Electrical and Electronic Systems
Electrical and Electronic Systems
This domain covers basic circuits, batteries, charging and starting, lighting, sensors, actuators and serial communication. The exam expects you to apply Ohm's law, read wiring diagrams, and distinguish a power-side fault from a ground-side fault.
Core rules
- Ohm's law: V = I × R. Power: P = V × I.
- Series: current is common, voltages add, resistances add. Parallel: voltage is common, currents add, total resistance is less than the smallest branch.
- Voltage-drop testing under load is more reliable than measuring resistance on an isolated component.
- A relay uses a small control current to switch a larger load current; identify coil and contact terminals before testing.
- CAN bus and other serial networks need termination resistance and clean differential signals; a shorted bus can disable multiple modules.
Worked example
A starter draws 200 A. The positive cable and ground path together measure 0.02 Ω. Voltage drop = I × R = 200 × 0.02 = 4.0 V. On a 12 V system this leaves too little voltage at the starter and can cause slow cranking. Measure the drop across the cable while cranking, not open-circuit voltage. A good cable should show a small fraction of a volt, typically well under 0.5 V total.
Traps
- Replacing a battery when the fault is a voltage drop in the ground cable.
- Measuring resistance with the circuit powered.
- Assuming a blown fuse is the cause rather than a symptom of a short.
- Ignoring module communication codes after disconnecting the battery.
- Using a test lamp that draws too much current on a sensitive electronic circuit.