How to Tell if the Alternator Is Bad

What does the alternator do?

The alternator keeps your battery charged while the car is on. Every modern vehicle has two ways of generating the electricity your car needs to run: the battery and the alternator. The battery stores charge so that your vehicle turns on when you crank the ignition, but after the vehicle starts, the alternator takes over. The alternator converts the mechanical energy from your engine into electricity so your car can stay on. Without the alternator, your battery would have no way of recharging itself while the vehicle is running.

Signs of a Bad Alternator

Car starts but dies shortly later The alternator’s primary function is to keep the vehicle on after the battery has done its job starting the car. So, if the alternator is bad, your vehicle will likely die out after it has started. Your car is unlikely to shut down 100% right on the spot with a bad alternator. You’re more likely to feel the car slowly dying over a few seconds when the alternator cuts out. How long will the car stay on? It depends on how bad the alternator is. Your car could die after 5 minutes or 5 seconds of running.

Vehicle stalling A stall occurs whenever the vehicle seizes up and loses power unexpectedly. The vehicle will lose power and you’ll have trouble turning the wheel since the power steering will go out. This will typically occur after your car begins to die. In some cases, if your alternator is only partially damaged or dying, your car may kick back on after stalling out temporarily. What should I do if my vehicle stalls while I’m driving? Put your hazards on if you still have power and do your best to coast to the shoulder of the road, a parking space, or a parking lot. If you lose power entirely, throw the car in neutral. You may need to push the car while someone else steers to move it safely.

Whining or screeching noises The alternator is on the side of your engine where the drive belt (aka the serpentine belt) is located. If the alternator isn’t cranking correctly or there’s something wrong with the drive belt, the friction will cause a screeching noise. If the noise goes away intermittently, it’s a sign you probably need to change your drive belt.

Dimming or fluctuating cabin lights The alternator keeps your vehicle’s electrical systems charged, so if it’s malfunctioning your vehicle won’t have enough power to keep everything running properly. As a result, you’ll likely notice that anything that lights up in your car (dash lights, cabin lights, indicators, etc.) dims and brightens randomly. Slowly dimming lights are a big sign that your vehicle is on the verge of stalling, so pull over if you notice the lights fluctuating lowly and not bouncing back.

Accessory malfunctions Your traction control may turn off randomly, your ABS light may pop on unexpectedly, or your radio may act up. With a damaged alternator, your vehicle may automatically cull the charge to any unnecessary or supplemental features to preserve energy. Alternatively, with an inconsistent charge, your vehicle may struggle to power all of its features. Many of these indicators are probably false; it’s likely the power to the sensors that monitor your accessories are the issue, not the accessories themselves.

Burning rubber odors The serpentine belt is made of rubber and it wraps around your alternator’s pulley, so if the alternator isn’t turning the belt may start to burn from the friction. Can I drive if I smell burning rubber? Nope. If you keep driving the belt could snap and you’ll lose all power steering and acceleration. That can be dangerous if you’re driving at a high speed or you’re in heavy traffic.

Warning or check engine lights Unless you’re driving a very old car, there are sensors that monitor your alternator. These sensors will trigger a check engine (and/or a “check alternator”) light when they detect issues with your alternator. How do you diagnose a check engine light? Plug an OBD scanner into the OBD port underneath your steering wheel and turn the vehicle on. Read the code to determine why the light popped on.

Signs of a Bad Battery

Car won’t start A bad alternator will make it hard for your vehicle to continue running, but a bad battery won’t allow your car to start at all. Your car may crank repeatedly, or you may not even get a crank depending on just how dead the battery is. How old is your car battery? You typically need to change your car battery out every 5-6 years or every 30,000–50,000 miles (48,000–80,000 km). If you’re passing half a decade or you’ve put a ton of miles on the car, your battery may simply need to be swapped out.

Clicking noises or slow cranking when turning the key If you don’t hear anything at all when you turn the key, there’s a chance your starter is the issue. But if you hear a clicking or cranking and the car sounds like it’s trying to start but can’t, it’s a surefire sign your car battery is cooked. Does the car eventually start? If the engine cranks for a few minutes before finally turning on, your battery is more likely to be the issue than the alternator. But if the car starts then dies immediately, it’s probably the alternator.

Rotten egg odors Car batteries contain sulfuric acid, so if there’s a leak or your battery is so damaged that its interior cells are exposed, you may smell rotten eggs. Is your vehicle struggling to shift, as well? Rotten eggs and shifting problems actually points towards transmission issues.

Battery corrosion or swelling Physically inspect your battery. If there’s corrosion on the battery terminals, it’s a sign your battery is nearing the end of its life. Swelling is a sign it’s well past that point—do not drive your car if the battery is swollen. Why are swollen batteries so dangerous? Swelling indicates overcharging, which occurs when heat and hydrogen build up too fast for the battery. An overcharged battery will explode, so get your vehicle towed to a mechanic ASAP. Clean corrosion off of your battery. Disconnect the battery and use baking soda and water to apply a soft paste to the terminals. Then, use a stiff wire brush to scrub off all of the corrosion. This may not solve the underlying issue, but it should help in the short term.

Troubleshooting Tips

Swap your battery out if it’s close to replacement time anyway. Changing a car battery is a great option if you’re on the verge of needing a new battery to begin with, since this will 100% confirm whether the issue is the battery or alternator. If you still encounter electrical issues after changing your car battery out, the alternator needs to be repaired or replaced. As a reminder, you need a new car battery every 5-6 years or every 30,000–50,000 miles (48,000–80,000 km)—whichever comes first.

When your car runs into trouble will tell you everything. If your issues develop as you’re starting the car (or trying to start it), it points to the battery as the culprit. If the problems you’re encountering tend to pop up after the vehicle has been running for a while, it strongly indicates the alternator is to blame.

Can you drive with a bad alternator?

No, although you may be able to make short trips. A bad alternator makes it difficult for your vehicle to keep running, so you’re unlikely to get far. However, if you’re only encountering intermittent issues or your alternator seems like it’s barely hanging on, you should be able to drive to the mechanic if they’re nearby. Driving with a bad alternator? Go slow, take a route where there won’t be a lot of traffic, and have a backup plan if your car dies while you’re on the road.