Automotive Electrical and Electronic Systems
What this domain tests
This domain tests basic electrical theory, battery and charging systems, starting circuits, lighting, sensors, wiring diagrams and electronic control. Expect Ohm's law calculations, voltage-drop testing, and diagnosis of no-crank, no-charge and parasitic drain faults.
Core rules
- Ohm's law: V = I × R. Power: P = V × I. A 55 W lamp at 12 V draws about 4.6 A.
- A 12 V battery has six cells at about 2.1 V each. A healthy specific gravity is near 1.265; 1.120 or lower indicates a discharged cell.
- Charging voltage at the battery with the engine running is normally 13.8-14.5 V.
- Circuit voltage drop should stay below about 0.5 V on cables and earths. Test under load, with the meter across the connection, not with an ohmmeter.
- Parasitic drain is measured in series with the battery after all modules sleep, typically under 50 mA.
- Never use an ohmmeter on a live circuit, and never disconnect a battery with the engine running.
Worked example
While cranking, the battery posts show 12.6 V but the starter terminal shows 10.8 V. The drop is 1.8 V, far above the 0.5 V limit, so there is high resistance in the cable, the earth strap or the terminal. Connect a voltmeter between the battery negative post and the starter body and crank again. A reading above 0.5 V puts the fault in the earth path; a low reading puts it in the positive cable. Measuring resistance with the circuit off would not reveal the fault, because resistance rises only under cranking current.
Traps that cost marks
- Probing an airbag circuit with a test lamp.
- Checking resistance with power still applied.
- Reversing jump-start polarity and destroying the alternator diodes.
- Clearing codes before reading the freeze frame.