Ignition, Charging, Lighting, Battery and Basic Electrical Systems
What the exam tests
How the spark is produced and timed, how the alternator and regulator-rectifier charge the battery, and how lighting and basic electrical faults are found.
Ignition
- CDI: an exciter coil charges a capacitor, a pickup coil triggers a thyristor, and the capacitor discharges through the ignition coil. Reliable at high speed, weak spark when cranking.
- TCI: the coil is charged inductively and switched by a transistor. Timing advance with rpm is built into the CDI or ECU.
- Spark plug gap is typically 0.7-0.9 mm; resistor plugs and caps suppress interference with the ECU.
Charging and battery
A magneto or alternator feeds a regulator-rectifier that converts AC to about 13.5-14.5 V DC at 3000 rpm. A healthy 12 V battery rests at about 12.6 V; below 12.4 V it needs charging. Lead-acid, VRLA and AGM types differ in maintenance: only flooded types are topped up with distilled water, and electrolyte specific gravity is about 1.26-1.28 when fully charged.
Lighting
AC headlamps run directly from the alternator and dim at idle; DC systems run from the battery. LED and projector lamps need the correct regulator. A blown fuse points to a short, so find the short before replacing the fuse with a higher rating.
Worked example
A scooter starts but the headlamp dims and the battery goes flat. Measured: 12.4 V at rest, and only 12.1 V at 3000 rpm with lights on. Reasoning: the charging voltage should rise to 13.5-14.5 V, so the system is not charging. Check the regulator-rectifier connector and earth, then stator resistance between leads (typically 0.5-1.2 ohm, no continuity to earth). A failed rectifier is the usual cause.
Traps
Reversed jump-start polarity, fitting a higher-rating fuse, testing charging with a flat battery connected, and assuming a new battery is charged.