What Is a Car Wiring Harness?
A wiring harness (also called a wiring loom) is a bundled assembly of electrical wires, connectors, and protective sheathing that carries power and data signals throughout a vehicle. It's essentially the nervous system of a modern car — connecting the battery, alternator, ECU, sensors, lights, motors, switches, and every other electrical component.
Modern vehicles can have kilometres of wiring and hundreds of connectors in their harness. The main harness typically runs from the engine bay, through the firewall, along the underside or inside the vehicle, and out to the lights, doors, and rear of the car.
There are usually multiple sub-harnesses within a vehicle:
- Engine harness: Connects the ECU to sensors, injectors, ignition coils, and actuators
- Body harness: Runs through the cabin — controls windows, central locking, interior lights, audio
- Chassis harness: Runs beneath the vehicle — rear lights, fuel pump, sensors
- Instrument cluster harness: Connects gauges and dashboard electronics
What Causes Wiring Harness Problems?
| Cause | Detail |
|---|---|
| Age and heat cycling | Insulation becomes brittle, cracks, and flakes off — common on NZ Japanese imports 10–20 years old |
| Rodent damage | Rats and mice nest in engine bays and chew wiring — a significant issue in rural NZ |
| Water ingress | Moisture entering connectors causes corrosion, short circuits, and intermittent faults |
| Chafing | Wiring rubbing against sharp metal edges gradually wears through insulation |
| Poor repairs | Previous amateur wiring repairs with incorrect connectors or tape |
| Accident damage | Wiring crushed, cut, or melted in a collision |
What Are "Electrical Gremlins"?
The term electrical gremlin refers to intermittent, hard-to-reproduce electrical faults that seem to appear and disappear without obvious cause. They're notoriously difficult to diagnose because they may only occur when:
- The engine is hot (thermal expansion changes resistance at a crack in the insulation)
- It's raining (water bridges a gap between wires)
- The car is over a bump (a loose connector makes or breaks contact)
Common electrical gremlin symptoms:
- Random warning lights appearing and disappearing
- Windows, wipers, or central locking working intermittently
- Engine cutting out briefly while driving
- Instruments reading incorrectly or failing
- Battery draining overnight (a parasitic draw from a short circuit)
- Intermittent no-start despite the battery being fine
Rodent Damage in NZ
Rodent-chewed wiring is a growing issue in New Zealand, particularly for vehicles parked in rural areas, near paddocks, or in garages where rodents seek shelter. Rats and mice are attracted to the soy-based insulation used in wiring harnesses of many modern vehicles.
Signs: visible bite marks on wiring, nesting material in the engine bay, ECU fault codes appearing in clusters (often for multiple unrelated sensors simultaneously).
Prevention: rodent repellent sprays for engine bays, not parking near known rodent activity, and having the engine bay inspected if the car sits unused for extended periods.
Diagnosis
Wiring diagnosis requires a methodical approach:
- Fault code scan: A scan tool identifies which circuits are affected — pointing to the relevant wiring section
- Visual inspection: Check for obvious damage, chafing, corrosion, rodent damage
- Connector inspection: Cleaning and reseating corroded connectors often resolves intermittent faults
- Multimeter testing: Measure resistance, voltage, and continuity along suspect wiring
- Oscilloscope testing: For data signals (CAN bus, sensor signals) that require waveform analysis
Intermittent faults may require the vehicle to be driven while monitoring live data — labour-intensive work.
NZ Cost Estimates
Wiring diagnosis and repair costs vary enormously depending on what's failed and where:
| Service | Estimated Cost (NZD) |
|---|---|
| Electrical diagnostic scan and assessment | $100–$250 |
| Connector clean/repair (minor) | $100–$300 |
| Rodent damage repair (partial harness) | $300–$1,000 |
| Full engine harness replacement | $1,000–$3,500 |
| Full body harness replacement | $2,000–$5,000+ |
Body harness replacement is among the most labour-intensive jobs a workshop can perform — often requiring near-total disassembly of the interior.
WoF Implications
Wiring faults affecting lights (headlights, brake lights, indicators) will cause a WoF failure — these are directly inspected. Faults causing windscreen wiper failure or horn failure also result in a fail. The condition of wiring itself is checked if it's visible and obviously dangerous (bare wires in the engine bay, for example).
When to Book a Mechanic
Book a mechanic if:
- You have unexplained warning lights that come and go
- Electrical components (windows, wipers, locking) are intermittently failing
- Your battery is draining overnight
- You've found evidence of rodent activity in the engine bay
- A single ECU scan returns many unrelated fault codes (suggesting a wiring or ground issue)