Lighting and Accessory Systems
Lighting and Accessory Systems
Lighting circuits are simple in principle-power, switch or control module, load, and ground-but transit buses add multiplexed body controllers, LED lamp assemblies, PWM dimming, and lamp-out detection. Know how parallel loads behave: a bad ground affects every lamp sharing it, while an open feed affects only that branch. LED lamps are polarity-sensitive and draw low current; some body controllers interpret low current as a failed bulb and set a fault or cause hyperflash. Replacing an LED with an incandescent lamp, or vice versa, can change flasher operation and current monitoring.
Accessory circuits include wipers, fans, doors, horns, mirrors, and HVAC blowers. Many are relay- or module-controlled, so a dead accessory may be a control-side fault rather than a bad motor. Check both power and ground at the load under load, and confirm the module command.
Traps: condemning a lamp when the socket or ground is corroded; probing computer-controlled outputs with an incandescent test light; ignoring shared grounds that cause feedback (one lamp glows when another is switched on); assuming a flasher is bad when an LED conversion changed the load.
Worked example: a left front turn lamp is dim while all others are bright. Measure voltage from the lamp ground terminal to battery negative while the lamp is on; 1.8 V indicates a poor ground. Clean and tighten the ground, then retest. If ground drop is low, measure the feed side.
Rule of thumb: total voltage drop in a lighting circuit should stay under about 0.5 V, with each connection under 0.1–0.2 V.