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6 Symptoms of a Bad Alternator

An alternator, sometimes known as a synchronous generator, is a vital component of the charging system of a vehicle. Along with the battery, it ensures that your vehicle starts and continues working as it should.

Symptoms of a bad alternator make themselves apparent quickly. Keep reading to know what signs to look for, how long alternators last, and what the average replacement cost of an alternator is.

Bad Alternator

What Does an Alternator Do?

The alternator converts mechanical energy from the vehicle’s running engine into electrical energy that is used to power the vehicle’s electrical systems and charges the battery. It is so named because it creates AC (alternating current) power, which reverses direction periodically. 

Alternating current has been used in vehicles since the 1960s since it is more efficient and produces more voltage than DC (direct current) power, which is unidirectional. Vehicle batteries and accessories use DC power so the alternator’s electricity is transformed into DC power as it leaves the alternator. 


How Does an Alternator Work?


In order to understand how the alternator does its job, it’s helpful to first know how it’s put together.

This coconut-sized part usually sits near the top of the engine and consists of an outer housing covering a stator and rotor. The housing is made of aluminum because it cannot be magnetized and dissipates heat well.

The rotor has roughly 10 to 20 magnetic poles sticking off of a center prime mover like petals of a flower growing off of a stem. These magnetic poles alternate North and South.


The rotor is powered by the turning crankshaft of a running engine via a belt (usually the serpentine belt in modern vehicles), which connects to the prime mover and turns the entire rotor (following the previous example, the flower spins). As the poles move in this way, magnetic flux is generated.

Brushes, which produce the magnetic field in each pole, are located near the poles and send them a direct current via slip rings around the prime mover. This power comes from an external power source or from small generators attached to self-excited alternators.

The stator assembly is made up of a system of copper coils. Most often vehicles use a three-phase assembly, where there are three sets of copper coils spaced 120 degrees out of phase of each other. This makes three times the electricity as one set would. As the rotor spins, the magnetic flux generated creates an alternating current in the copper wire.

At the other end of the alternator, a diode (also known as a rectifier) converts the AC power into DC power which is the format that can actually be used by the vehicle and stored in the battery. 

Older vehicles have a voltage regulator between the alternator and the battery to make sure the battery doesn’t get too little or too much electricity. This can harm the battery or other electrical components. In newer vehicles, the ECU does this job.

Since cooling is critical to the proper function of an alternator, there are vents on the front and back to dissipate heat and cooling fans that are also powered by the serpentine belt. 

Several terminals on the back of the unit connect the alternator to the electrical system in order to turn it off and on and send data from sensors to the ECU.

6 Symptoms of a Bad Alternator

Some points of failure affecting the alternator are bad bearings (which allow the rotor to spin freely) causing the rotor to seize, problems with the coils, and a bad belt (the rotor doesn’t spin right, so the alternator isn’t powered).

Of course, the way the vehicle indicates a problem is different depending on the root cause. If you have any of the following signs of a bad alternator, make sure to get it checked out quickly. 

Vehicle Won’t Start/Dead Battery


The alternator doesn’t provide the power to start the vehicle – that’s actually the job of the battery. It does, however, charge the battery as the vehicle is running. If the battery can’t be charged, the car won’t start.

This is a tricky situation as the problem could be with the alternator or the battery itself, or even a plethora of other issues (bad starter, spark plugs, etc).

Squealing, Growling, or Other Noises From the Engine Bay


The alternator has spinning parts, and if the movement of these is restricted (such as with bad bearings) there will be some odd noises.

Seizing of the rotor will cause a loud grinding noise, so make note of the sounds you hear and what’s happening with the car when you hear them in order to tell your mechanic. This information can give good clues. 

Failing Electrical Systems


The electricity created by the alternator is also what powers the vehicle’s accessories such as air conditioner, power windows, power mirrors, stereo, heated seats, heated mirrors, headlights, entertainment, etc.

If you notice that any or all of these are not working well or at all, especially when the vehicle is at idle (since the RPM isn’t high enough to produce enough power if there are other issues), the alternator may be the cause.

You may also notice failing electrical gauges on your dashboard. Since those give you important information about safe driving conditions (i.e. speed) and the state of your vehicle (i.e. oil temperature) it’s imperative to stop driving as soon as possible and get the issue fixed. 

Sudden Dashboard Warning Lights


Also due to the lack of sufficient power, you may notice warning lights such as the battery light, “check engine”, “ALT”, and/or other phantom lights show up on the dashboard.

When the ECU is starved of electricity, the brain can’t think well (much like you may feel when you desperately need a snack) so signals may be sent that are incorrect. 

Stalling Engine


In newer vehicles, the engine may even cut entirely when it’s running because electronic fuel injectors need the power to behave properly.

It’s important for the right amount of fuel to be shot into the combustion chamber at the right time for the engine to run smoothly, so issues with this may stop the combustion cycle from continuing successfully. 

Burning Smell


In severe cases, rubbing of parts or electrical issues within a bad alternator can cause a burning smell. Since you probably don’t want a fire under the hood, stop driving, and get things checked out. 

It’s usually a better idea to replace the alternator entirely if it is malfunctioning. While new ones can be spendy, they are usually a better choice than remanufactured or rebuilt options. Those probably won’t last as long as a new one since the mechanical parts have miles on them already. 

If you can’t afford a new one, check for a remanufactured or rebuilt alternator at your local auto parts store. If a mechanic is doing the job, make certain that they are giving you a new alternator and not trying to cut costs by using a remanufactured or rebuilt one.

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