Automobile Electrical and Electronic Systems
Domain 5 tests the 12 V system: battery, starting, charging, ignition, lighting, sensors and basic circuit calculations.
Battery and charging
A lead-acid battery has six cells of about 2.1 V, giving 12.6 V at rest; a healthy cell shows specific gravity around 1.260–1.280. The alternator with built-in rectifier charges at about 13.8–14.2 V, and the warning lamp glows when it is not charging. Never disconnect the battery with the engine running - the alternator voltage spike can destroy the ECU.
Starting, ignition and electronics
The starter solenoid switches the heavy current and throws the pinion into mesh. Petrol ignition may be coil-and-distributor or coil-on-plug. The ECU reads crank (CKP), cam (CMP), MAP or MAF, throttle (TPS), coolant (ECT), oxygen and knock sensors. Fuses protect circuits, relays switch heavy loads, and the body earth is part of every return path. CAN bus lets modules share data on two twisted wires.
Worked example
A 60 W headlamp on 12 V: I = P ÷ V = 60 ÷ 12 = 5 A. Two lamps need 10 A, so a 15 A fuse is correct and a 7.5 A fuse would blow. If a 0.4 V drop appears across a connector at 5 A, the wasted power is V × I = 2 W, which appears as heat and dim lights - a high-resistance joint, not a faulty bulb.
Traps that cost marks
Measuring current by placing the meter across the battery (it must be in series); reading specific gravity straight after topping up with water; condemning a merely discharged battery; ignoring earth straps; fitting a bigger fuse to stop it blowing.