If you buy connectors but do not design harnesses, a harness drawing can look deliberately forbidding. Branches, circuit numbers, cavity letters, gauge callouts — and somewhere in all of it, the part you actually have to order. The good news: you do not need to read the whole drawing — only four or five specific pieces of information, each in a predictable place. This guide walks through them in the order a buyer needs them.
What Is a Harness Drawing Actually Telling You?
A harness drawing does three jobs at once, and it helps to separate them. First, geometry: overall lengths, branch points and routing — the physical shape of the assembly. Second, electrical: which circuit goes where and what conductor size each circuit uses. Third, components: the connectors, terminals, seals and plugs that terminate the wires. As a buyer, the first two layers are the harness engineer’s problem. Your entire job is the third layer — identifying each component precisely enough to source it or a validated equivalent.
Which Symbols on the Drawing Represent the Connector?
Connectors are drawn as simple blocks with a designator next to them, most commonly labels like X1, X2 or X101. The block itself carries almost no visual information; the real data is in the text around it. Two conventions matter. First, the designator identifies which connector this is: the same X1 label must appear in the bill of materials, in the cavity table, and on the mating device. Second, gender is usually implied by the mating partner’s designator or a short note, not by the block’s shape — never assume the drawing’s block style reliably distinguishes plug from receptacle.
The callouts you will meet most often, and what each one actually controls:
| Callout on the drawing | What it means | Why it matters when ordering |
|---|---|---|
| X1, X2, X101 … | Connector designator — the name of this specific connector | Links the block, cavity table and BOM entry; quote it in every enquiry |
| A, B, C … or 1, 2, 3 … | Cavity positions in the housing | Sets the number of ways; cavities are printed on the real part |
| 0.5, 0.75, 20 AWG … | Conductor cross-section per circuit | Constrains the terminal’s wire range |
| Box around a segment / “sealed” | Sealed region of the harness | Determines sealed housing, wire seals and cavity plugs |
| TPA / secondary lock note | A retention device is required | Narrows the housing family; affects assembly order |
| Part number in title block | Reference to a specific component | Often outdated — treat as a cross-reference starting point |
What Do the Numbers Next to a Connector Callout Mean?
This is where most misreads happen, because two numbering systems coexist on the same drawing. Circuit numbers identify the electrical function — they follow the wire from end to end. Cavity designators identify the physical position in the housing — typically letters like A, B, C printed on the connector itself. A callout such as “X1, cav B, 0.5” tells you: connector X1, cavity position B, 0.5 mm² conductor. The circuit number for that same wire may be something entirely different, like 0354. When you order a housing, you order by cavity positions and count — not by circuit numbers. Confusing the two is the most common way buyers specify a housing with the wrong number of ways.
How Do You Match the Drawing to a Physical Connector?
The drawing tells you what the connector must do, not which catalogue part satisfies it. The extraction sequence looks like this:
- Count the used cavities at each connector block — the number of wires landing there sets the minimum number of ways.
- Note the conductor sizes per cavity — this constrains the terminal’s wire range.
- Check for a sealed region — a box drawn around part of the harness, or a note like “sealed”, tells you whether the housing needs wire seals and cavity plugs.
- Look for secondary lock or TPA notes — retention requirements narrow the housing family further.
- Record any part number in the title block — but treat it as a cross-reference starting point, not as gospel; drawings outlive the parts they name.
With those five items you can walk a catalogue like the LANXI product catalog and shortlist candidates — and know exactly which question to ask when two look similar.
Which Details Decide Which Housing You Need?
Housing selection follows almost mechanically from the drawing once you know what to look at. The number of ways comes from the cavity count, and housings are organized accordingly — the connector shell category groups families by pin count for exactly this reason, from simple 2-pin shells upward. Sealed versus unsealed comes from the sealed-region check. Keying and colour come from anti-mis-mating requirements. A concrete example: a shell such as the LX7561 series shell is specified by its own drawing-level parameters (ways, pitch, locking), which is why the five-item extraction above matters more than any single number on the harness print.
What Does the Drawing Not Tell You?
Just as important is what a harness drawing conventionally omits — decisions someone else has to close:
- Terminal plating — tin or silver is usually a service-environment decision, not a drawing callout.
- Seal material details — the drawing says “sealed”; the elastomer compound lives in the component specification.
- Terminal series — the conductor size constrains but does not uniquely determine the terminal; browse the terminal category by wire range and tab size to complete the match.
- Current rating basis — the drawing assumes the circuit was sized correctly; the connector datasheet still needs to agree.
When Should You Stop Interpreting and Ask the Drawing’s Author?
Three situations should always go back to the drawing’s owner rather than to your best guess. First, partially populated housings: when a 6-cavity connector shows only 4 wires, someone must decide whether to plug the empty cavities or order a 4-way housing — the answer affects sealing and future service. Second, obsolete part numbers: if the title-block number does not resolve to anything current, order by extracted parameters and request a cross-reference instead of substituting silently. Third, any conflict between the cavity table and the graphic — drawings get revised, and half-updated revisions exist in the wild. When any of these appear, send the drawing section with your question: a specific question plus the callouts takes minutes to answer, a wrong order takes weeks.
Frequently Asked Questions
Do I need the original connector part number to order a replacement?
No. A part number speeds things up, but a competent cross-reference can work from ways, pitch, sealed/unsealed, terminal series and lock features. Measure the physical part or read the cavity table — the drawing contains enough information in most cases.
The drawing shows fewer wires than the housing has cavities. What now?
Common and intentional: housings often carry spare positions. Check whether the drawing calls for cavity plugs in the unused positions, and confirm with the harness owner whether a smaller housing would be acceptable — never just drop the spare positions on your own.
How can I tell a sealed connector from an unsealed one on a drawing?
Look for a boundary box around the connector or harness segment, notes like “sealed” or an ingress-protection callout, and wire-seal symbols at the connector entries. If no such marks appear and the connector sits inside the protected zone, it is usually unsealed — but confirm: a missed seal requirement is expensive to discover late.
Can I read the connector pitch from a harness drawing?
Only if the drawing dimensions the connector block or the cavity table lists pitch. More often it does not — pitch then comes from measuring the physical part or mating device. Never infer pitch from a scaled printout; print scaling silently changes dimensions.