Daytime running lights was voted into a Canadian law several years back and required all new vehicles to be equipped with running lights that were automatically turned on during daylight hours. This automotive technology was developed with safety in mind.
In the United States General Motors was one of the first companies to also make this standard equipment on all GM vehicles that were sold in North America. My 2004 Chevrolet Blazer is equipped with daytime running lights. And they have not been completely trouble-free.
For one thing they are on so much that they experience more bulb failures than any other bulbs all my truck. There is also quite a lot of heat generated from the bulbs being on all the time. This can eventually melt the bulb sockets and the plastic lenses and housings that they are mounted in.
How daytime running lights work
On some vehicles they use the high beam lights as dedicated daytime running lights. The control circuit is connected directly to the vehicles ignition switch so the lights are turned on whenever the vehicle is running. The circuit is usually equipped with some type of module that lowers the power to the high beam so that it can burn with less intensity.
When the headlamp switch is moved to the headlight position the module is deactivated and the lights work with their normal intensity and brightness. On my 2004 Chevrolet Blazer there is an optical sensor mounted on the top side of the dash panel. During the day the sunlight is detected and the daytime running lights are turned on automatically.
When it starts to get dark the ambient light sensor turns on the headlights on low beam intensity automatically. My 2004 Chevrolet Blazer also has an override in case the system malfunctions.
Also note on Chevrolet vehicles that when you apply the parking brakes this will also deactivate the daytime running lights even if the vehicle is running. So if you set your parking brake and try to check your daytime running lights this will not work.
On some vehicles the daytime running lights may not come on until the vehicle is put in drive. Keep these quirks in mind when you are trying to check the operation of your daytime running lights. As for me when I check them I usually pull up to a wall or a garage door so I can see the reflection of the lights. This will work on most models and is a good way to tell if one of your daytime running lights is blown.
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