Lighting and Accessory Circuits
Lighting and Accessory Circuits - what the exam tests
Headlamp, tail, brake, indicator, reverse and interior circuits; relays and fuses; bulb types; wipers, horn, windows and central locking; and legal headlamp aiming.
Rules
- High-current lamps are fed through a relay; the switch carries only coil current. A burned switch or melted connector usually means the relay was bypassed or an uprated bulb was fitted.
- Indicators flash 60–120 times per minute. LED conversions need an electronic flasher or ballast resistors, otherwise hyperflash and bulb-out warnings appear on monitored circuits.
- Dual-filament stop/tail bulbs share a ground. A shorted filament or corroded ground lets tail and brake circuits back-feed, so lamps glow dimly or the brake lights stay on.
- Headlamps are aimed at kerb weight, correct tyre pressures, on level ground; a dipped beam typically drops about 1% over distance.
Worked example
A fuse blows the moment the lights are switched on. Do not fit a bigger fuse. Disconnect the lamp connectors one at a time and re-test; when the fuse stops blowing, that branch holds the short. Measure resistance from that branch's feed wire to ground - near zero confirms a short to the body. Reasoning: the fuse protects the cable, so a repeat blow is a dead short, not a load fault.
Traps that cost marks
- Fitting 100 W bulbs to a standard harness.
- Assuming the bulb is faulty when the ground is corroded - most lighting faults are ground faults.
- Using a low-impedance test lamp on a module output.
- Adjusting aim with a loaded boot or wrong tyre pressures.
- Reversing polarity on LED lamps.