Automotive Electrical and Electronic Systems
This domain tests the 12 V electrical system and the electronics above it: battery, starting, charging, lighting, sensors, ECU and vehicle networks. Expect battery ratings, alternator parts, starter circuit, fuse and relay use, sensor functions and diagnostic trouble codes.
Battery and charging
- Lead-acid battery: six cells of about 2.1 V each, lead and lead-dioxide plates in dilute sulphuric acid.
- Ah is capacity at a stated 20-hour rate; CCA is cold cranking current at minus 18 degrees C.
- Alternator: rotor field winding, three-phase stator, diode rectifier, regulator. Never run the engine with the battery disconnected.
- Cells in series add voltage; cells in parallel add capacity.
Starting, lighting and electronics
- Starter: the solenoid closes the heavy cable and throws the pinion; the motor can draw 100-300 A.
- Lighting runs through fuses and relays; the dash switch controls the relay coil, not the full lamp current.
- Sensors include MAP, TPS, ECT, IAT, crank position, lambda (O2), knock and vehicle speed. The ECU sets injection and ignition from them and stores a DTC when a signal leaves its expected range.
- OBD-II code families: P powertrain, B body, C chassis, U network. A scan tool reads and clears codes, but a code points to a circuit, not always to the failed part.
Worked example
Two 60 W headlamps and two 5 W tail lamps on 12 V. Total power = 120 + 10 = 130 W. Current = 130 / 12 = 10.8 A. Select a 15 A fuse and cable rated above that current, allowing margin for inrush. Reasoning: P = V x I gives running current; the fuse must protect the wiring, so choose about 25-40 percent above the running current.
Traps that cost marks
Reversing battery polarity or charger connections. Assuming a flat battery always means a faulty alternator. Treating a P-code as proof of a failed component. Running an alternator with no battery connected, which can destroy the regulator.