Automotive electrical and electronic systems
Automotive electrical and electronic systems
The exam tests basic circuit laws, battery, charging and starting, sensors, actuators, multiplexing and safe measurement.
Core rules
- Ohm's law: V = IR. In series, current is common and voltage divides; in parallel, voltage is common and current divides.
- Voltage-drop testing under load is more reliable than resistance testing for high-current circuits. The limit for a starter cable is often under 0.5 V total.
- Battery: CCA is rated at −18°C; a rested 12.6 V is about 100% state of charge. Charging should be 13.8–14.8 V at the battery with loads off.
- Sensors: 5 V reference, signal return and ground. A signal shorted to ground reads 0 V; an open circuit reads 5 V on many circuits.
- CAN bus: two wires, 120 Ω termination; measure about 60 Ω across CAN H and L with power off. Do not probe with a low-impedance test lamp.
Worked example
A battery drains overnight. Measure parasitic draw with a clamp meter or a series ammeter after all modules sleep. Acceptable is often under 50 mA. If 0.35 A is measured, remove and refit fuses one at a time to isolate the circuit. A glovebox lamp staying on is a common cause. Do not disconnect the battery to test alternator output on a modern vehicle; voltage spikes can damage modules.
Traps
- Measuring resistance on a live circuit.
- Assuming a new battery is charged; it may need commissioning.
- Replacing an alternator when the belt or sense wire is at fault.
- Back-probing connector terminals with a probe that spreads the pins.