Nighttime driving is inherently more dangerous than daytime trips. Despite the artificial light provided by streetlamps and headlights, the darkness can conceal road hazards, pedestrians, and nocturnal creatures. Driving after sunset becomes even more challenging when your own headlights are not performing as they should. If you notice your headlights are suddenly dimmer or the brightness is fluctuating, there is likely a problem lurking under the hood of your car that needs addressing.

Dim Headlights: 5 Reasons You May Struggle to See at Night

(tookapic / pixabay)

Headlight Basics

Because you are accustomed to changing bulbs inside the house when they stop giving off light, you may assume that the same solution will work in your car. Changing the bulbs in your headlights may inadvertently solve your problem, but should it be the first step you take? A variety of different issues can cause headlights to dim. Understanding how your headlights work from the 10,000-foot level can help diagnose the problem.

Car headlights are powered by the car’s battery when the vehicle is off. After you start the car, the engine begins providing power for all of the car’s electrical systems, including charging the battery. The alternator carries energy from the engine to the electrical components of the vehicle. The headlights are connected to the engine by a wiring harness and grounded to the car’s chassis. Any disruption to the flow in this electrical circuit can cause the lights to dim or go out entirely.

It is important to note that headlight bulbs are usually on or off. If they are still good, they should light up completely. If they are broken, they will not shine at all. If yours are dimly lit, they are likely not the source of your problem. You will need to look elsewhere for the underlying cause of your problem.

The Usual Suspects

1. Aging Headlight Lenses

Modern headlights have a clear plastic cover over the headlights to protect the bulbs from damage. Over time, this plastic can become yellowed with exposure to sunlight. Rocks, dirt, and other road debris can scratch the surface of the plastic, making it appear white or cloudy. This discoloration obscures the lens and inhibits the passage of light. Your headlights may appear dim even though they are working correctly.

Often you can extend the useful life of the plastic using a DIY headlight restoration kit, which polishes out the blemishes to restore the plastic to its transparent state. Many Salt Lake City auto repair shops offer headlight restoration services for those who shy away from DIY projects. Keep in mind that the gains from restoration are temporary, and your headlights will likely need replacing down the road.

2. Discolored Bulb

Most factory headlights come with halogen bulbs. Over time, the gas inside these bulbs gradually creates a film on the inside of the glass. If the bulbs in your headlights look dark, it is likely just build-up on the inside of the bulb, preventing light from escaping. Unfortunately, the only way to fix this problem is to replace the bulbs themselves.

3. Ground Wire Corrosion

The wire connecting the headlights to the chassis is vital to their performance. This grounding wire is part of the circuit that allows electricity to flow to them, providing power. If the wire becomes damaged or inhibited by dirt or corrosion, electricity cannot flow freely. The headlights will receive less power and shine less brightly. The flow of electricity may be cut off completely, causing your headlights to go out even though they are still in good working condition.

Resolving this issue involves replacing the ground wire. The wire is not an expensive part, but it will take your mechanic some time to replace it. The majority of your cost for this fix will be for labor, not parts.

4. Failing Alternator

A functioning alternator is not only responsible for maintaining proper headlight brightness, but it provides electrical power to nearly all of the car’s bells and whistles. The power steering, windshield wipers, radio, power windows, and dash instruments all need electricity to run. The alternator receives energy from the engine and turns it into usable electrical energy to power all of these different components.

When the alternator fails, it stops providing power to the backup camera, the power windows, the headlights, and the battery. Now, the battery must meet the electrical needs of all the vehicle. Car batteries are not designed to provide that much power. The vehicle’s needs will quickly drain the battery leaving you stranded by the side of the road.

How can you tell if your alternator is going? Pay attention to the headlights when you start your car. When you first turn the key in the ignition, the lights are running off the battery and should be bright. If your headlights dim when they switch over to power from the alternator, that is an indication that it may not be providing enough power, and you may need to have your alternator checked.

5. Worn Alternator Belt

Headlights that appear to brighten and dim at random intervals may indicate that the alternator belt needs replacing. This belt attaches to a pulley that connects the alternator to the engine. If the belt is old and worn, it may alternate between slipping and gripping. When it slips, the headlights lose power, causing them to dim. When the belt grips the pulley again, the headlights receive more power and get brighter. Belts stretch and wear out with time. If you believe yours needs replacing, your mechanic can check its condition and make any necessary repairs.

Unfortunately, dim headlights are not a problem that drivers notice until they need them. Driving at night with little more than the light from a couple of flashlights can be dangerous. If your headlights are not as bright as they should be, it is time to investigate the usual suspects, identify the culprit, and fix the problem.


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