Lighting System Diagnosis and Repair
Concepts
Know the lighting circuits: headlamps (high/low beam), turn signals, brake lamps, tail/marker/clearance lamps, backup lamps, and interior lighting. Understand series vs. parallel lamp behavior, the role of relays, flashers, and the multifunction switch. School buses have extensive marker and clearance lighting, so shared grounds and corrosion are common.
Diagnosis Approach
Isolate by circuit: does the lamp work on one function but not another? A single lamp out points to the bulb or its local ground; multiple lamps out points to a shared fuse, relay, or ground. Use voltage-drop testing on the ground side. For turn signals, verify the flasher and the multifunction switch before replacing lamps.
Traps
Questions often present dim or fast-flashing lamps. Fast flash usually means a bulb is out (reduced load), while slow or no flash can mean a flasher or ground fault. Another trap: assuming a lamp is bad when the socket or ground is corroded. Watch for LED vs. incandescent differences-LEDs draw less current and may need compatible flashers.
Rule of Thumb
"One lamp, suspect the lamp; many lamps, suspect the circuit."
Worked Example
Left turn signals flash fast; right side is normal. One left bulb is out, reducing circuit load and speeding the flasher. Replace the bulb and confirm normal flash rate-do not replace the flasher.