how to tell which you have.

Which side of the plug is the diagram showing?

Most 7-pin wiring mistakes come from reading a diagram from the wrong side. A plug and a socket mate face to face, so the socket viewed from the front (under the flap) is the mirror image of the plug viewed from its front. The socket viewed from the front matches the plug viewed from the wiring side, and the other way round.

The simple answer is to ignore the diagram’s orientation and read the numbers. Every 12N plug and socket insert has the numbers 1 to 7 moulded next to the terminals on the wiring side: 6 terminals in a ring and 1 in the centre. Wire to the moulded numbers and the orientation takes care of itself.

How to wire a 7-pin trailer plug

The plug goes on the trailer cable. You need 7-core trailer cable, a wire stripper, a small flat screwdriver and side cutters. Ferrules and a crimper make a neater, longer-lasting joint than bare twisted strands.

1. Thread the parts on first. Slide the plug body, cable gland and any rubber boot onto the cable before you strip anything. Forgetting this is the most common reason for rewiring a plug twice.
2. Strip the outer sheath. Remove about 30 mm to 40 mm of outer sheath so the cores reach the terminals with a little slack, without nicking the core insulation underneath.
3. Strip each core. Bare about 6 mm to 8 mm of each core and twist the strands tight. Crimp on a ferrule if you have them. Avoid tinning the ends with solder under a screw terminal, because the solder creeps and the screw loosens.
4. Connect by number. Working on the wiring side of the insert, connect each core to the terminal with the matching moulded number: yellow 1, blue 2, white 3, green 4, brown 5, red 6, black 7. Start with the centre terminal so the outer cores don’t get in the way.
5. Check each screw. Tug each core gently. No loose strands should touch a neighbouring terminal.
6. Clamp the sheath. Tighten the cable clamp on the outer sheath, not on the cores. The clamp takes the pull when the plug is yanked out by a tight turn; the terminals should never take it.
7. Reassemble and seal. Fit the insert into the body, tighten the gland and fit the boot.

How to wire a 7-pin trailer socket

The socket is on the towing vehicle. On a tractor it is usually factory-wired into the loom at the rear, and the normal job is replacing a broken or corroded socket like for like.

1. Disconnect the battery or remove the trailer lighting fuses.
2. Photograph the old socket from the wiring side before disconnecting anything, then label each core with tape.
3. Prepare the cores as for the plug, cutting back any green, corroded copper until the strands are bright.
4. Connect by number on the wiring side of the new socket, using the same moulded numbering.
5. Seal the back. Fit the rear gasket or a rubber boot, and fill the cable entry with a sealing grommet. Water entering from behind fills the socket and corrodes the pins from inside.
6. Mount it firmly on its bracket with the flap hinge at the top so the flap closes by gravity and the spring holds it shut.
7. Earth the white core to the tractor’s chassis or loom earth with a clean, tight ring terminal. Don’t rely on the hitch or towing pin to carry the earth.

When fitting a new socket to a vehicle that never had one, the feeds must come from the vehicle’s own lighting circuits. On modern cars and vans with CAN bus wiring, that usually means a vehicle-specific or bypass relay towing module rather than tapping into lamp wires. Browse our 7-pin plugs and sockets and the wider range of 12V plugs, sockets and connectors.

How to test a 7-pin socket and plug

Test the socket first, then the plug and trailer. A plug-in socket tester with an LED for each pin is the quickest tool. A multimeter or a 12V test lamp works just as well.

1. Set the multimeter to DC volts and put the black probe on pin 3, or on a known good chassis earth.
2. Switch on each function in turn and probe the matching pin: left indicator on pin 1, rear fog on pin 2, right indicator on pin 4, sidelights on pins 5 and 7, brake on pin 6.
3. Expect close to battery voltage, around 12V to 12.8V with the engine off and up to about 14.4V with it running. A reading a volt or more below the battery points to a poor connection.
4. Check the earth: with the meter on resistance, measure from pin 3 to the chassis. It should read close to zero. A few ohms will dim every lamp on the trailer.
5. Plug the trailer in and repeat the checks at the lamp boards.

A bulb-type test lamp is better than an LED tester for finding a high-resistance joint, because it draws real current and will glow dim where a meter still shows full voltage. If any test fails, our guide to trailer lights not working takes you through the faults one by one.

Common 7-pin wiring mistakes

• Mirror-image wiring. Wiring from a diagram of the wrong face swaps left and right and puts the stop lamps on the wrong pin. Wire to the moulded numbers.
• Swapped indicators. Yellow (pin 1) is left and green (pin 4) is right. Swapped, the trailer signals the opposite way to the tractor.
• Rear fog on the tail circuit. Linking pin 2 to a tail lamp feed leaves the trailer’s fog lamp on with the sidelights, which dazzles drivers behind and is not allowed outside seriously reduced visibility.
• Weak earth. A thin or loose earth, or relying on the tow hitch, causes lamps to glow when other functions are used. Run a proper earth core to every lamp.
• Clamping the cores. If the cable clamp grips the cores instead of the sheath, the first hard pull breaks a core inside the plug.
• Leaving out the seal. An unsealed plug or socket fills with water, slurry and salt.

The legal requirements the 7-pin circuits have to meet are covered in our guide to agricultural trailer lighting requirements.

Frequently asked questions

What colour wire goes to which pin on a 7-pin plug?

On a 12N plug: pin 1 yellow (left indicator), pin 2 blue (rear fog), pin 3 white (earth), pin 4 green (right indicator), pin 5 brown (right tail), pin 6 red (stop lamps), pin 7 black (left tail). Always confirm by the moulded pin numbers, because cable colours aren’t always standard.

What is the difference between 12N and 12S?

12N is the normal lighting socket for indicators, tail, stop and fog lamps, and is usually black. 12S is a supplementary socket for reversing lamps, caravan battery charging and fridge power, and is usually white or grey. They are fitted side by side and cannot be swapped.

What is pin 2 on a 7-pin trailer plug?

On UK 12N wiring, pin 2 (blue, marked 54G) is the rear fog lamp. Some imported and older vehicles use it for a permanent feed or another auxiliary circuit, so test it with a meter before connecting a trailer to an unfamiliar tractor.

Is the wiring diagram viewed from the front or the back?

It varies between diagrams, which is why mistakes happen. The socket’s front face is the mirror image of the plug’s front face. Ignore the drawing and wire to the pin numbers moulded on the wiring side of the insert.

Can I fit a 13-pin plug to a 7-pin socket?

Not directly. Use a 13-pin to 7-pin adapter, or a 7-pin to 13-pin adapter for the other way round. The adapter carries the lighting functions on pins 1 to 7. Reversing lamps and power feeds need the 13-pin socket or a 12S socket.