7-PIN type S connector (24V)

Updated: 09/12/2025
By: Kevin van Mierlo

The 7-Pin S-plug is used in 24V systems as an additional connector to the N-plug. Where the N-plug provides the basic lighting, the S-plug provides additional functions such as:

Constant power supply
Battery supply for semi-trailer functions
Reversing lights
Power supply for cooling motors or relays

This configuration is defined in ISO 3731 and was the standard in heavy transport and agricultural equipment for a long time, before the introduction of the 15-PIN connector (ISO 12098).

Wiring diagram of the 7-PIN S (24V) connector

PIN	Function	Colour
1	Battery charging current	Grey
2	Reversing lights	Black
3	Mass for PIN 1	White
4	Free / auxiliary function	Green
5	Free / auxiliary function	Brown
6	Mass for PIN 2 (constant current)	White
7	Constant power supply	Red

ISO 3731 standardisation for 7-PIN S connector 24V

The (24V) 7-PIN connector is defined in ISO 3731 and was mainly used for older vehicles and semi-trailers with additional electrical features. Read more about ISO 3731

Applications of the 7-PIN connector

Although the 7-Pin S connector is increasingly being replaced by the 15-PIN standard, it still remains relevant in specific niches of Heavy Transport, Agriculture and Industry. The S connector is essential in places where a separate, additional power supply is required in addition to basic lighting:

Special transport: trucks and tractors equipped with separate power circuits for functions not directly covered by the basic lighting.
Additional power supply: agricultural machines or implements that require continuous current (PIN 10) or relay-controlled power supply for monitoring, small hydraulic pumps or charging of onboard batteries, for example.
Older systems: The connector is necessary for compatibility with older semi-trailers, semi-trailers or containers still in use where conversion to 15-PIN is not cost-effective.

Check out TRALERT®'s lighting for heavy agricultural vehicles and semi-trailers:

The switch to 15-PIN connector (ISO 12098)

The functions of the N and S plug are combined in modern vehicles in the 15-PIN connector. It is more reliable, waterproof and prevents errors.

15-PIN connector (ISO 12098).

Adapter use is possible, but not recommended in professional applications.

CAN-bus interference and LED lighting

Are you using LED lighting on a semi-trailer with S-connector and a modern vehicle with CAN-bus system? Then problems may arise, such as:

Lighting not working
Fault codes on the dashboard
Overheated cabling in case of faulty load

CAN-bus solutions for LED lighting
Blog: resolving CAN-bus interference with LED lighting

Frequently asked question

Why have N and S plugs been replaced by the 15-PIN connector?

The double plug configuration (N + S) was error-prone and inconvenient. With the advent of ISO 12098, a single combined, reliable connection suitable for all functions in heavy-duty vehicles and semi-trailers was chosen.

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Author

Kevin van Mierlo

Lighting expert at TRALERT® - Kevin tests lighting and finds out everything about legislation. He can tell you all about it.

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