How Do Automatic Headlights Work?

by Edmund Gary
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1961 chrysler 300m angled headlights image by Jorge Moro from

Automatic headlamps are a modern convenience in many of today's cars. They eliminate the need for the driver to manually switch on or off the headlamps in most driving situations. The names of the automatic headlamp option differ between car manufacturers, but they perform the same service for the driver. Their secondary features set one automatic headlamp option apart from the others.

Operation of Automatic Headlamps

The automatic headlamps are activated through a photoelectric sensor which is embedded into the instrument panel. The sensor is located at the base of the windshield under the defogger grill. The sensitivity of the sensor is either set by the auto manufacturer or the driver. The sensor is activated by the lighting conditions at dawn or dusk. The lights may switch off up to five minutes after the engine has been turned off.

The driver has the ability to bypass the functions of the automatic headlamps by operating the light switch or some other device in the car. If the driver inadvertently leaves the lights on, a reminder chime will ring so the driver can turn them off.

Switch Controls

The light switch of an automatic headlamp controlled car resembles the switch of a car with conventional headlamps. The conventional light switch has positions that read "off", "park" and "headlamps." The light switch of an automatic headlamp car has positions which read "auto," "park," and "headlights." Some car models have an "off" position so the driver can bypass the automatic headlamp function when starting the vehicle.

General Motors' Automatic Light Control

The Automatic Light Control system (ACL) is a standard feature for 1999-2004 Oldsmobile vehicles and an option for some Pontiac models. The sensitivity of the sensor and headlamp delay are set at the factory and cannot be adjusted. Automatic Light Control turns the lights on whenever the sensor, in the instrument panel, senses dark conditions. The system will activate the lights when the car is driven in a enclosure, such as a parking garage. Heavy overcast conditions can cause the lights to switch on. The headlamp off delay is twenty seconds. Fog lamps, if equipped, will switch off when the driver turns off the engine.

Bypassing the system can be accomplished by engaging the parking brake before starting the engine. Also after turning the engine off, one can move the light switch from "auto" to "headlamps" and back to "auto" again.

Ford's Auto Lamp System

Lincoln-Mercury vehicles have used a system called "Auto-Lamp." A photocell is used to to activate the automatic headlamps whenever the conditions warrant.

Its two secondary features differentiate Auto-Lamp from General Motors' Automatic Light Control system. The automatic headlight system can be bypassed at the light switch. Secondly, Auto-Light has an exit delay feature, which keeps the interior and exterior lights illuminated so the path to the driver's dwelling can be illuminated. This was an optional feature (circa 1981 and 1982 model year) later made a standard on the Mercury Grand Marquis.

Twilight Sentinel

Twilight Sentinel is one of the earliest forms of automatic headlamp features for General Motors (Cadillac and Buick) and Chrysler cars. It dates back to 1964. Twilight Sentinel uses an amplifier and a single photocell to gauge the light intensity before switching the lights on. The 1980 Buick Buyer Guide states the headlights will "stay on to light a path ahead of the car for three minutes," when the engine is turned off. The driver can set the time length for the headlamp delay. According to Imperial, the Chrysler version of Twilight Sentinel will illuminate the reverse lamps along with the headlamps.

According to Chip Lamb, one uses the turn signal to illuminate the corresponding side of the car, provided that the car is equipped with side cornering lamps (Cadillac, Buick Riviera and Electra and Chrysler Imperial feature). Cornering lamps are white lights on the leading portion of the front fender of some luxury cars.



The automatic headlamp system is a mainstream feature in many cars. It was once a feature seen exclusively on luxury cars. It is a handy luxury feature.

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