5 Warning Signs of a Failing Alternator (And What to Do About It)

From dimming lights to dead batteries, a failing alternator can spell disaster on the open road if you don’t catch it in time. Whether you're hauling up I-77 or idling outside a job site, knowing the warning signs could save you from costly downtime and roadside headaches.

We asked our team at NextGen Diesel to identify five red flags that scream “alternator trouble” and, more importantly, what you should do about them.

1. Dimming or Flickering Lights

What’s happening?

Your alternator powers your truck’s electrical system while recharging the battery. If it’s struggling, the voltage output drops, leading to flickering interior or exterior lights, especially under load (such as when idling at a red light or revving up the AC).

What to do: Test the voltage at the battery terminals with a multimeter. A healthy alternator should read around 13.5 to 14.5 volts with the engine running. Anything below that? It’s time for a replacement.

2. Weak or Dead Battery

What’s happening?

When the alternator fails, the battery picks up the slack to keep your truck running. But that’s not its job, and it can’t keep up. Eventually, you’ll be left with a battery that’s drained faster than a cold drink in July.

What to do: Check your battery’s health, but also inspect the alternator output. If the alternator isn’t feeding enough power back into the battery, you’ll kill even a brand-new one in no time. A professional truck diagnostic service can pinpoint which one’s the real culprit.

3. Strange Noises Coming From the Engine Bay

What’s happening?

Your alternator pulley spins via a serpentine belt, which rotates the internal turbine and creates electricity. If the bearings inside the alternator are shot, you’ll hear telltale noises that rise and fall with RPMs.

What to do: Pop the hood and listen closely. If the noise is localized around the alternator, it’s a good bet the bearings are worn. Swap it out before it locks up and takes the belt—and possibly other belt-driven components—with it.

4. Electrical Gremlins

What’s happening?

A dying alternator can't support all of your truck’s electrical accessories. That means everything from your HVAC system to your ECU and fuel injectors can start behaving unpredictably.

What to do: Don't start yanking fuses or blaming the wiring harness. First, check the alternator’s voltage output under load. If it dips below 13 volts when accessories are running, that’s a big ol’ red flag.

5. Dashboard Warning Light

What’s happening?

Modern trucks use sensors to detect charging system irregularities, and they’ll illuminate a warning light when things go sideways. This is often the first and clearest sign that your alternator isn’t doing its job.

What to do: When that light comes on, don’t just clear the code and hope for the best. Get a full charging system test done. If it confirms low alternator output or intermittent charging, replace the alternator before you get stranded.

What Causes Alternators to Fail?

There’s no single smoking gun, but common culprits include:

• Worn-out bearings due to age
• Heat damage from poor ventilation
• Overloaded electrical systems
• Faulty voltage regulators
• Belt slippage from a worn serpentine belt

Heavy-duty alternators deal with massive electrical loads, from powering ECMs and DEF heaters to regenerating DPF systems. In high-demand setups, failure is often just a matter of time, especially if maintenance is skipped.

What to Do if Your Alternator Fails

Caught the signs early? Here’s your roadmap:

1. Run a charging system diagnostic. Use a voltmeter or have a tech do a full system scan.
2. Check the serpentine belt. If it’s cracked or loose, replace it.
3. Inspect wiring and terminals. Corrosion or loose connections can mimic alternator failure.
4. Replace the alternator. Don’t cheap out, go for an OEM or high-output model if you’ve got a lot of accessories.

And don’t forget, your alternator and battery work in parallel, not in sequence. So replacing just one might not solve the issue if the other’s gone bad too.

Wrapping It All Up

If you spot flickering lights, hear odd noises, or your battery keeps biting the dust, don’t wait. A failing alternator might not stop your truck immediately, but it will eventually and suddenly. And you don’t want that happening on I-485 during rush hour.