Bad Alternator Symptoms, Causes, and Replacement Cost

The alternator is one of the most important components of the vehicles. Its primary role is to convert the mechanical energy generated by the rotating crankshaft of the engine into electrical power using the principle of induction. Additionally, the alternator ensures the proper charging of the vehicle’s battery, supplying the necessary power to start the car. When the alternator goes bad, it produces different signs. This article explains the bad alternator symptoms, causes, and how to test it. 

Symptoms of A Bad Alternator

A bad alternator produces different symptoms such as a dead battery, dim lights, slow accessories, hard starting, unusual noise, burning rubber smell, battery warning light, or engine misfiring.

The following are the major causes of a bad alternator:

1) Dim or Overly Bright Lights

Dim lights, symptoms of bad alternator

The dim headlights are one of the clear symptoms of a bad alternator. As the battery’s charge diminishes, the headlights progressively lose their brightness.

On the other hand, if the headlights brighten while you increase the RPMs of the engine, it suggests that some current is being generated by the alternator. However, this power alone is insufficient to maintain a charged battery and keep the lights functioning as usual.

2) Malfunctioning or Slow Accessories

Car Electrical Accessories

Insufficient power supply from the alternator to the electronic components of your vehicle often leads to sluggish or non-functional accessories.

When you observe that the car’s windows are moving slower than normal or experiencing delays when rolling upward or downward, notice abnormal behavior from the seat warmers, or witness erratic behavior from the speedometer and other devices, it could indicate a problem with the alternator.

In the latest vehicle models, there is a prioritized list of equipment installed into the onboard computer of your vehicle, determining which accessories to cut power to first in case of inadequate electricity supply from the alternator. This prioritization ensures that non-essential accessories like the radio are powered off before essential components like the headlights.

3) Dead Battery

Dead Battery due to a bad alternator

In some cases, a dead battery is simply the result of its natural lifespan after several years of use, or it could be due to accidentally leaving the car lights on overnight. But in other instances, a dead battery may indicate a bad alternator.

A faulty alternator fails to adequately charge the battery while the engine is running, leading to a quick depletion of the battery’s charge. An easy method to determine whether the issue lies with the alternator or the battery is by jumpstarting the vehicle.

If the vehicle remains running after a jumpstart, it suggests that the battery may require replacement soon. Conversely, if the vehicle dies shortly after jumpstarting, it indicates that the alternator isn’t providing sufficient electricity to the battery.

4) Hard Starting

Hard Starting

As mentioned earlier, hard starting the engine is one of the clear signs of a bad alternator. Instead of hearing the expected engine purr while turning the key in the ignition, you may only hear a clicking noise.

However, if your vehicle regularly stalls when accelerating, it could indicate insufficient power from the alternator reaching the spark plugs, resulting in the engine struggling to operate.

5) Battery Warning Light Illumination

Battery Warning Light

An illuminated battery warning light is often misconstrued as an issue related to the battery. But the presence of the battery warning light indicates a potential issue within the broader electrical system of the vehicle, such as the alternator.

The alternator of your vehicle is engineered to operate within a specific voltage range, typically between 13 to 14.5 volts. If your vehicle’s alternator is bad, its voltage output may fall less than the designated capacity, triggering the battery warning light to activate on the dashboard. Similarly, if the alternator surpasses its voltage limit due to excessive strain, the battery light may also come on.

If you notice the battery warning light flickering on and off while using different car accessories like the radio, wipers, or headlights, it could be a sign of alternator issues. It’s important to have your vehicle checked for potential issues with the alternator instead of risking a breakdown on the highway.

6) Burning Rubber Smell

Burning Rubber Smell, signs of bad alternator

The presence of a bad smell, like burning rubber or wires, may signify the wear and tear of certain components within the alternator. Due to the constant tension and friction endured by the drive belt of the alternator, it may gradually deteriorate with time, emitting an unpleasant burning rubber smell, particularly as it is situated close to the hot engine.

Likewise, if the alternator is under excessive strain or if its wires are damaged, you can perceive a burning odor akin to an electrical fire. An overburdened alternator attempts to move an excessive amount of electrical power through its wires, leading to unsafe heat buildup. Frayed wires also introduce resistance to the flow of electrical power, causing them to heat up and emit an unpleasant odor.

7) Unusual Noise

Unusual Noise

Your vehicle may produce various unusual noises, some harmless while others indicate significant mechanical issues. If you notice whining or growling sounds originating from under the hood of your car, it may indicate an issue with the alternator. It’s important to have it checked by a professional right away.

The whining or growling noise typically occurs when the belt responsible for turning the pulley of the alternator is misaligned or rubs against the pulley’s sides. This sound can also indicate deteriorating bearings that rotate the rotor shaft.

8) Flickering Lights

Flickering Lights

If you observe a pattern where the vehicle lights alternate between being bright, then dull, and then bright again, whether in a regular rhythm or seemingly randomly, it is a clear indication of an issue with the electrical system of your vehicle, with the alternator being the most probable reason.

The underlying reason is similar to the scenario where the lights are consistently dim—the alternator is unable to generate sufficient power to maintain the lights at their full brightness. Instead of uniformly lowering the brightness of all the lights in response, your vehicle is attempting, but failing, to sustain the desired lighting output.

To confirm that the issue indeed lies with the alternator and not another component of the electrical system, pay attention to the pattern of the fluctuating lights. Identifying a discernible pattern can provide further evidence pointing toward an alternator malfunction.

9) Engine Misfiring

Engine Misfiring, signs of a faulty alternator

The primary role of the alternator is to charge the battery. However, if the alternator is not functioning properly, it will fail to charge the battery effectively.

Consequently, if the battery is not adequately charged, it will be unable to provide adequate power to the spark plugs, resulting in a weak spark. This weak spark leads to improper combustion of the air-fuel mixture and can cause misfiring issues.

Read More: Causes of Engine Misfires 

Causes of Bad Alternator

Your vehicle alternator may go bad due to one or more of the below-given reasons:

1) Bearings Getting Jammed

 The jamming of bearings in the alternator can occur due to normal wear and tear over time or as a result of poor repair work or incorrect assembly/disassembly of the casing.

2) Stator Winding Burnout

Burnout of the alternator can happen due to various causes. One primary factor that can lead to an unforeseen decrease in temperature is the interaction of water with the alternator, which can occur when driving through puddles or washing the engine. This situation can result in the degradation of the insulation varnish on the windings, potentially causing damage to the alternator.

When water or other liquids are present on the alternator, they can act as conductors for electric current, resulting in a short circuit.

3) Brush Wear

The alternator brushes are in continuous contact with the slip rings and undergo friction because of the rotation of the alternator shaft. As a result, the brushes eventually wear out over time.

The replacement process for worn-out brushes can vary according to the alternator’s design, particularly the type of voltage regulator employed.

In certain cases, the brushes can be replaced individually, separate from the more expensive relay. However, in other instances, it may be necessary to replace the entire voltage regulator.

4) Overheating

The heat generated by the rotating components of the alternator can be substantial. If these parts become excessively hot, it can result in the malfunctioning of the alternator.

Read More: Causes of Engine Overheating

5) Faulty Diodes

The primary role of diodes is to convert the AC voltage produced by the alternator into DC voltage. This direct current is then used to operate the electrical components of the car.

When the diode goes bad, it badly affects the overall performance of the alternator, and it doesn’t produce an adequate amount of power.

What is an Alternator?

While the engine is operating, the alternator plays a vital role in charging the car battery. As you turn on the vehicle, the battery requires a significant amount of power to initiate the engine. This power is replenished by the alternator, which functions as a generator.


In addition to recharging the battery, the alternator supplies electricity to power various electrical systems and accessories in the vehicle while the engine is running. This includes climate control, taillights, interior lights, audio systems, power windows, headlights, and more. The alternator ensures that these systems receive a consistent and sufficient power supply.  

The operation of the alternator involves the conversion of mechanical energy into electrical energy.

When the engine operates, a drive belt powers the pulley coupled to the alternator. The rotation of this pulley causes the rotor shaft within the alternator to spin, in turn causing the magnet surrounding a coil to rotate. This spinning motion of the magnets generates alternating current (AC) around the coil. The AC power is then directed to the rectifier, where it is converted into direct current (DC) and transferred to different electrical accessories.

Alternator Location

The alternator is typically positioned in the engine compartment of the car, usually fastened to one side of the engine. It exhibits a cylindrical design and is noticeably large in size.

Alternator Location

In some cases, there is a fan attached to one end of the alternator, serving the purpose of providing cooling.

The side of the alternator with the fan is attached to the drive belt, which is powered by the engine. As the drive belt rotates, the alternator crank also rotates to generate electrical power.

To find the alternator fast, you can trace the drive belt. Look for a diagram under the hood that shows the path of the belt and the location of the alternator.

If you’re having trouble finding the alternator or the belt’s path, check the service manual for your vehicle model. It will have easy-to-understand instructions and diagrams to assist you.

How to Test an Alternator?

  1. Switch off the engine and remove the negative terminal of the battery.
  2. Set your multimeter to voltmeter mode, specifically to an operating voltage of 20 V.
  3. Attach the multimeter probes to the relevant battery terminals. The voltage reading should fall between 12.4-12.8 V, indicating the battery’s charge level.
  4. Reconnect the negative terminal of the battery and switch on the engine. Check the voltage again by using a multimeter. A normal reading should fall between 13.8 to 14.6 V, although in some costly car models, it may reach up to 14.8 V.
  5. To further test the alternator, increase the electrical load by turning on the headlights, radio, interior lights, and other accessories. Measure the voltage once more. It shouldn’t be less than 13.6 V. Any significant divergence from this range may represent an issue with the alternator.

Alternator Replacement Cost

The average replacement cost of the alternator is from $570 to $840. In this cost, the labor costs are from $100 to $140, while the estimated costs of the parts are from $470 to $600.

FAQ Section

How do you know if it’s a bad Alternator or Battery?

If the battery of your vehicle is dead and you successfully jump-start it, the car should continue to operate as long as the battery can hold a charge and the alternator is functioning correctly.

Can a car run with a bad alternator?

In the event that the battery is properly charged, the vehicle will run until the battery’s charge is depleted. Once this happens, restarting the vehicle without the aid of a jumper box or jumper cables becomes impossible. Therefore, it is advisable to change a faulty alternator as soon as any signs of trouble arise.

How does a car act when the alternator is going out?

However, certain indicators may suggest potential issues with the alternator. These include the battery warning light illuminating, unusual behavior from electrical accessories, flickering lights, stalling, strange noises, and difficulties starting the vehicle. These warning signs often arise as the alternator begins to fail.

Do alternators go out suddenly?

While the warning signs mentioned above can provide an indication of an impending alternator failure, it’s important to note that an alternator can also fail suddenly without any prior warning. It’s crucial to be prepared for such an occurrence and be ready to change the alternator promptly.

How long does an alternator last?

Typically, alternators have a lifespan of around 80,000 to 150,000 miles or approximately seven years. But various factors, such as driving habits and environmental conditions can lead to premature wear and tear.

Can you jumpstart a car with a bad alternator?

In some conditions, it is possible to jumpstart a car with a defective alternator, but the battery must have sufficient charge to continue the car running. Nonetheless, it is crucial to change the alternator quickly to ensure proper charging of the battery.

What does a bad alternator sound like?

When an alternator is faulty, it can produce various sounds depending on the issue at hand. A slipping belt may result in a whining sound, while internal failure can cause groaning or growling noises when the rotor shaft or bearings deteriorate.

Can a weak alternator cause poor engine performance?

Indeed, a faulty alternator can impact acceleration. The engine’s ability to run highly relies on the proper functioning of computer components, ignition systems, and sensors, all of which can be affected by a faulty charging system caused by a faulty alternator. 

What are the signs of a bad alternator?

A bad alternator produces different signs such as a dead battery, dim lights, slow accessories, hard starting, unusual noise, burning rubber smell, flickering lights, battery warning light, or engine misfiring.

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