Electrical and Electronic Systems Diagnosis
Fundamentals first
Ohm's law and series/parallel circuit behaviour explain almost every electrical fault. Understand that resistance increases with heat, that a voltmeter draws negligible current (so it can read 12 V on a circuit that cannot carry load), and that a test lamp or loaded voltmeter reveals the truth. Learn to read wiring diagrams: power flow, grounds, splices, connectors and module pinouts.
Networked vehicles
Modern vehicles use CAN, LIN and other buses. A network fault can disable communication between modules, producing multiple unrelated symptoms. Check battery voltage and grounds before condemning modules; low voltage causes spurious codes across many systems. Terminating resistors and bus topology matter when testing CAN resistance (typically about 60 ohms across the bus with the battery disconnected).
Testing methodically
Bisect the circuit: verify supply at the fuse, then at the component, then ground. Use voltage drop under load rather than open-circuit resistance. For intermittent faults, wiggle-test looms, check for chafing near hinges and heat sources, and use freeze-frame data. Parasitic drain testing requires waiting for modules to sleep.
Traps
Questions often present a blown fuse and expect the candidate to replace the fuse only; the correct answer recognises the fuse blew because of a short and requires finding the cause. Another trap is misreading a high resistance ground as a faulty component. Watch for confusion between short-to-ground and open-circuit symptoms: a short-to-ground fuse blows immediately, an open circuit gives no operation with intact fuse.
Rule of thumb
If a fuse blows instantly, look for a short to ground on the load side. If it blows after a delay, suspect a motor or actuator drawing excess current.
Worked example
Headlights dim and brighten with engine load. Measure voltage at the bulb: 10.5 V with engine running. Measure from battery negative to engine block: 0.9 V drop. The engine earth strap is corroded; replacing it restores 13.8 V at the bulb.