Lighting, Horn, Wiper, Indicators and Body Electrical Accessories
What the exam tests
Lamp circuits, switches, flashers, wiper systems, horns and body accessories.
Core
- Halogen bulbs need correct wattage and clean glass; HID lamps use a ballast that generates high voltage at start-up; LED lamps are polarity-sensitive and draw low current.
- Many body circuits switch the ground rather than the positive feed. The switch may be upstream or downstream of the load, so check both sides with a meter or scope.
- Indicators use a thermal or electronic flasher. LED conversions can hyperflash because the flasher expects a higher current; the fix is an electronic flasher or load resistors, not a bigger bulb.
- Wiper motors often have a park switch that keeps the motor running until the park position. A motor that will not park is not necessarily a faulty motor.
- Horns use a relay and are grounded through the horn button or clock spring. Never test a clock spring by turning the wheel with the airbag circuit live.
- Central locking, windows and mirrors share door modules and grounds; a single door fault is usually local.
Worked example
Both brake lights are dead, but the tail lights work. The fuse is shared with other working circuits, so the fuse is probably good. Check the brake pedal switch: with the pedal pressed, one terminal should have battery voltage and the other should feed the lamps. If voltage enters but does not leave, the switch is faulty. If only one brake light fails, suspect that bulb, holder or local ground, because a fuse or switch would affect both.
Traps
Fitting an HID kit without a ballast; diagnosing hyperflash as a wiring fault; replacing a wiper motor for a park switch; testing airbag wiring with a powered test light; assuming all lamps share one ground.