Automotive Electrical and Electronic Systems
Domain focus
This domain covers battery, charging, starting, ignition, sensors, actuators, ECU control, wiring, and serial data networks. The exam tests Ohm’s law, voltage-drop diagnosis, and safe use of scan tools.
Core rules
- Ohm’s law: V = I × R. Power = V × I.
- A 12 V battery at rest is about 12.6 V fully charged; 12.0 V indicates significant discharge.
- Voltage-drop testing finds high resistance under load; resistance testing with the circuit powered can damage a meter.
- CAN bus uses twisted-pair wiring and typically 120 Ω termination at each end.
- Sensors provide input; ECU processes data; actuators carry out commands. DTCs indicate a circuit or plausibility fault, not always the failed part.
Worked example
A starter draws 200 A through a cable with 0.005 Ω resistance. Voltage drop = I × R = 200 × 0.005 = 1.0 V. Power loss in the cable = I²R = 200² × 0.005 = 200 W. If battery open-circuit voltage is 12.6 V, the starter sees about 11.6 V before other losses. A 2 V drop would leave 10.6 V and cause slow cranking; the cable or connection is then suspect.
Traps
Measuring resistance on a live circuit, replacing a battery without checking charging voltage, and assuming a DTC names the faulty component. CAN bus faults often come from wiring, connectors, or termination, not the ECU.