Electrical and Electronic Systems Servicing
This domain tests battery, starting, charging, lighting, and basic circuit diagnosis with a multimeter.
Core rules
- Ohm's law: V = I × R; power P = V × I. A 12 V, 60 W lamp draws 5 A and has about 2.4 Ω when hot.
- Resting battery: about 12.6 V at 100% state of charge; below 12.4 V is discharged.
- Cranking: usually not below 9.6 V at the battery at 20 °C.
- Charging: about 13.5–14.5 V at the battery with loads off.
- Voltage-drop testing under load is more reliable than resistance on a live circuit; keep each connection under 0.1 V and the total near 0.5 V.
- Fuses protect circuits; test with a meter, never by fitting a larger fuse.
- Disable SRS before working near wiring, and never disconnect the battery with the engine running.
Worked example
Headlamps are dim. The battery reads 12.6 V and the lamp connector reads 11.7 V with the lamp on. Reasoning: 0.9 V is dropped between battery and lamp, far above the 0.5 V guideline. Work backwards: battery positive to fuse, fuse to switch, switch to lamp; the largest drop identifies the bad connection. If the drop is on the ground side, the lamp is also dim, so test the ground return too.
Traps
Measuring resistance on an energized circuit; reverse-connecting a battery; charging a frozen battery; ignoring parasitic draw; replacing an alternator without checking the belt and grounds; probing airbag connectors.