Electrical and electronic systems
This domain tests 12 V electrical fundamentals, battery and charging circuits, starting, ignition, lighting and basic electronic controls such as EFI sensors and relays.
Core rules
- Ohm's law: V = I × R. Power: P = V × I. A voltage-drop test with the circuit loaded finds resistance better than an ohmmeter.
- Battery: 12.6 V rested is fully charged; about 10 V while cranking is a rough load test. Charge in a ventilated area.
- Charging: the stator produces AC; the regulator-rectifier converts it to DC and limits voltage. At 3,000 rpm, battery voltage should rise to about 13.5–14.5 V.
- Starting circuit: battery, fuse, starter relay, starter motor, earths. A click and no crank points to the relay, battery or earth.
- Measure resistance only with power off. Back-probe connectors; do not pierce insulation.
- LED indicators need a suitable flasher relay; incandescent bulbs do not.
Worked example
A motorcycle battery goes flat overnight. The battery is new and holds charge. Reasoning: something drains it. With everything off, connect an ammeter in series at the negative terminal and measure parasitic draw. If it is high, disconnect circuits one by one. A common cause is a stuck brake-light switch or an alarm. If draw is normal, test charging voltage at 3,000 rpm; low voltage means the stator or regulator-rectifier.
Traps
Disconnecting the battery with the engine running; testing resistance on a live circuit; assuming a new battery is charged; fitting a higher-wattage bulb without checking the circuit; reversing polarity.