Headlights are integral to safe driving in poor visibility; anything that interferes with your headlights' illumination interferes with your safety. While cloudiness of headlights is a well-known cause of poor illumination, it isn't the only thing to worry about. Here are three other major things that can interfere with your headlights' illumination:
Your headlights will be dim if their bulbs aren't getting the right voltage. A fully charged battery should give a reading of 12.66 volts on the voltmeter. At 75% charge, the reading is 12.45 volts, and this is where lead-acid batteries should be maintained. Anything less than 12.45 volts is low; fortunately, driving the car at 40 mph (or faster) for at least 15 minutes should charge the battery.
If the battery doesn't charge, then you are either dealing with a damaged battery or a malfunctioning alternator. It's likely to be a damaged battery if:
- It charges but quickly drains
- It doesn't charge, and you are sure the charging system hasn't malfunctioned
- You have been using it for more than four years
To confirm your suspicions, take your car to an auto mechanic or auto parts store for testing.
As for the alternator, you should suspect a malfunction if the alternator warning light (it is shaped like a battery and may have a "GEN" or "ALT" sign) lights up on the dashboard or the headlights dim when you rev the engine.
Dirt on the outside of the headlights can reduce their brilliance by blocking or reflecting some of the light rays. This may happen if you have been driving on dusty or muddy roads, and some dust or mud gets stuck in the headlights. Some of the debris may even get inside the headlight assembly. Luckily, this is something you can easily get rid of by normal cleaning using a glass cleaner (soap and water also works).
Cleaning the inside of the headlight assembly involves dismantling the assembly, and cleaning the lens and housing using the glass cleaner. Take care not to touch the glass bulb while doing this; touching the bulb contaminates it with oils from your hands, and the oils shorten its life.
Lastly, poor headlight aim may also prevent proper illumination of the road. You should suspect that your headlights are too high if other drivers are signaling to you (by flashing their lights) that your lights are blinding them. If they are too low, then the road ahead will only be illuminated for a short distance, say 20 feet or so.
A common cause of poor headlight aim is a suspension problem that interferes with the car's ride height. The problem can also arise due to mechanical damage, such as that caused by a collision, that interferes with the headlight assembly. The solution depends on the cause of the problem. For example, if it is suspension-related, then a trip to an auto repair shop is necessary to diagnose and fix it.