Auto Electrical and Electronic Systems
Circuits and charging
Candidates must apply Ohm's law and understand series and parallel circuits, battery construction, starting systems, charging systems, relays, fuses, and automotive electronics. In a series circuit, current is the same and voltages add; in parallel, voltage is the same and currents add. The battery supplies starting current, the alternator charges it, and the starter motor converts electrical energy to mechanical torque. Relays use a small control current to switch a large load current. Voltage drop testing locates high resistance in cables, earths, and connectors. Electronic control units process sensor signals and control actuators.
Traps in questions
A common trap is diagnosing a flat battery as a faulty alternator without checking for parasitic drain. Another is confusing an open circuit with a short circuit. High resistance causes low current and voltage drop, while a short to earth blows fuses. Questions may ask why a relay clicks but the load does not run: the control circuit works but the power contacts or load circuit are faulty. Do not forget that a bad earth can make several systems behave strangely.
Rule of thumb
Use a voltmeter for voltage drop under load: more than about 0.5 V across a cable or earth path suggests excessive resistance.
Worked example: a 12 V circuit draws 4 A. Resistance is R = V ÷ I = 12 ÷ 4 = 3 Ω. If a connection adds 2 Ω, current falls to 12 ÷ 5 = 2.4 A and the load may not operate.