Electrical and electronic systems
Electrical and electronic systems
This domain covers battery, starting, charging, lighting, relays, sensors, and basic network diagnostics. The exam tests safe measurement and logical circuit testing.
Core rules and concepts
- Ohm’s law is V = IR; power is P = VI. Use a digital multimeter, not a test light, on computer circuits.
- Voltage-drop testing under load finds high resistance. A good connection usually drops less than 0.1 V; cranking circuit total drop is often specified below 0.5 V.
- Battery: open-circuit voltage near 12.6 V is fully charged; perform a load test and clean terminals. Alternator charging is typically 13.5–14.5 V.
- No-crank: check battery, fuses, ignition switch, neutral or clutch safety switch, relay, starter solenoid, and grounds.
- Relays: identify coil and contact pins; test coil resistance and contact voltage drop.
- Network faults: check power, ground, and twisted-pair CAN wiring before condemning a module.
Worked example
A car does not crank, but dash lights are bright. Battery rest voltage is 12.5 V. While the key is turned, voltage at the starter solenoid control wire is 0 V. Check the relay coil and neutral safety switch; an open relay ground is found. Repair the ground and retest.
Traps that cost marks
Measuring resistance with the circuit powered; using a test light on ECU circuits; assuming a battery is good because it reads 12 V; fitting the wrong relay; and failing to clean grounds.