Why are my Headlights Dimming?

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VM-1 Voltage Meter displaying Low Voltage.
Utilizing the VM-1 to monitor your voltage will heklp you prevent headflight dimming.

Are your headlights dimming after installing your first car amplifier?

Use this guide to pinpoint the problem of your headlight dimming.

You’ve installed your first amplifier and are enjoying music at reasonably loud volumes in your car.

One evening, you notice the headlights dimming to the beat of your music.

If you have a stock electrical charging system, there may be a voltage issue.

Turn your stereo and air conditioner on full blast while idling at night. You may notice the headlights dim slightly.

Next, roll the windows down and pay close attention to your headlights as the windows retreat into the doors- They will probably be dimming a little bit more now.

If you’ve noticed the issue starting or becoming worse after installing your first amplifier and subwoofer, the charging system may be to blame.

Headlight dimming is typically limited to large systems in the thousand watt and up range but it can also happen with smaller builds when playing music.

Your charging system may not be capable of producing the power that your amplifier needs.

It is a matter of power. When the bass hits, the amplifier is drawing more power than your vehicle’s electrical charging system can create.

This is happening because of the voltage and current that your amplifier is drawing.

The entirety of your electrical system may be suffering. Since your eyes are more sensitive to changes in light than your ears are to changes in sound, you will notice the dimming first. Ultimately, halogen bulbs will vary in their brightness, regardless of what is in your vehicle. But it’s good to err on the side of caution.

Let’s make sure there’s not an issue with the install before taking expensive measures that may not be necessary.

  1. Ensure that power and ground wires are connected to the clean bare metal of the car. It is essential to have good power and ground connections.
  2. Check the battery connections and make sure that the battery terminals are crimped and fastened tight. Use a Digital Multimeter to confirm that your connections are solid using the method described here.
  3. Have your battery load-tested at your local auto parts store. This is something they will normally do for free and will help suggest a replacement if your battery is the root issue.

Car batteries, particularly lead acid batteries, do not have a long shelf life. They rarely last for more than four years. An old battery may have enough charge to start your vehicle but may not have enough juice to safely power your amplifier when it’s pulling high voltage.

If replacing your battery does not correct the headlight dimming, there are still a few things to consider.

  1. Examine your headlight wiring. Make sure that the connections are solid.
  2. Inspect the fuse panel. Blown fuses can sometimes complete the circuit and vibration from driving and deep bass can break that circuit enough for the lights to dim.
  3. Check the headlight relay. If the relay is beginning to fail, it will switch on and off, causing a headlight flicker.
  4. Go over the electrical charging system.
    • Examine the alternator belt. If it is fraying or loose, tighten or replace it.
    • Test the alternators voltage and amperage with a Clamp Meter.

If you found the issue in your electrical system and corrected it, utilize the VM-1 Voltage Meter to monitor your voltage in the future.

If all of the above checks out and the problem persists, verify that your battery and alternator can supply the necessary power for your amplifier.

As a general rule, for every thousand watts of amplification power, you will need about 100 charging amp hours from your battery and alternator.

Stock bateries and alternators may not be powerful enough to provide reliable and clean power for large, 1,000+ car stereo systems.

Next time, we’ll cover ways to provide your system with better voltage!