Reading a wire harness drawing is a fundamental skill for anyone involved in electrical design, manufacturing, or maintenance. It’s the blueprint that translates an electrical schematic into a physical, functional assembly. Think of it as the instruction manual for building the nervous system of a vehicle, aircraft, or industrial machine. To truly master it, you need to move beyond just recognizing symbols and understand the language of the drawing itself, including the critical standards that govern its creation. A great resource for diving deeper into these guidelines is this article on wire harness drawing standards.
Deciphering the Title Block: Your Starting Point
Before you even look at the lines and symbols, the title block is your first and most crucial stop. Located typically in the lower-right corner, this section is packed with metadata essential for correct interpretation and traceability. Ignoring it is like trying to assemble furniture without checking the model number on the instructions.
Key elements in the title block include:
- Part Number & Name: The unique identifier for the harness assembly. For example, a drawing might list “P/N: 84A-12345-AB, Name: Engine Bay Main Harness.”
- Revision Level: Indicates the version of the drawing. A change from rev ‘B’ to rev ‘C’ could signify a critical wire gauge update. Always use the latest revision.
- Scale: While not always to scale for the entire length, certain detailed views or connector depictions might be scaled (e.g., 1:1, 2:1).
- Sheet Size & Format: Tells you the physical or digital drawing size (e.g., A-size, B-size, or A4, A3).
- Company & Drafter Info: Identifies the originating organization and the engineer responsible.
- Applicable Standards: This is vital. It lists the industry or company standards the drawing adheres to, such as IPC/WHMA-A-620 for requirements and acceptance of cable and wire harness assemblies or specific automotive standards like Ford’s WSF-M, which dictates everything from solder sleeve requirements to tape wrap methods.
| Title Block Data | Why It Matters | Example |
|---|---|---|
| Part Number (P/N) | Unique identifier for ordering and tracking. | 84A-12345-AB |
| Revision | Ensures you are building to the most current design. | Rev: C |
| Scale | Helps interpret dimensions in detailed views. | 2:1 (Detailed View) |
| Standard | Defines the quality and acceptance criteria. | IPC/WHMA-A-620, Class 3 |
Understanding the Bill of Materials (BOM)
The Bill of Materials, often a table on the drawing or a separate linked document, is a comprehensive list of every single component needed to build the harness. It’s your shopping list. A typical BOM will break down items into categories for clarity.
| Component Type | Description | Part Number Example | Quantity per Harness |
|---|---|---|---|
| Wire (20 AWG) | Primary color, 105°C rated, thin wall | M22759/16-20-9 | 15.5 meters |
| Connector Housing | 12-pin, sealed, black | 15324987-1 | 2 |
| Terminal (Female) | Gold plated, for 20-18 AWG wire | 15324988-1 | 24 |
| Seal Plug | For unused cavity in connector | 15324989-1 | 4 |
| Heat Shrink Tubing | 3:1 ratio, adhesive lined, red | SHR-25-3/64 | 0.3 meters |
| Convoluted Tubing | 1/2″ diameter, split loom | CT-0.50-BLK | 2.0 meters |
Notice the wire quantity is given in meters (or feet). This is the total cut length, not the length of a single wire. The harness manufacturer will cut individual wires to the lengths specified in the wiring table. The BOM ensures you have all the terminals, seals, and connectors that match the specified housings.
Interpreting the Schematic View and Wiring Table
This is the heart of the drawing. It shows the electrical connectivity and the physical routing of the wires. The schematic view uses symbols to represent components, and lines to represent wires. Each wire is labeled with a unique circuit identifier, often called a “wire number” or “circuit code.”
Key symbols you’ll encounter:
- Connectors: Drawn as rectangles with pins. The connector part number is usually labeled nearby (e.g., C123). Each pin is numbered.
- Splices: Represented by a dot or a specific symbol, indicating where two or more wires are joined together. They may have a unique splice code (e.g., S101).
- Ground Points: Shown with a specific ground symbol, labeled with a location code (e.g., G102).
- Components: Sensors, relays, and other devices are shown with their standard symbols and part numbers.
The wiring table is your cross-reference guide. It lists every wire in the harness by its circuit identifier and provides the essential details for fabrication.
| Wire ID / Circuit Code | From (Connector.Pin) | To (Connector.Pin) | Wire Gauge & Color | Length (mm) |
|---|---|---|---|---|
| 101 | C101.1 | C102.A | 20 AWG, RED | 1250 |
| 102 | C101.2 | S101.1 | 18 AWG, BRN | 800 |
| 103 | S101.2 | C102.B | 18 AWG, BRN | 600 |
| 104 | C101.3 | G101 | 16 AWG, BLK | 400 |
Reading the table: Wire 101 is a 20-gauge red wire that runs from pin 1 of connector C101 to pin A of connector C102, and it needs to be cut to a length of 1250 mm. Wire 102 and 103 are part of a splice (S101); a brown 18-gauge wire runs from C101.2 to the splice, and another continues from the splice to C102.B.
Mastering the Assembly Drawing and Topology View
While the schematic shows electrical connectivity, the assembly drawing (or topology view) illustrates the physical layout of the harness. This is critical for understanding how the harness will be routed through the vehicle or machine, where it needs to be secured, and how it avoids interference with other components.
This view will show:
- Harness Branches: The main trunk line and its various branches leading to different connectors.
- Length Dimensions: Exact measurements for each segment between breakout points, splices, or connectors. These dimensions are crucial for building the harness on a board of the correct size.
- Component Placement: The location of clips, grommets, tape wraps, and protective sleeving (like convoluted tubing or braided loom) along the harness length.
- Bend Radii: May specify minimum bend radii to prevent damage to wires, especially important for high-voltage cables or shielded wires.
For instance, the drawing might specify that a branch to a door connector must have a service loop of 150mm to allow for door movement. Or, it might indicate that a section of the harness passing through a metal panel must be protected by a grommet with a specific part number.
Reading the Notes and Flag Notes
Never, ever skip the notes section. This is where the drafter communicates critical information that doesn’t fit into the schematic or table formats. Notes can override standard practices or provide specific instructions.
Common types of notes include:
- General Notes: Apply to the entire harness (e.g., “All splices shall be crimped and insulated with adhesive-lined heat shrink,” or “Harness must be assembled per IPC/WHMA-A-620, Class 2 requirements.”).
- Local Flag Notes: These are notes with a leader arrow pointing to a specific location on the drawing. They provide hyper-local instructions. For example, a flag note might point to a connector and say, “Apply dielectric grease to all cavities before terminal insertion,” or point to a wire and say, “Shield drain wire to be grounded at this end only.”
- Testing Requirements: Notes often specify post-assembly tests, such as “100% continuity test required,” or “Hi-Pot test 1500 VAC for 60 seconds.”
These notes are legally binding instructions. Failing to follow a flag note about a specific sealing method could lead to a warranty failure in the field. The notes tie the design intent directly to the manufacturing process, ensuring reliability and compliance.