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A vehicle’s alternator is a device that generates an electrical current to provide its power requirements. When the engine is running, it converts mechanical energy into electricity that’s distributed to safety systems like airbag modules and anti-lock brake system parts as well as components your car needs in order to run like fuel injectors and spark plugs. Some of the power is stored in the battery so there’s energy available to start the car when the engine is off, and reserve power to use electrical devices when the motor isn’t running. As well, the convenience systems you use every day like power windows, the radio, and heated or ventilated seats all require significant amounts of power to operate, and it all starts with the alternator.  

An alternator lasts for years, quietly spinning under the hood, driven by the alternator belt or serpentine belt. After thousands of hours of runtime, the alternator may not work as well as it once did, or worse yet, will fail. If that’s the case, your car’s battery won’t charge properly and there isn’t enough power production to work all your car’s systems reliably. When that happens, the alternator needs to be replaced. It could be around 60,000 miles or less, or it could be more than 200,000 miles. Here’s what you need to know about an alternator’s lifespan. 

How is an Alternator Made? 

Regardless of which car make and model you drive, an alternator’s general design has remained the same for decades.  

An external case is made of aluminum to contain all the critical workings, and the choice of material is intentional. Aluminum does not magnetize so it won’t interfere with the alternator’s function.  

The alternator pulley is mounted on the front of the assembly. Cars with a V-belt for the alternator will have a single deep groove in the pulley while cars that use a serpentine belt have a flatter surface with multiple shallower grooves. 

The stator is a bundle of copper wires fashioned into three windings and fixed to the inner surface of the case or shell. It delivers the generated power to the battery.  

The rotor is the magnetized rotating mass attached to the pulley by a shaft. It’s connected to power, creating an electromagnet that generates the alternating current electrical field in conjunction with the stator. 

The rectifier converts power from an alternating current (AC) to direct current (DC) that your car’s devices need. 

A voltage regulator controls how much power can pass from the windings to the conductor to power your car. On older vehicles before 1980, many voltage regulators were external, mounted somewhere in the engine compartment. Since then, they have been internally located within the alternator.  

Other parts such as bearings, wiring, brackets, and brushes ensure the alternator continues to function reliably for years. 

Why an Alternator Can Fail 

Designed for longevity and dependability, an alternator can seem indestructible. For some cars, the alternator can and does last its complete lifetime but that isn’t always the case. Inside, there are parts that can wear down or there can be sudden failures. It might generate less electric energy than it’s supposed to, it can be sporadic, or it can be completely out of commission with zero output. 

  • Worn brushes reduce the alternator’s conductivity and can be the source of many alternator problems.  
  • Excessive energy demands from the battery and electrical system can make the alternator charge at maximum capacity, shortening its lifespan.  
  • Decreased output from failed or corroded sections of the rotor can mean the alternator doesn’t function well enough, requiring a repair.  
  • A regulator issue can mean an overproduction or underproduction of electrical energy.  
  • Corroded or broken electrical connections inside the alternator can interrupt current flow.  

Sometimes, a failure doesn’t mean that the alternator isn’t charging. It’s possible that bearings can fail from an overtightened belt or due to wear, which starts as an annoying noise, but can lead to mis-alignment of moving parts, and catastrophic failure.  

Symptoms of a Bad Alternator 

When an alternator goes bad, symptoms are sure to appear. They can vary depending on the cause of failure.  

bad alternator

A battery light on isn’t actually related to a battery problem. Rather, it’s indicating a charging system fault when the voltage is above or below the normal operating range. It comes on when the alternator output is above 15 volts or below 12 volts. 

Dim headlights and interior lights are a common symptom since they’re receiving less than the normal 12 volts of power. 

Whining under the hood can occur when the alternator is struggling to keep up with the car’s demand for power. 

A burning smell is another common occurrence when the belt is slipping on the alternator pulley and when there are overheating electrical connections in the alternator. 

The engine can stall while you drive when your car’s critical systems don’t have the required voltage to maintain their operations.  

A dead battery may be directly related to an alternator that’s unable to produce power, thus it doesn’t charge the battery.  

Many of the symptoms of a bad alternator can be caused by other issues too, so an accurate diagnosis is important to prevent spending money on repairs unnecessarily. 

How Do You Change an Alternator? 

The alternator is driven by a belt from the front of the engine, so it’s almost always located in an accessible location. Some cars have other components in the way that must be removed or relocated to access the alternator but, in many cases, the job can be done in around an hour or so.  

While the exact process to replace an alternator varies by model, begin by disconnecting the negative cable from the battery for safety. Then, remove the serpentine or alternator belt. Disconnect the wiring from the alternator itself being careful not to damage any plastic connectors, then remove the fasteners that hold the alternator to the engine. Install the new alternator, making sure you’ve chosen an exact replacement based on the car’s year, make, model, and engine size. After connecting the battery cable, start the car and test that your alternator is working properly.  

Wondering where you can find the right parts for the job? Whether you need a new alternator or Trustworthy Advice for a successful DIY job, an AutoZone associate is always ready to help.  

Also, if changing an alternator isn’t a job you want to tackle, check out our list of Preferred Shops in your area to help you complete the job.  

Advice, how-to guides, and car care information featured on AutoZone.com and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

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