Automotive Electrical and Electronic Systems
Automotive Electrical and Electronic Systems
This domain tests battery, starting, charging, ignition, lighting, sensors, ECU and wiring fault-finding. Expect Ohm's law, series and parallel circuits, voltage drop, battery testing, alternator output, relays, fuses and CAN communication.
Core rules and concepts
- Ohm's law is V = IR; power is P = VI. In series, current is common and voltages add. In parallel, voltage is common and currents add.
- Voltage-drop testing under load is better than resistance testing for high-current circuits. A poor earth can show normal open-circuit voltage but fail under cranking.
- A healthy battery rests near 12.6 V and has a specific gravity near 1.265. Charging voltage is typically 13.8–14.5 V.
- Never disconnect a battery with the engine running; the alternator can produce a voltage spike.
- Fuses protect wiring. Relays use a small control current to switch a larger current.
- Sensors include potentiometers, Hall-effect, inductive and piezoelectric types. Many use a 5 V reference.
- CAN bus uses a twisted pair with termination resistors. Do not probe it with a low-impedance test lamp.
Worked example
A 21 W brake bulb at 12 V draws I = P ÷ V = 21 ÷ 12 = 1.75 A. Its hot resistance is R = V ÷ I = 12 ÷ 1.75 = 6.86 ohm. A measured 0.6 V drop on the earth side at 1.75 A wastes 1.05 W and leaves only 11.4 V at the bulb. Clean and tighten the earth.
Traps that cost marks
Measuring resistance on a live circuit; using a test lamp on an ECU circuit; fitting a higher-wattage bulb; disconnecting the battery with the engine running; and assuming a 12 V reading proves the battery is good.
Key points
- Use V = IR and P = VI for circuit calculations.
- Voltage-drop test high-current circuits under load.
- Charging voltage is about 13.8–14.5 V.
- Never disconnect the battery with the engine running.
- Use a high-impedance meter around ECUs and CAN.