Automobile Electrical and Electronic Systems
This domain covers battery, starting, charging, lighting, sensors, and electronic control. The exam tests basic circuit calculations, component function, and safe use of a multimeter.
Core concepts
- A lead-acid battery stores energy chemically and supplies high current for starting. Capacity is stated in ampere-hours (Ah) and cold-cranking amps (CCA).
- The starter motor converts electrical energy into mechanical torque. The alternator charges the battery and powers loads when the engine runs.
- Ohm’s law: V = I × R. Power: P = V × I. A 12 V, 60 W bulb draws 5 A and has about 2.4 Ω resistance when hot.
- Sensors feed the engine control unit (ECU); actuators carry out commands. CAN bus allows control modules to share data on two wires.
- A multimeter measures voltage, resistance, and current. Use the correct range and function; never measure resistance on a live circuit.
Worked example
A 12 V battery supplies a 60 W headlamp. Current = P ÷ V = 60 ÷ 12 = 5 A. Resistance = V ÷ I = 12 ÷ 5 = 2.4 Ω. If a connector adds 0.2 Ω, the circuit resistance rises and current falls, dimming the lamp. The reasoning is that supply voltage is fixed, so resistance changes current; voltage-drop testing locates the bad connection.
Traps that cost marks
- Using an ohmmeter on a circuit that still has power.
- Connecting an ammeter in parallel instead of in series.
- Confusing CCA with Ah capacity.
- Assuming a charged battery means the charging system is good; test alternator output.
- Ignoring earth connections when diagnosing dim lights or slow cranking.