Electrical Fault Diagnosis and Repair Procedures
This domain tests a repeatable diagnostic process and repair quality. Expect questions on fault types, voltage-drop testing, intermittent faults, parasitic draw, and acceptable repair methods.
Core rules
- Verify the complaint, gather conditions, check the battery and basics, obtain the wiring diagram, and understand how the circuit should behave.
- Fault types are open circuit, short to ground, short to power, high resistance, and intermittent. A short to ground usually blows a fuse; high resistance causes dim lamps, slow motors, and heat.
- Test under load. Voltage drop across each connection and cable should be low; an open circuit shows full source voltage at the break, which can mislead.
- Intermittent faults need wiggle tests, heat or cold application, water spray, and road-test data. A bypass test can confirm a suspect conductor.
- Repair with the same gauge and insulation type, correct terminals, crimps, adhesive heat shrink, and proper routing. Replace damaged terminals rather than piercing insulation.
Worked example
Headlamps are dim and the fuse is intact. With the lamps on, measure battery positive to lamp positive: 0.2 V, acceptable. Measure lamp ground to battery negative: 0.9 V, too high. That means the ground side is dropping nearly a volt. Trace the ground terminal, find corrosion at the body ground, clean and tighten it, then re-test. The drop falls to 0.05 V and brightness returns. Replacing the lamps would not have fixed the cause.
Traps
A test light that lights does not prove a ground can carry load. Checking voltage at an open circuit can show full voltage and fool you into thinking the circuit is good. Replacing parts without testing wastes money. Not verifying the repair under the original conditions lets the fault return. Piercing insulation or using wire nuts creates future failures. Always confirm with a voltage-drop or loaded test.