Automotive Electrical and Electronics
This domain tests the 12 V lead-acid battery, charging and starting circuits, ignition, lighting, and the electronic control units and sensors that manage a modern engine.
Battery and circuits
Six cells at about 2.1 V each give 12.6 V at rest; a healthy cell reads about 1.26–1.28 specific gravity on a hydrometer, corrected for temperature. Top up with distilled water only, never acid. Remove the negative terminal first and refit it last so a spanner cannot short the positive post to the body. Ohm's law and power: V = IR and P = VI.
Charging, starting and ignition
An alternator with a built-in rectifier and regulator keeps the battery charged; the warning lamp should go out above idle. The starter draws hundreds of amps, so check voltage drop across the earth strap. Spark plug gaps are set in millimetres with a feeler gauge, typically 0.7–0.9 mm.
Electronics
The ECU reads crank position, throttle position, manifold pressure and oxygen sensors and drives actuators over a CAN bus. OBD-II fault codes point at a circuit, not automatically at the component.
Worked example
A 55 W headlamp bulb on 12 V draws I = P ÷ V = 55 ÷ 12 ≈ 4.6 A; its resistance is R = V ÷ I = 12 ÷ 4.6 ≈ 2.6 Ω. A much lower measured resistance suggests a short; a much higher one suggests a poor connection. Reason from the formulas instead of guessing.
Traps
Reversing battery polarity and destroying alternator diodes; reading specific gravity straight after adding water; gapping a plug in centimetres; replacing an ECU on the strength of one code without testing the wiring.