Wire Harness Assembly: From Drawing to Verified Build

What Is a Wire Harness Assembly?

A wire harness assembly is an installation-ready interconnection that organizes conductors, terminations, and interfaces so electrical power or signals can be routed through a product. Examples of common interconnection methods include crimps, mechanically secured connections, and soldered connections. IPC/WHMA-A-620F provides materials, methods, tests, and acceptance requirements for cable and wire harness assemblies manufactured using such interconnections.

The wire harness design may be relatively simple or highly branched depending on the product, and components such as splices, seals, shields, coverings, clips, labels, and secondary locks may also be added based on the product’s specification. The finished outline alone is not enough to define the assembly.

Wire Harness Components and Materials

Wire harnesses and their components comprise electrical paths, interfaces, routing features, and any protection needed for the installation environment. The type of material has a significant impact on the performance and characteristics of the wire harness system. Examples include current-carrying capacity, flexibility, temperature capability, abrasion resistance, moisture protection, signal integrity, and serviceability.

Why Are Wire Harnesses Used?

Using a harness to consolidate multiple wires into one assembly streamlines installation, improves repeatability of connector and branch locations, decreases loose-wire routing mistakes, and allows for easier identification of service locations. In addition, where sleeves, clips, seals or other protective wraps may be needed, harnesses can help protect the wire from damage due to abrasion, vibration, moisture, or localized mechanical stress within the application.

Some of the more practical elements that are typically associated with the design of a harness include electrical load, space available for routing, bend radius, connector mating requirements, exposure to environmental conditions, and the method of inspection or testing employed with the completed wire harness assembly (PCA overview; Epec wire harness overview).

What Defines a Wire Harness Before Assembly?

Prior to performing any cutting or terminating of wire, the design package will contain the documents needed for the design and construction of the wire harness. These documents include the wiring diagram, the BOM, the master wire list, the connector and contact identification, the pin map, and the relevant specifications for the design, including the revision status. The wiring diagram provides a graphical representation of the geometry of the finished harness and notes, while the BOM provides the materials needed to build the harness. The master wire list and the pin map are used to identify where to connect each of the conductors within the harness assembly to their respective endpoints.

The design package will typically also contain specifications regarding the conductor gauge, stranding, insulation type, contact plating, ferrules, lugs, grommets, backshells, shield drains, twist rate, service loops, breakout locations, clocking, keying, color coding, labels, and mating references for the wire harness assembly. In addition, IPC-D-620A explains how design and critical-process requirements can be invoked through drawings and accompanying documentation.

Prior to commencing work, a list of manufacturing information is required and should cover the following four primary checks:

• Verify that the drawing, BOM, wire list, and pin map use the intended revision.
• Confirm that part identities, endpoint assignments, dimensions, and labels are not ambiguous.
• Identify the applicable acceptance criteria and test instructions.
• Differentiate overall harness requirements from features used only at specific locations.

A formboard can provide physical visualization of the harness geometry at a one-to-one scale. Siemens identifies formboards for locating bundle forks, connector holders, clip holders and tape positions, while relevant manufacturing documentation may provide connector and wire tables (Siemens Formboard Design; Siemens Wiring Harness Designer Essentials).

How Is a Wire Harness Assembled?

The manufacturing sequence may vary with the product and will depend on which items need verifying prior to their concealment during assembly.

1. Work Package Verification – The drawings, BOM, wire list and pin maps should match the applicable revision of the harness before any materials are released.
2. Prepare Conductors – Identify, cut and strip all specified wires while controlling length and avoiding damage to the conductor or insulation.
3. Create Terminations – Create the terminations specified by crimping, soldering or utilizing mechanical means with compatible tooling and materials.
4. Populate Connectors – Insert contacts into the identified cavities, verify the orientation and seating of contacts and engage necessary locking devices.
5. Form Harness – Route conductors and branches in accordance with specifications provided on the fixture or formboard, controlling all breakouts, bend paths and connector orientations.
6. Add Identification and Protection – Apply all identification labels, seals, sleeves, shields, clips, ties or coverings, as defined for the harness assembly.
7. Inspection and Testing – Perform visual, dimensional, mechanical and electrical inspections and tests as required for the harness assembly, prior to release.

Crimping and Terminal Insertion

The proper crimp is a crimp applied with a terminal and wire of the correct range, the proper conductor location with respect to the strip length, and, where the terminal design provides it, insulation support with no strands cut or missing. The measurement of crimp height is a nondestructive process measurement when defined, while pull force is a different mechanical property and is used when required by the applicable terminal system or testing procedure.

After the crimp process, the contacts will be inserted into their respective connector cavities, and the contacts will be checked for orientation and whether they are fully seated, and any secondary lock defined by the connector family will be engaged prior to final bundling or protection of the termination area (i.e., prior to covering it with tape) (Molex Quality Crimp Handbook; TE Connectivity EP II Application Specification).

Manual vs. Automated Wire Harness Assembly

Automated methods can be used to perform cutting, stripping, crimping, twisting, selected insertion operations, and electrical testing when the product to be assembled and the production volume are suitable for the automated methods. However, routing flexible conductors through three-dimensional branch configurations, applying sleeves or tape, and working with high-mix custom assemblies will generally continue to be performed manually, as the configurable form and material characteristics of flexible conductors make it difficult to wholly automate the entire process.

Many production lines use automated wire-processing methods as described by EMA and Altium, but operators still complete routing of flexible conductors, branch formation, protection and other manual operations (EMA Wire Harness Assembly Process; Altium Wire Harness Automation Overview).

How Is a Wire Harness Tested?

Wire harness verification incorporates visual, dimensional, mechanical and electrical inspections, as these different inspection types can detect a range of defects. The harness design, the terminal systems used with the harness, the connector systems employed in the harness, and the product test procedure defined in the specifications should be considered to ensure that the proper combination of inspection types is being used.

Continuity testing can identify open circuits, short circuits, or incorrect wiring connections, but does not assess crimp geometry, mechanical retention, or insulation withstand. Insulation resistance and dielectric withstand tests also provide insight into different properties of insulation, so the limits, connections, and test coverage are based on product-specific test procedures (Cirris Cable Testing Glossary; Cirris Testing Cables with High Voltage).

Wire Harness, Cable Harness, or Cable Assembly?

Wire harness, wiring harness, cable harness, cable assembly, and loom are overlapping terms in electrical engineering and purchasing. For the purposes of IPC/WHMA-A-620F, cable and wire harness terms are used interchangeably; however, individual companies and industries may use these labels more narrowly.

Common Wire Harness Types by Application

Grouping by application is a practical approach as there is no single universal taxonomy of wire harness types. How a wire harness will be installed can change the way that conductor sizes, connector styles, protection, routing, identification, and verification are prioritized.

An automotive engine-bay harness typically uses sealed connectors and abrasion sleeving to withstand heat, oil, and constant vibration, while the interior harness of that same vehicle may use lighter unsealed connectors since the environment is less demanding than the engine bay; as such, the application priority is only a starting point for each section of the design, not one global specification covering the entire product.

Common Wire Harness Defects and Prevention

Crimped terminals may look acceptable at first glance, but they may actually fall outside the specified crimp-height or pull-force limits. For that reason, crimp measurement and pull testing are established as separate checks from crimp visual inspections. Practical harness assembly guidance also documents hard-to-see termination and cavity defects across production settings (Soulin cable harness assembly guide).