Automotive Electrical and Electronic Systems
What the exam tests
This domain tests basic electricity, battery and charging systems, starting, lighting, fuses, relays, sensors, electronic control units and simple circuit diagnosis.
Core rules
- Ohm's law: voltage equals current times resistance. Series circuits share current; parallel circuits share voltage across branches.
- A charged 12 V battery typically reads about 12.6 V engine off and 13.8–14.4 V with the engine running and charging.
- Fuses protect circuits; replace with the same rating. A relay uses a small control current to switch a larger load current.
- Disconnect the battery negative first and reconnect it last to reduce spark risk. Never disconnect the battery with the engine running; the alternator can be damaged.
- Use a multimeter correctly: voltage in parallel, current in series, resistance only on an isolated circuit.
Worked example
Headlights are dim at idle and brighten when the engine is revved. Reasoning: the battery alone is supplying the load, so charging is weak. Check the alternator belt, battery terminals, earth straps and charging voltage. If running voltage stays near 12.2 V, diagnose the alternator or regulator. Do not assume the battery is faulty until charging is tested.
Traps that cost marks
Testing resistance on a live circuit, fitting a higher-rated fuse, reversing jumper leads, probing ECU pins with a test lamp, and ignoring poor earth connections.