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 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 S connector is specified in ISO 3731 and was mainly used in older vehicles and trailers with additional electrical equipment. 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 to ensure compatibility with older trailers, semi-trailers or containers that are still in use and where conversion to a 15-PIN connector is not cost-effective.

View TRALERT® lighting for heavy-duty agricultural vehicles and agricultural 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

Do you use LED lighting on a trailer with an S-connector and a modern vehicle with a CAN-bus system? If so, 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 dual-plug configuration (N + S) was error-prone and cumbersome. With the introduction of ISO 12098, a single, combined, reliable connection has been adopted that is suitable for all functions in heavy-duty vehicles and trailers.

Share this article:

Author

Kevin van Mierlo

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

ABOUT
TRALERT®

TRALERT® is the leading manufacturer and partner in high-quality LED lighting solutions for commercial vehicles. By taking care of everything from drawing board to realisation, you and your customers can hit the road safely, carefree and professionally lit.

VISIT
EXPERIENCE CENTRE

The TRALERT® Experience Centre is an interactive showroom with training and truck rooms. Experience TRALERT® LED vehicle lighting in practice. Experience light performance in different situations.

Plan visit

PARTNER
TO BE

TRALERT® works closely with its B2B partners and does not supply end users directly. Are you a manufacturer, reseller or company active in first-fit installation? Then register as a partner. Are you an end user? Then please refer to a TRALERT® partner.

Discover the possibilities

Author