Menu Close

14 Reasons why your Alternator is Going Bad


What causes an alternator to fail? If the engine’s undercover shield is broken (or absent) alternator could have gotten soaked. Driving a vehicle through water deep enough to spray or flood the alternator shaft bearings, as well as the brushes and electronics within the device, will harm them.

A lot of things could go wrong with your alternator, anything from a bad battery to mechanical issues. Not to mention, it could also be nearing the end of its lifecycle.

If the alternator fails unexpectedly, you are left with no power source for your vehicle’s electrical systems. You will be able to get by on the battery for a bit, but once it runs out of juice, the engine will stall and refuse to start.

To charge your battery and supply electricity to your electrical devices, your alternator should be able to produce at least:

a voltage of 13.5 volts or higher

Test the voltage on the battery terminals with the motor going for the alternator. If the voltage drops below 12.9-13 volts at idle, it may be a sign of a problem with the alternator.

Before you have your mechanic replace the alternator, have him try it out with a digital voltmeter and a fully charged battery. At idle, the alternator should produce about 13.8-14.4 volts with lights and accessories turned OFF. A load test will also aid in the detection of alternate faults.

Now, let’s dive into details on what can cause an alternator to go bad:

#1. Bad battery

Is your charging system being overworked due to a low or bad battery? Not all alternators are intended to charge a battery that has been fully discharged. And yes, charging a battery for an hour could kill the alternator!

The majority of alternators are only built to achieve the highest-rated power for a brief amount of time. It is designed to charge the battery for less than five minutes before dropping to a charge rate of less than 10% of the battery’s full-rated capacity.

If the alternator is put under more strain by a dying battery, it could damage the stator coil and the regulator (or rectifier). If your battery is dead, don’t try to charge it with an alternator, but simply remove it, check the water levels, and use an external charger to recharge it.

A bad battery is often the cause of alternator failure. The alternator sends 5 amp-hours to the battery at about 3000 RPM, and the rest of its power goes to the rest of the car’s systems.

A car battery’s primary purpose is to start the engine. If charging takes longer than necessary (especially on a hot summer day), the alternator will overheat. This is why you should never drive with a dead battery unless it’s an emergency.

A battery will take as much current as it requires which is proportional to its charge state. A discharged or dying battery will need a lot of charging to keep it going. The voltage regulator compensates by passing more current through the rotor.

At idle, the alternator cannot provide all the necessary current needed for a weak battery. As a result, the voltage regulator pushes the current through the rotor at the maximum level.

The wear occurs when the load is at its highest and the speed is at its lowest. The condition improves as the RPM is raised above 2000 so further cooling is available and the current through the rotor reduces.

The alternator can operate even harder if the battery has an internal electrical short (as I mention below), which is normally because one of the plates has fallen loose and is hitting its adjacent plate. The alternator’s life can be shortened as a result of this.

#2. Battery short

You can also have the battery inspected to ensure that no shorted plates are present. The battery functions like a very large capacitor. It stores DC energy.

If a short circuit is left unattended, the battery could become extremely hot and possibly combust, spraying hot sulfuric acid everywhere. That, however, is extremely rare. If active materials come into contact, the medium normally has a high resistance, and the short drain the cell slowly.

Since charging will also overcome the rate of short discharge, the battery can operate for a limited time, but it will ultimately fail to start the car. If you do manage to start the car with a battery in this condition, the alternator will be forced to run at full capacity for long periods of time and it is not designed to do so.

Constant full production of electricity actually overheats the rectifiers, and will eventually cause them to malfunction. Heat is generated when the alternator starts to operate harder than necessary.

The rotor windings’ insulation can be damaged and the grease in the bearings may become overheated.

#3. Loose terminals of the battery (open circuit)

The problem with alternator failure could be loose or corroded (due to acid buildup) battery terminals. In this case, the alternator will either not start charging at all or will erratically bounce from low to high voltage. This will also cause the regulator to fail.

Double-check that the battery cables are securely wired and free of corrosion.

#4. Battery ground problem

Your battery ground could be loose! Stop changing alternators and look for a ground problem!

Between the engine and the body, there is also a ground strap. It connects the negative car battery terminal to the vehicle’s frame. It’s usually very easy to find as it is connected right to your battery and looks similar to this.

Here is a ground strap (paid link) if anybody needs it, and it is conveniently available from Amazon. If after cranking your car the ground strap produced any heat or sparks at the negative battery terminal or the bolt attached to the body, you have a ground problem.

Be careful, it can get extremely hot! A continuity test between the battery and the chassis is another good way to detect this problem.

#5. Alternator is overworked

When the alternator gets installed in a car, it is usually the smallest alternator that the manufacturer can get away with! However, when the alternator loses some of its operating power over time it will not be able to support the electrical system the way it was intended.

If you measure the voltage at the battery terminals to check the alternator’s performance (at full electrical load when the engine is running), you’ll be able to tell if it can keep up with all the power that you are requesting. If the battery voltage drops below 12 volts, it’s time to get a larger alternator!

Wrong-capacity alternators will wear out rather quickly and you will experience problems.

#6. The alternator shorted (insulation failure)

If insulation fails in any part of the alternator, it could create a short. This can be caused due to an overcapacity fuse or melted wire shorting with the ground.

One way to recognize a short is when your fuse keeps blowing after being replaced. Also, if your ECM just blew out, it is a good sign of the alternator short.

So, stop going through batteries and look for the alternator short! If you are changing one battery after another, this may be your problem!

#7. Bad wiring within the alternator (open circuit)

Inside an alternator, there are two different winding sets. One set is wound around the rotor, which spins. The magnetic field generated by these windings induces electricity that flows to the other set of windings (the stator).

If any of these wires break or get loose, the alternator will not function correctly or completely stop working. Not to mention the risk of having a short.

The external wire connections could also be corroded or simply loose, and that means that the alternator’s output may be disrupted. Voltage drop testing is a sure way to detect extreme resistance within wires.

Since the alternator’s wire windings contain just a thin layer of insulation, they can fail and not produce any magnetic field if contacted by any solvent (like carb cleaner).

#8. Reversing the polarity of the jumper cables

Is it possible to destroy an alternator by jump-starting a car? Only if you do it incorrectly! This normally happens in the jumping vehicle because the alternator is already ON.

Jumping the vehicle and reversing the polarity of the jumper cables will cause almost instant harm, particularly to the computer-operated regulator and diodes. Also, the actual jumping process (connecting and disconnecting) generates current surges, which can destroy the alternator of either car.

#9. Reversing polarity when installing the new battery

If you installed your new battery backward, you may notice a frightening spark, and then your car dies! Fixing something like this is not a DIY project and it is very complicated due to the many problems you just created. Simply take your car to the repair center.

Cranking the engine again (in this condition) may even cause it to turn in the wrong direction! The harm is done as soon as the reverse battery connection is made. The number of fuses is already blown, and any unprotected car’s components have already been destroyed.

When you attached the battery in reverse, immediately rectifying diodes on the alternator are blown, and that may cause the alternator to overheat and fail.

#10. Extreme temperatures

The most significant factor influencing how well the alternator is working and how long it will last is heat! The reason for this is that heat raises electrical resistance, requiring parts to work at maximum level to achieve the same performance.

Excess heat also puts a strain on mechanical and electrical components, as well as lubrication systems. When the alternator is working hard, it has a tendency to produce lots of heat, as well as absorb heat produced by the engine.

Just by warming up, an alternator loses about 10% of its efficiency. A hard-working alternator puts unnecessary strain on its drive belt, as well as the engine. The lifecycle of bearings will be reduced as well.

Belts and bearings can wear out over time as a result of this strain (especially in hot weather). It’s also a good idea to perform a voltage test on your battery on a regular basis to make sure it’s capable of receiving a complete charge.

If it’s not, the regulator will never shut off, and the alternator will continue to charge the whole time the engine is running.

#11. Dust and dirt contamination

Keeping the alternator clean and dry is the single most important thing you can do to keep it in good working order. Dust and debris will get into the alternator, causing shorts or brushes to wear out over time.

Dirt may also cause bearing failure. To guarantee a long service life for a vehicle’s alternator, try keeping it clean and free of debris.

#12. Oil or coolant leaks

Oil leaks will shorten the life of an alternator as well, so make sure you get them repaired as soon as possible! It’s difficult to tell how much oil will be needed to cause the alternator to fail. However, oil leaking from leaking valve cover gaskets may cause the alternator to fail.

If the alternator is leaking oil, it may be a good idea to replace it as well along with a valve cover gasket. A coolant leak may also damage the alternator.

#13. Have you been in an accident recently?

If you are recently been in some kind of accident and have an alternator problem, the actual impact may have something to do with it. Most alternators have regulators built-in that control the output of the rectifiers and even though these are very durable components, they can be damaged due to impact.

#14. Damaged alternator components

Life is not perfect, and things go bad once in a while… The best part, in most cases they can be fixed!

Some of the components of the alternator that can fail include:

  • Bearing failure. Malfunction of the shaft bearings (that enable the rotor to spin) is often caused by poor servicing and maintenance. They could also corrode as a result of sitting in water for an extended period of time. Worn bearings can be replaced.
  • Rotor brushes can wear out. If the brushes (if you have them) are worn or corroded, the flow of electricity is disrupted, resulting in a reduction of the alternator output. Worn brushes can be replaced with new ones.
  • Bad diodes. A voltage spike may cause the diodes to short out. High temperatures while the car is idling for an extended period of time may cause diodes to fail as well. One or two bad diodes in the back of the alternator are one of the most frequent triggers of low or no charging output from the alternator. When diodes fail, they will allow AC to flow into the regulator, which can damage it as well.
  • Damaged rotor or stator windings. Insulation could be lost due to constant vibration. Rust buildup could be another problem in this situation.
  • Serpentine belt too tight, too loose, or misaligned. If your serpentine belt’s friction is incorrect, your alternator will malfunction and fail. You will notice this by the growling or whining sounds.

So, you figured out your problem and wondering what to do now? You can take both the alternator and the battery to AutoZone for inspection. It’s completely free and they will tell you what needs to be replaced.

After that, you could bring your car to a shop that specializes in electrical repairs. They should be able to repair the old alternator and may even be cheaper than buying a new one.

Here are some alternators from Amazon (paid links). After selecting your alternator  make sure it is compatible with your vehicle by clicking “select your vehicle” (on the top):

Alternator for Nissan

Alternator for American-Made Trucks

You could also check out this article on “How to Fix the Charging System Failure“. Whatever the cause of a bad alternator may be, you still need to take care of it ASAP:

Attention! This article is for informational purposes ONLY and is NOT a replacement for professional advice! ALWAYS consult your local specialist for an appropriate solution to your problem. All statements, prices, contact information, recommendations, and reviews contained herein came from sources that we believe to be reliable, but the accuracy or completeness thereof is not guaranteed. Please contact the service provider for complete details and updates.

Leave a Reply

Your email address will not be published. Required fields are marked *

Hit Counter
wp stats