ISO 3731:2003

Overview

ISO 3731:2003 defines the dimensional characteristics, contact allocation and performance requirements for the 7‑pole connector type 24 S used for electrical connections between towing and towed road vehicles with a 24 V nominal supply voltage. The standard ensures interchangeability for a supplementary 7‑pole connector intended to be used in addition to the 24 N connector (ISO 1185) where more than seven poles are required.

Key topics and technical requirements

• Connector geometry and contacts
  • Plug: six spring tubes (contacts 2–7) and one pin (Contact 1).
  • Socket: six pins (2–7) and one spring tube (Contact 1).
  • Dimensions and mating features specified to ensure interchangeability.
• Contact allocation (functions & core colors)
  • 1 - Common return (White)
  • 2 - Reserved for future allocation (Black)
  • 3 - Reversing light (Yellow)
  • 4 - Permanent power supply (Red)
  • 5 - Sensing device with common return (Green)
  • 6 - Power supply controlled by ignition (Brown)
  • 7 - Rear fog light (Blue)
• Electrical and mechanical requirements
  • Terminal cable capacities: Contacts 1, 4, 6 → 2.5 mm²; Contacts 2, 3, 5, 7 → 1.5 mm².
  • Contact designation permanently marked on socket cover and terminal faces (character height ≥ 2 mm where space allows).
  • Protection/storage provision required for disconnected plug to prevent ingress and damage.
• Performance and testing
  • Test sequences and procedures reference ISO 4091 (visual, dimensional, connection/disconnection, locking strength, current capacity, endurance, temperature/humidity, salt spray, etc.).
  • Mismating requirement: no unintended contact between specified contacts is permitted.
  • Mechanical forces: connection/disconnection = (150 ± 50) N; cable retention = (500 ± 5) N.
  • Endurance: 1 000 cycles; temperature/humidity cycling includes −25 °C to +75 °C steps per the standard.

Applications and users

ISO 3731:2003 is primarily used by:

• Vehicle OEMs (tractors, trailers, semi-trailers) and bodybuilders specifying towing/towed electrical interfaces.
• Connector and cable harness manufacturers designing compliant 24 V inter-vehicle connectors and coiled/uncoiled cable assemblies.
• Fleet maintenance, repair shops and test labs for verification, replacement and interoperability checks.
• Standards and compliance engineers implementing vehicle electrical interface specifications.

Related standards

• ISO 1185 - 7‑pole connector type 24 N (normal) for 24 V vehicles
• ISO 4091 - Definitions, tests and requirements for towing/towed connectors
• ISO 4009 - Location of electrical/pneumatic towing connections
• ISO 4141 - Multi‑core connecting cables
• ISO 12098 - 15‑pole connector for 24 V (alternative when >7 poles required)

Keywords: ISO 3731:2003, 7‑pole connector, 24 S, 24 V, towing and towed vehicles, connector dimensions, contact allocation, interchangeability, performance tests.