Automotive Electrical and Electronic Systems
This domain covers the 12 V electrical system, starting and charging, lighting, fuses, relays, sensors, electronic control units and basic circuit testing. Expect Ohm's law and voltage-drop questions plus safe testing practice.
Core rules
- A 12 V lead-acid battery has six cells, each about 2.1 V. Capacity is rated in ampere-hours (Ah); cold-cranking amps (CCA) indicate starting ability at low temperature.
- The alternator generates alternating current and rectifies it to direct current. The voltage regulator typically holds charging voltage around 13.8-14.8 V.
- The starter motor uses a solenoid to engage the pinion and switch the heavy current. A click with no crank suggests low battery, poor terminals or solenoid contacts.
- Fuses protect circuits; relays use a small control current to switch a larger load. The ECU reads sensors and controls actuators; OBD-II stores diagnostic trouble codes.
- A multimeter must be set correctly: volts in parallel, amps in series, ohms only on an isolated dead circuit.
Worked example
A 60 W headlamp runs at 12 V and the supply wire has 0.1 ohm resistance. Reasoning: current = power ÷ voltage = 60 ÷ 12 = 5 A. Voltage drop = current × resistance = 5 × 0.1 = 0.5 V. The lamp receives 11.5 V and is dimmer. This is why voltage-drop testing is more useful than resistance testing on a loaded circuit.
Traps
Disconnecting the battery with the engine running can damage the alternator. Reversing jump-lead polarity can destroy electronics. Testing resistance on a live circuit gives false readings. Replacing an ECU before checking power, ground and sensor supply is a costly error.