Lighting, Signalling and Auxiliary Electrical Systems
Lighting, Signalling and Auxiliary Electrical Systems covers headlamps, indicators, hazards, brake and reverse lamps, horns, wipers and the relays and fuses that support them. The test checks circuit operation, switching, protection and legal aim.
Core rules
- High-current lamps use relays; the switch controls the relay coil, not the lamp current.
- Headlamp circuits have main and dip beams, often with separate fuses for left and right; a single fuse failure can cause a one-side outage.
- Flasher units are thermal or electronic; LED conversions can cause hyperflash because current draw is lower.
- HID lamps need ballasts and high voltage; do not touch with power on. LED replacements must suit the lamp housing and preserve the beam pattern.
- Brake, reverse and indicator circuits are checked for correct colour, function and flash rate.
Worked example
Two 55 W headlamps on a 12 V system draw I = P/V = 55/12 = 4.58 A each, total 9.17 A. A 10 A fuse would be loaded at 92% and may nuisance-blow; a 15 A fuse is more suitable if the cable is rated for it. The fuse protects the cable, so never fit a larger fuse to stop blowing without checking cable size.
Traps
Fitting a bigger fuse for a short circuit. Assuming a relay is faulty when the ground is bad. Fitting LED flasher bulbs without a compatible flasher. Ignoring headlamp aim after replacement. Confusing terminal numbers on a relay.