A connector choice looks small on a bill of materials, but it sets the terminal, the wire range, the sealing method and, often, the failure mode of the whole harness. This guide lists the questions to answer before a connector is written onto a drawing. It applies to the automotive, rail and solar harnesses built by APAR, whose pages for automotive and EV, locomotive and solar project harnesses show the range of applications.
| Question | Why it matters | What to record on the drawing |
|---|---|---|
| What current does each contact carry? | The contact rating must exceed the circuit current with margin, and the rating falls as ambient temperature rises. | Contact current rating and the circuit current |
| What wire size range does the terminal accept? | A terminal crimped on a wire outside its range gives a weak or loose crimp. | Terminal part number and its wire range |
| What is the contact plating? | Plating must suit the signal level and the mating contact. Tin is common for power and general circuits. Gold is common for low-level signals. | Plating on both halves of the pair |
| Is the position sealed? | An exposed connector in a wet or dusty zone needs wire seals, cavity plugs and a mating seal. | Seal and plug part numbers, target IP rating |
| How is the connector keyed? | Keying stops mis-mating where several connectors of the same size sit close together. | Keying code or colour |
| How is it locked? | Vibration can back out an unlocked connector. A secondary lock gives a second line of defence. | Primary and secondary lock type |
| What temperature will it see? | The housing and the seal both have temperature limits, and they can differ from the wire. | Housing temperature class |
| How many mating cycles? | A connector opened often in service wears faster than one fitted once on a line. | Expected cycles |
| Can it be supplied at the volume needed? | A connector on long lead time controls the lead time of the harness. | Approved alternates |
Common mistakes
Specifying the connector housing but not the terminal is the most common one: the housing part number does not define the terminal, and several terminal variants usually fit the same housing. A second is mixing plating, for example a tin terminal against a gold mate, which can wear and corrode. A third is matching the housing to the wire outer diameter but not to the seal, so the seal does not grip the insulation.
The connector sealing chart and the failure modes chart show what happens when these choices go wrong, and the keying FAQ covers the mis-mating case.