General Electrical/Electronic System Diagnosis
Foundations
Every electrical diagnosis starts with the same discipline: verify the complaint, then test the circuit in a logical order rather than replacing parts. Understand Ohm's law (E = I × R), series vs. parallel behavior, and the difference between voltage, current, and resistance measurements. Know that voltage is measured in parallel across a load and current in series through a conductor.
Tools and Techniques
A DMM, a test light, a wiring diagram, and a voltage-drop test are the core toolkit. The voltage-drop method is the single most important skill: measure across a connection or cable while the circuit is loaded. More than about 0.1 V per connection or 0.5 V total on a high-current path signals excessive resistance. Do not use a low-impedance test light on computer-controlled circuits; it can damage drivers.
Traps
Questions love to blur the difference between an open circuit (no current, full source voltage at the open point) and high resistance (circuit operates poorly, heat, dim lights). Another trap is assuming a blown fuse is the root cause-fuses protect; something downstream drew excess current. Watch for ground-side faults masquerading as component failures, and for shared grounds that cause multiple unrelated symptoms.
Rule of Thumb
"No power, test toward the battery; no ground, test toward the chassis." Load the circuit before trusting a voltage reading-a meter draws almost no current and will show good voltage through a nearly broken wire.
Worked Example
A stop-arm lamp is dim. Measure voltage at the lamp: 11.8 V with the lamp connected. Measure from the lamp ground to battery negative while loaded: 0.9 V. That drop exceeds 0.5 V, so the fault is a corroded ground, not the lamp.