Lighting Systems Diagnosis and Repair
Lighting Circuit Basics
Lighting questions test understanding of series and parallel bulb circuits, switches, relays, and grounds. Know that most truck lighting is wired in parallel so each lamp gets full system voltage, and that a single dim lamp in a parallel circuit points to a high-resistance connection on that lamp's feed or ground, not a system-wide fault. Understand headlamp types-sealed beam, halogen, HID, and LED-and that LED and HID systems may use ballasts or drivers and can be sensitive to voltage and polarity.
Diagnosis by Symptom
Be able to reason from symptoms: all lights dead (feed or ground or switch), one light dead (bulb or local connection), lights dim (voltage drop), lights bright and burning out (overvoltage or wrong bulb). Know how to use voltage drop on both the power and ground sides of a lamp. Understand turn signal and hazard circuits, flasher operation, and that a fast-flashing signal often indicates a burned-out bulb on older thermal flashers, while modern electronic flashers may not change rate.
Traps
A classic trap is a lamp with good voltage at the socket but no light-check the ground path, since a floating ground can show voltage through the filament. Another is assuming a relay is good because it clicks; the contacts can still be burned. Questions also probe daytime running lamps, brake/turn combination bulbs, and trailer lighting, where a bad ground at the trailer causes cross-feeding and weird symptoms like lights that glow when another circuit is energized.
Rule of Thumb
For any single-lamp problem, measure voltage across the lamp with it connected and operating; if it is significantly below battery voltage, perform voltage drop on feed and ground to locate the resistance.