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Auto Electricals: Wiring, Connectors, Harnesses, and Circuit Protection

Auto Electricals · Work through your exam, one topic at a time.

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Wiring, Connectors, Harnesses, and Circuit Protection

This domain tests wire selection, terminal and connector repair, harness routing, and circuit protection. Expect questions on ampacity, voltage drop, fuse and relay selection, crimping standards, and the difference between protecting a wire and protecting a device.

Core rules

  • Wire is sized by cross-sectional area (AWG or mm²) and by ampacity, insulation temperature rating, bundling, and ambient temperature. Voltage drop limits are typically 2–3% for sensitive loads.
  • Fuses protect conductors, not devices. Select a fuse at about 125% of continuous load current but never above the wire's ampacity. A fuse must have a DC voltage rating equal to or above system voltage.
  • Fusible links protect main feeds; circuit breakers may be automatic or manual reset. Relays switch high current with a low-current control.
  • A good crimp uses the correct tool and terminal for the wire gauge, is sealed where exposed, and passes a pull test. Solder splices need adhesive-lined heat shrink and strain relief.
  • Harnesses must be routed away from heat, sharp edges, and moving parts, with grommets through panels, service loops, and proper clips. Connectors need terminal tension, secondary locks, and weather seals.

Worked example

A 15 A pump is 5 m from the fuse box, one-way, and the system is 13.8 V. A 3% drop allows 0.41 V, so the loop resistance limit is 0.41 ÷ 15 = 0.027 Ω. The loop is 10 m, so resistance must be below 0.0027 Ω/m. 10 AWG at about 0.0033 Ω/m fails; 8 AWG at about 0.0021 Ω/m passes. A 20 A fuse (125% of 15 A) protects 8 AWG, whose ampacity is far higher. The result is correct wire, correct protection, and acceptable drop.

Traps

Upsizing a fuse to stop nuisance blowing overheats the wire. Using pliers instead of a crimp tool creates a high-resistance joint. Wire nuts and twist-and-tape splices are not acceptable in vehicles. Ignoring voltage drop produces dim lamps and slow motors even when the fuse holds. Routing through a bare metal hole without a grommet causes an intermittent short.