Automotive Electrical and Electronic Systems
This domain covers battery, starting, charging, lighting, sensors, ECU inputs and outputs, multiplexing and electrical diagnosis. Expect Ohm's law, voltage-drop testing, meter connection and safe probing of electronic circuits.
Core rules
- Ohm's law: V = I × R. In a series circuit current is common; in parallel circuits voltage is common. Resistance rises with a poor connection and causes heat and voltage drop.
- Battery: about 12.6 V at rest for a fully charged lead-acid battery. Cranking voltage below about 9.6 V suggests a weak battery or high starter draw. Charging voltage is typically 13.8–14.4 V.
- Voltage drop: connect the voltmeter across the cable or connection while the circuit is loaded. More than about 0.5 V on a high-current circuit indicates excessive resistance.
- Multimeter use: volts in parallel, amps in series, ohms only on a dead circuit. Do not use an ohmmeter on a live airbag circuit.
- ECU sensors often use a 5 V reference and a dedicated ground. CAN bus faults need correct termination resistance and no shorted or open data lines.
Worked example
A headlamp is dim. With the lamp on, the voltage drop from the battery negative post to the lamp ground is 0.8 V while the circuit carries 10 A. Resistance = V ÷ I = 0.8 ÷ 10 = 0.08 Ω, which is excessive for a ground path. Clean and tighten the ground, then retest; the drop should fall well below 0.5 V.
Traps
- Measuring resistance on a live circuit and damaging the meter.
- Disconnecting the battery while the engine runs and causing a load-dump spike.
- Using a high-current test lamp to probe an ECU circuit.
- Replacing a battery without checking charging voltage and parasitic drain.