Automotive Electrical and Electronic Systems
This domain tests batteries, starting and charging systems, ignition, lighting, sensors, actuators, wiring and basic electronic control. Expect Ohm's law, series and parallel circuits, voltage-drop testing, CAN bus and safe use of meters.
Core rules
Current flows only in a closed circuit. In series, current is common and voltages add; in parallel, voltage is common and currents add. Resistance is measured with the circuit dead; voltage drop is measured with the circuit operating and the meter in parallel. A battery stores chemical energy and is rated by cold-cranking amps and reserve capacity. The alternator rectifies AC to DC and regulates voltage. Sensors provide voltage, resistance or digital signals to the ECU; actuators convert electrical commands into movement. CAN uses a twisted pair with differential signalling, so both lines should be checked for shorts and termination.
Worked example
A starter draws 200 A and the measured voltage drop across the positive cable is 0.40 V. Resistance is V / I = 0.40 / 200 = 0.002 ohm. Power lost as heat is V x I = 0.40 x 200 = 80 W. A common limit is 0.5 V total for the starter circuit, so this cable is acceptable, but 0.9 V would be excessive and would cause slow cranking.
Traps
Using an ohmmeter on a live circuit; placing an ammeter in parallel; confusing a voltage drop with open-circuit voltage; and testing a CAN bus with the wrong termination. A 12 V lamp rated 60 W draws 5 A and has a hot resistance of about 2.4 ohm.