Why Do Headlights Flicker When Listening to Music?

The simple answer is power: your amp is drawing a lot of it, and the charging system in your car just can’t keep up. If it happens only when your car is idling, and only with really loud bass notes, then your situation is probably an edge case. You may be able to get away with installing a car audio capacitor, or stiffening cap.

If it happens more often than that, or if you notice your headlights dimming even when the engine is “revved up” when you’re driving down the road, then something’s going to have to give.

Flickering lights in a car.
Mark Horn / Photographer's Choice / Getty

Feeding Your Hungry Amplifier

Your powerful new amp is hungry, and what it craves is electrical current. The good news is that most cars produce more of that than they need, which is how your car can keep its battery charged even if you have accessories like headlights, windshield wipers, or your car stereo running.

The bad news is that your alternator isn’t an infinite smorgasbord of juice. There comes a point where the rubber meets the road, and that point is often the installation of an amplifier — especially a powerful, dedicated subwoofer amp.

When you have a big subwoofer, and a powerful amplifier, the amount of current it draws is variable. If you listen to music that doesn’t have a lot of bass, then the amp isn’t going to have much of an appetite.

That means you can crank up your AM talk radio stations or classical music all you want and probably never have a problem. If, on the other hand, you cue up your favorite Pandora radio dubstep station, that amp is going to get very hungry very fast.

How to Stop Flickering Headlights

A lot of things can cause flickering headlights, but when the problem happens right in time with your music, the root cause of the problem is that your amp is writing checks that your charging system can’t cash, and everything else is suffering. In effect, your headlights dim and flicker because your amp is starving them. That leaves you with two basic solutions: fix your sound system, or alter your charging system.

If you like listening to loud music, and you probably do if you’re having this problem in the first place, the first solution is also the easiest. It’s also probably the least ideal from the point of view of someone who likes their music loud, since it only has one step, and that step is “don’t turn the volume up.”

If you leave the volume down low, your amp won’t ever try to draw more power than the charging system is capable of putting out.

The other sound-system-related solution is to downgrade your amp. In the same vein as leaving the volume down low, installing a lower-powered amp will avoid the sticky problem of a charging system that isn’t quite ready for prime time. This is why it’s a good idea to check into the capacity of your charging system before you start upgrading your car audio, but you’re far past that point if you’re asking this question.

If you want to crank your music up with reckless abandon — without your headlights flickering — then you either need to upgrade your alternator or install a stiffening cap.

The best solution is a bigger alternator, but you’ll have to talk to a qualified technician to verify that installing a high-performance alternator in your car is a viable option. Since other problems — like a failing alternator or bad wiring — can also cause dimming or flickering headlights, it’s probably a good idea to check with your mechanic anyway.

What if You Aren't Sure It's Music Related?

Since there are a lot of things that can cause flickering headlights, you should make sure that a power-hungry amp is really to blame before you try to take any corrective action.

Here are some basic troubleshooting steps that can help you track down the source of flickering headlights:

  1. Open your hood and check your headlight wires.

    If the connections are loose, or you notice any frayed wires, repair those issues before you tackle the potential issue of an overpowered amp.

  2. Check your fuse panel.

    If you see any frayed or burnt wires, or the headlight fuse is loose or partially blown, that could be your culprit. Fuses sometimes blow in such a way that they are still able to make a complete circuit, and the jostling from driving can break that circuit to create a flickering effect.

    If someone replaced your headlight fuse with a circuit breaker, that can also cause flickering if your headlights are constantly drawing just enough amperage to trip the breaker.

  3. Check your headlight relay.

    The headlight relay may be located on your fuse panel or elsewhere. If it's starting to fail, it may switch rapidly on and off, causing your headlights to flicker. Check to see if you can find an identical relay being used elsewhere in your car's electrical system, and swap the relays. If the flickering goes away, and you develop a different problem elsewhere, replace the relay.

  4. Check your charging system.

    A weak or frayed belt is a sign that your alternator may not be charging up to its full capacity. Tighten or replace the belt, and it may work better.

    You can easily check the voltage output of your alternator with a basic multimeter, but you may want to take a trip to a mechanic or a parts store and have them check the amperage output to make sure the alternator is really working properly.

  5. Suspect your amplifier.

If everything else checks out, and your headlights still seem to flicker in time to your music, then your amp is probably drawing too much power. Consider downgrading your amp, upgrading your alternator, or installing a stiffening cap.