Automotive Electrical and Electronic Systems
What this domain tests
Electrical and electronic questions check your grasp of basic circuit laws, the battery–starter–alternator loop, lighting and accessory circuits, and the sensor/ECU networks that control a modern engine.
Core rules
- Ohm's law: V = I × R. Power: P = V × I = I²R.
- A lead-acid battery cell is about 2.1 V; six cells give a nominal 12 V. Specific gravity and open-circuit voltage indicate state of charge.
- The alternator rectifies three-phase AC to DC and regulates voltage to roughly 13.8–14.4 V; the warning lamp usually indicates a field or charging fault.
- The starter motor draws hundreds of amperes; cable size and clean earths matter more than almost anything else.
- Sensors are usually three-wire: reference voltage, signal and earth. A 5 V reference with a signal stuck at 0 V or 5 V points to an open or shorted circuit.
- CAN bus carries digital messages between ECUs; a shorted CAN line can disable many systems at once.
Worked example
A 60 W headlamp runs on 12 V. Current = 60 ÷ 12 = 5 A; lamp resistance = 12 ÷ 5 = 2.4 Ω. The feed and earth cables together have 0.1 Ω resistance. Voltage drop = 5 × 0.1 = 0.5 V, so the lamp sees 11.5 V and produces about 11.5 × 5 ≈ 57.5 W - a dim lamp with no faulty bulb, explained by circuit resistance.
Traps that cost marks
Measuring resistance on a live circuit, assuming an alternator charges at idle with all loads on, disconnecting the battery with the engine running, and treating a blown fuse as the fault rather than as a symptom of a short.