Lighting, Signaling, and Vehicle Accessory Circuits
This domain covers exterior and interior lighting, turn and hazard signaling, stop lamps, and accessory circuits such as wipers, windows, horns, and relays. The exam tests bulb types and control strategies, flasher operation, relay and switch diagnosis, and body-control-module behavior.
Core rules
- Halogen bulbs use a tungsten filament and halogen cycle; never touch the glass, because oils create hot spots. HID uses a ballast to strike an arc, with high starting voltage; treat it as hazardous. LED lamps need a driver, are polarity sensitive, and are often pulse-width modulated for dimming and thermal control.
- A thermal flasher depends on current through the bulbs; replacing bulbs with LEDs reduces current and causes hyperflash. An electronic flasher or load resistors restore normal flash rate.
- Relays use a low-current coil to control a high-current contact. Know terminal numbers: 30 supply, 87 output, 86 and 85 coil, and 87a the normally closed contact.
- Body control modules often drive lamps directly or through relays, monitor bulb current for failure detection, and may pulse lamps for bulb-check functions.
- Accessory motors draw high inrush current; a circuit that is marginal may blow fuses only when cold or stalled.
Worked example
A customer installs LED turn-signal bulbs and the indicators flash rapidly. The original thermal flasher expects about 2 A per side; the LEDs draw about 0.2 A, so the bimetal does not heat enough. Measuring current at the flasher confirms low current. Fitting an electronic flasher rated for LEDs, or adding a resistor in parallel at each lamp, restores the load. The resistor must be mounted to metal because it dissipates heat.
Traps
Probing an HID ballast output with a standard meter can damage the meter and injure you. Assuming an LED bulb is faulty when it is inserted backwards ignores polarity. Reading a PWM lamp circuit with a basic meter gives an average voltage, not the peak. Adding accessories to an existing circuit without checking wire and fuse capacity causes repeat failures. Never fit a larger fuse to stop blowing; the fuse protects the wiring.