Automotive Electrical and Electronic Systems
What this domain tests
Battery, starting, charging, ignition, lighting, sensors, ECU and multiplexed networks. Questions often use Ohm's law, voltage-drop reasoning and sensor function.
Core rules and concepts
- Ohm's law: V = I × R. Series resistances add; parallel resistance is lower than the smallest branch.
- Battery Ah is capacity; CCA is cold-cranking ability. They are not the same rating.
- A healthy alternator typically holds about 13.8-14.4 V at the battery with the engine running and loads on.
- Voltage-drop testing under load finds high-resistance cables and grounds better than a plain resistance test.
- Fuses protect wiring; relays let a small switch current control a large load current.
- Key sensors: crankshaft and camshaft position, MAP/MAF for load, oxygen sensor for mixture feedback, coolant temperature.
- The ECU controls fuel, ignition and emissions; CAN bus uses a twisted pair for module communication.
Worked example
A starter draws 200 A through a cable of 0.02 Ω. Voltage drop = I × R = 200 × 0.02 = 4 V. With 12.6 V at the battery, only 8.6 V reaches the starter, so cranking is slow. Reason: high current makes even small resistance important; repair the cable or ground rather than replacing the starter.
Traps that cost marks
Measuring resistance on a live circuit; confusing Ah with CCA; condemning an alternator when the belt slips; ignoring engine-to-body ground straps; fitting a higher fuse to stop it blowing.