Vehicle Electrical and Electronic Systems
Domain 4: Vehicle Electrical and Electronic Systems
This domain tests basic electrical theory, battery and charging circuits, starting, lighting, sensors, wiring and electronic control. The emphasis is on measurement and logical testing rather than part swapping.
Core rules
- Ohm's law: V = I x R. Power: P = V x I. A circuit needs a supply, a load, a return path and protection.
- Measure voltage with the meter in parallel; measure current in series; measure resistance with the circuit isolated and de-energised.
- A healthy 12 V battery rests near 12.6 V. Charging voltage is typically 13.8-14.8 V, but follow the manufacturer's specification.
- Voltage drop tests find high resistance under load. A good cable or connection usually drops less than 0.1 V; long circuits may allow slightly more.
- Parasitic draw is measured after modules sleep, often with a clamp meter. A common limit is around 50 mA, but check the vehicle specification.
- Do not probe airbag (SRS) circuits with a test light or ordinary multimeter. Wait for the specified disable time.
- CAN and other networks carry data; a shorted or open network can disable many systems. Check powers, grounds and termination before replacing modules.
Worked example
A battery goes flat overnight. After the vehicle sleeps, a clamp meter shows 0.35 A (350 mA). That is far above a typical 50 mA limit. The technician pulls one fuse at a time and watches the meter. When the draw falls to 20 mA with the interior lamp fuse removed, the circuit is isolated. The boot lamp switch is found stuck on. Reasoning: the meter quantifies the fault, and fuse isolation narrows the circuit without guessing.
Traps
- Using a test light on sensitive electronic circuits.
- Disconnecting the battery while the engine runs.
- Measuring resistance on a live circuit.
- Replacing an alternator when the fault is a corroded ground.
- Ignoring the sleep time when checking parasitic draw.