General Electrical/Electronic System Diagnosis
Foundations of Electrical Diagnosis
Every diagnosis on a medium/heavy truck starts with understanding the three basics: voltage, current, and resistance, and how they behave in series and parallel circuits. Candidates must be fluent with Ohm's law (E = I × R) and power (P = I × E), and must know that voltage is measured in parallel while current is measured in series. Know the difference between a loaded and unloaded voltage measurement-an open circuit can show full voltage with no load and collapse under load.
Tools and Techniques
Be comfortable with the DVOM, test light, and ammeter/clamp, and know when each is appropriate. A test light draws current and can damage sensitive electronic modules; a high-impedance DVOM is safer for computer circuits. Understand voltage drop testing-measuring across a connection or cable while current flows-as the preferred method over resistance testing for high-current circuits like starter and ground cables.
Common Traps
Questions love to blur the line between "no voltage" and "no current." A circuit can have voltage present at an open point yet carry no current. Another trap is assuming a ground is good because the meter reads continuity to battery negative; a corroded ground can pass a low-current meter test but fail under load. Watch for questions about parasitic draw-measured in milliamps with everything off and modules asleep-and the need to let modules time out before measuring.
Rule of Thumb
Limit voltage drop to about 0.1 V per connection and 0.3 V total on a 12 V high-current circuit; anything more points to resistance. Worked example: a starter cranks slowly; measure voltage drop from battery positive to starter stud while cranking and see 1.2 V-that exceeds the 0.3 V guideline and indicates a bad cable or connection, not a bad starter.