AUTOMOTIVE LIGHTING
by Lance Wright

Very few vehicles use the standard sealed beam headlamp that has seen service for years. Most vehicles today use a halogen bulb headlamp system. This system provides excellent lighting potential while giving automotive engineers the opportunity to design streamlined vehicles without having to allow for the shape of a sealed beam. Most vehicle manufacturers use similar wiring systems for headlamp. Fused or fusible link battery power is supplied at all times to the headlamp switch. When the automotive headlamp switch is closed current is delivered through the beam select switch to the headlamps.

Some vehicles are equipped with automatic headlamps. These systems use photo-electric sensors to detect twilight or night conditions. The signal from the sensors is delivered to the headlamp controller which automatically turns on the headlamps. Automatic headlamp dimming systems are also used. A sensor or sensors can detect lighted headlamps from oncoming vehicles and automatically change the headlamps from high to low beam. Daytime running lamps have become standard equipment on many vehicles. These systems illuminate the headlamps only when the engine is running. To extend headlamp life full voltage is not usually supplied to the headlamps. Daytime running lamps will normally be overridden when the headlamp switch is moved to the on position.

Parking lamps are similar to headlamps in operation. Fused battery voltage is supplied to the headlamps switch at all times. The parking lamps can be illuminated using two headlamp switch positions. The first position illuminates the parking lamps only. The second position in which the headlamps and the parking lamps are illuminated.

Turn signals can be found in two different configurations. The first configuration, used by domestic manufacturers for years, combines the stop lamp and the rear turn signal into one lamp. The second configuration, first seen on Japanese and European vehicles, uses separate bulbs and lenses for brake and turn signal lamps. Both systems operate in similar fashion. Fused ignition voltage is supplied to the turn signal switch. When the turn signal lever is moved to the left or right position, current is supplied to the turn signal bulbs through the flasher unit. When the bulb illuminates, current flow causes the bi-metallic contact in the flasher unit to heat rapidly. When a bi-metallic spring or contact is heated, it is caused to distort because of the different expansion rates of the two combined metals. The distortion will cause the contact to open, opening the circuit to the turn signal bulbs. When the circuit is open, current flow is stopped. The bi-metallic spring will cool almost immediately causing the circuit to close. Current flow will resume and cause the bi-metallic spring to open again as it heats. This cycle of heating and cooling is what causes the turn signals to flash on and off. If the turn signals will illuminate and not flash on side of the vehicle but flash normally on the other, the cause is usually a failed bulb or open circuit, as most automotive repair troubleshooters will attest. A turn signal circuit may not flow enough current to properly heat the turn signal flasher if one or more of the bulbs is inoperative.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)