Electrical and Electronic Systems
Ohm's law to network diagnostics
Electrical questions are usually circuit-reasoning questions. Apply Ohm's law (V = I × R) and the series/parallel rules confidently: series resistance adds and current is common; parallel branches share current and total resistance falls. Most modern loads are controlled on the ground side by a module, so a suspected faulty component is often a faulty driver, ground, or connector.
The professional diagnostic sequence is: verify the complaint, check the battery state of charge (about 12.6 V at rest, 13.5–14.5 V charging), test fuses under load rather than by eye, then perform a voltage drop test with the circuit operating. Chasing resistance with an ohmmeter on a dead circuit hides problems that only appear under current flow. Keep drops to roughly 0.1 V per connection and 0.5 V total per circuit.
Know the components: relays and their pin numbering, blade fuse ratings, grounds and ground straps, sensors (three-wire potentiometer with a 5 V reference, Hall-effect digital, inductive magnetic, thermistor), actuators, and communication buses. On a CAN bus, two 120-ohm terminators in parallel read about 60 ohms across the data lines, and a shorted or open leg takes down multiple modules. U-codes point to communication faults, not necessarily the module that set them.
Trap: condemning an ECU before checking powers, grounds, and terminal tension. Trap: measuring parasitic draw with the meter on the wrong scale or before modules sleep.
Worked example: A 12.6 V supply drives a 3-ohm load. I = V/R = 4.2 A. If a 0.1-ohm ground connection sits in series, drop = 4.2 × 0.1 = 0.42 V - the load sees only about 12.2 V and may perform poorly.