Vehicle Electrical and Electronic Systems
Fundamentals
Ohm's law, series and parallel circuits and power relationships underpin everything. In series, current is common and voltage divides; in parallel, voltage is common and current divides. Most vehicle circuits are chassis-grounded, so a poor ground is a genuine fault path, not a detail. Battery ratings cover cold cranking amps and amp-hour capacity; charging systems must hold roughly 13.5 to 14.8 V at idle with loads on, and excessive AC ripple indicates diode failure.
Testing method
Voltage drop testing under load is the definitive check for cables, connectors, switches and grounds: measure across the component while current flows, not to earth. A healthy circuit typically shows less than 0.5 V total drop, with each connection under about 0.1 V. Resistance measurement is only valid on an isolated, unpowered circuit. Use a wiring diagram to identify feeds, fuses, relays, grounds and splice points, and understand that relays use a small control current to switch a large load current.
Electronics and networks
Hall-effect and inductive sensors, PWM control, LEDs and multiplexed CAN buses are standard. Modules share information, so a faulty wheel speed sensor can disable speedometer, ABS and transmission logic. Parasitic drain testing must allow modules to sleep first, else the reading is meaningless.
Rule of thumb: for a no-crank, check battery state of charge, then both battery terminals and the engine earth strap with a voltmeter under load before replacing a starter - high-resistance cables mimic a dead motor. Traps include measuring resistance with the circuit powered and misreading a series circuit's current.