Automotive Electrical and Electronic Systems
Electrical basics and charging
The exam covers Ohm’s law, series and parallel circuits, battery, starting, charging, lighting, sensors, ECU and CAN bus. A fully charged lead-acid cell has a specific gravity near 1.260; 1.120 indicates a low state of charge. Alternator output is typically 13.8–14.4 V at idle with loads off. Test voltage drop on cranking rather than only open-circuit voltage. Fuses protect wiring, so replace with the same rating, never a higher one. Relays switch high current with a small control current. Use a multimeter correctly: volts in parallel, amps in series, ohms only on an isolated circuit.
Electronics and safety
Sensors supply ECU inputs; actuators are outputs. CAN bus wiring needs correct termination and twisted-pair repair. Before welding, disconnect the battery and ECU as instructed. Airbags and high-voltage hybrid systems require maker’s isolation procedure; orange cables are high voltage. Do not probe airbag circuits with a test lamp.
Worked example
A 60 W headlamp at 12 V draws I = P/V = 60/12 = 5 A. A 10 A fuse gives a safe margin. If measured current is 8 A, suspect a wrong bulb, poor earth or partial short. If the fuse blows instantly, do not fit a bigger fuse; isolate the circuit and find the short.
Traps
Resistance must not be measured on a live circuit. Never use an ohmmeter on an airbag or ECU pin without the maker’s procedure. A battery that reads 12.6 V at rest can still fail a load test.