Wiring Harnesses, Connectors, Terminals and Circuit Protection
Harness and connection principles
Wire is sized by cross-sectional area (mm²) and by AWG, chosen for current capacity and for voltage drop - a common target is no more than about 3% drop on a 12 V circuit, roughly 0.3–0.4 V. Routing matters: harnesses must be clipped, protected from chafe and heat, kept clear of moving parts, and not strained at connectors. Twisted pairs (CAN, sensor signals) must stay twisted to reject interference, and shields must be earthed at the specified point only.
Terminals and connectors fail more often than the component they serve. Look for fretting corrosion, spread female terminals, pushed-back pins, water ingress, broken locks and green copper. Crimp with the correct tool for the terminal type, inspect the crimp, and use adhesive-lined heat-shrink on repairs. Probe from the rear only with the correct adaptor.
Circuit protection
A fuse protects the wire, not the device. Fuses are rated against the wire's current capacity and the load's continuous current (typically about 1.25–1.5× the load); time-current curves mean a fuse tolerates brief surges. Fusible links and maxi fuses protect high-current feeds; circuit breakers are self-resetting or manual. Relays let a small switch current control a large load - know the coil and contact pins, and the difference between normally open and changeover.
Traps. Fitting a bigger fuse to stop it blowing (the fault stays, the wire burns); testing a fuse with an ohmmeter only, when it can pass a low current but fail under load; crimping onto insulation; leaving a splice unsealed; and condemning a component when the real fault is a 0.8 V drop across a corroded connector.
Worked example. Measure voltage across the fuse itself while the circuit runs: a good fuse shows about 0 V. Any appreciable drop means resistance - replace it.