Automotive Electrical and Electronic Systems
Automotive Electrical and Electronic Systems
This domain tests battery construction and testing, charging and starting circuits, lighting, fuses, relays, sensors and basic multimeter use. Questions often ask what a 12.2 V open-circuit reading means.
Core rules
- Lead-acid battery: six cells in series, each about 2.1 V, giving 12.6 V at rest and full charge. Specific gravity of electrolyte is about 1.265 fully charged; a hydrometer reading below 1.200 indicates discharge.
- Charging: alternator with rectifier and regulator, belt-driven; the warning lamp stays on if charging fails. Never disconnect a battery with the engine running - the alternator's voltage spike can damage the ECU.
- Starting: battery → ignition switch → starter relay → solenoid → starter motor.
- Circuits: fuses protect wiring, relays switch high current, earth returns complete the circuit. Use a multimeter correctly: measure voltage in parallel, current in series.
- Sensors: MAP, MAF, crank position, cam position, coolant temperature, oxygen (lambda). OBD-II stores fault codes for diagnosis.
Worked example
A car cranks slowly and the battery reads 12.2 V at rest, falling to 8.5 V while cranking. Reasoning: 12.2 V at rest is roughly 50% state of charge, so the battery is partly discharged. A cranking voltage below about 9.6 V on a 12 V system suggests a weak battery, a poor connection or a failing starter. Clean and tighten terminals, recharge and test with a conductance tester; then check alternator output, which should be about 13.8–14.5 V at idle with loads off. If charging is correct and the battery still fails, replace it.
Traps
- Reading voltage with the meter in series.
- Connecting a booster cable negative to the flat battery's negative instead of to an earth point away from the battery.
- Ignoring a loose or corroded earth strap.
- Testing specific gravity immediately after adding water.