Lighting and Signalling Systems
Lighting and Signalling Systems
This domain tests headlamp, side, indicator, hazard, brake, reverse, horn and interior lighting circuits, including relays, flashers and earth returns.
Core rules
- Lamps are rated in watts at nominal voltage. Current I = P/V; a pair of 55 W headlamps at 12 V draws about 9.2 A total.
- Headlamp main and dip beams are separate filaments or lamps. A relay carries the high current; the switch carries only relay coil current.
- Indicator flashers are thermal, electromechanical or electronic. LED conversions may need a load resistor or LED-compatible flasher because current is too low to operate thermal types.
- Hazard lamps often use a separate flasher or a combined unit and must work with the ignition off.
- A poor earth can cause cross-lighting: current feeds back through another lamp filament, so several lamps glow dimly.
- Fuses protect wiring, not the lamp; fit the specified rating.
Worked example
Two 55 W headlamps on a 12 V system draw 55/12 = 4.58 A each, total 9.17 A. A 15 A fuse is acceptable because continuous load is about 61% of rating; a 7.5 A fuse would nuisance-blow. If the owner fits 100 W bulbs, total current becomes 16.7 A, exceeding the 15 A fuse and possibly the wiring capacity. The correct fix is a relayed harness of adequate cable size, not a larger fuse.
Traps
Fitting higher-wattage bulbs and a bigger fuse; diagnosing a dim lamp as a bulb fault when the earth is corroded; assuming a flashing indicator means the hazard circuit is healthy; fitting LEDs without checking flasher compatibility; confusing side-light and brake-light filaments in a double-filament bulb; ignoring lamp aim and legal colour or position requirements.