Bad Alternator: Diagnosis, Symptoms & Fixes

Bad Alternator: Diagnosis, Symptoms & Fixes
alternator power supply for the electrical circuits of automotive

It is a common belief that the battery powers the electric components in your car. This is actuallynot the case. The battery provides power to start the engine but only continues to run it for a little while. Once the engine is started and running, the battery takes its leave and the alternator takes over.

It is the alternator which powers the electricals as you drive. It also charges the battery as you drive to ensure it has enough juice to start the engine again when you stop and start off again.

The battery only powers electricals when the engine is off.

How the alternator makes electricity

The alternator is made up of 4 main components.

  • A voltage regulator
  • The stator
  • The rotor
  • The diode

When the car is first powered by the battery, the alternator belt (V-belt) spins a pulley which makes the rotor spin at high speed. The rotor is basically a group of magnets encased in copper wires known as the stator.

The process in which magnets spin along a set of copper wires at incredibly high speed is what generates electricity.

It is called electromagnetism. Electricity produced travels through the copper wires to a diode which then converts it from AC (alternate current) to DC (direct current) which is the form in which the battery utilizes it.

The final step is in the voltage regulator which works as a gate keeper. It restricts the flow of power to the battery when voltage rises beyond 14 ½ volts.  This ensures the battery doesn’t overcharge and burn out.

As soon as some power from the battery is drained, the voltage regulator allows more power into it from the alternator and then cuts it off when the battery is fully charged.

Electricity produced is what is used to power electric components such as headlights, dashboard lights, power windows and the radio.

Signs of a Failing Alternator

Alternators rarely break down suddenly. They usually start to show signs of an eminent breakdown in the near future. You will probably notice warning signs well in advance. Some common signs that your alternator is dying include:

Difficulty starting: It is the battery which provides energy used to crank up the motor. How does a faulty alternator cause starting problems? It is because the battery’s power is replenished by the alternator so if it is doesn’t replenish it sufficiently, then the battery lacks sufficient power to start the engine.

When your car doesn’t start as it should, the battery is usually the first suspect. Don’t forget to check the alternator as well.

Failing electrical components: It may not be as obvious as failure to start the engine but it gets more obvious as the alternator gets weaker.

At first you may imagine that you noticed headlights get dim then dashboard lights also start to dim. Next you notice that the power windows are still working but seem to drag as the window rolls up. If nothing is done these components eventually fail completely.

Growling noise: The alternator is driven by a crankshaft pulley which works alongside a belt. The alternator pulley spins at extremely high speed in order to produce power. When the pulley and belt are misaligned, as often happens with age, the alternator produces growling noises. It may also happen when bearings are worn out.

Burning rubber smell:When the belt and pulley are misaligned, excess friction may produce a burning rubber smell. If wires in the alternator are scorched, a similar smell may be produced.

Warning light comes on:In some vehicles, the ALT or GEN light in the instrument panel comes on when there is a problem with the alternator.

Causes of a Bad Alternator

These are some common reasons behind a faulty alternator

Worn out alternator:A typical alternator lasts anywhere between 5 and 7 years. If your alternator is failing and has served you for about this period of time, it is time to let it go. It has done its time and a replacement is due.

Damaged wires: Energy produced is delivered to the alternator via wires. If one or some of these wires are damaged or worn out, the alternator does not function as it should even though it is in itself in good condition.  When the alternator fails, then the battery is not charged and lacks sufficient power.

Make a point to check all the wires feeding the alternator to rule out this possibility. If it is indeed a damaged wire, this is a quick fix.

Computer system failure: Modern cars come with an in-built computer system which is responsible for managing virtually all vehicle components, including the alternator. When there is a problem with the computer system, the alternator may fail to work even if it is in good condition.

It will probably be easy to identify a computer-related problem because failures will arise in other computer controlled components. You will probably need a professional technician to fix car computer problems.

Blown fuse:Alternators use fuses to work. Like all other fuses, these blow out after continued use. If the alternator fuse blows, it doesn’t recharge the battery. To fix this, simply replace the fuse.

What makes an alternator fuse blow?Like all other fuses in electric circuits, this fuse is placed in the alternator designed to melt and separate into 2 pieces in the event of excessive current. If there is a power surge, the fuse blows.

Another reason the fuse could blow is if the current demands placed on it are more than it can support. If for instance you have a dead battery so you jump start the car then switch on many electric components as you drive such as lights, air conditioner and entertainment system.

Can A Bad Alternator Ruin A Car Battery?

The short answer is yes. A bad alternator can kill a battery. However it depends on a few factors. To understand how this happens it is necessary to understand how the car battery which is constantly recharged by the alternator works.

Some types of batteries are designed to fully discharge while others are designed such that they should always have some charge in them. Car batteries fall in the latter category.

How car batteries work: Inside the battery is a pair of lead plates suspended in a bath of sulphuric acid which is the electrolyte. Every time the battery is used, the plates are coated with lead sulphates. This sulfation process is reversible and is reversed when the battery is recharged by the alternator.

If for some reason the battery is excessively discharged, lead sulfate crystalizes. When it does even recharging cannot reverse sulfation. Lead sulfate crystals remains on the plates. This causes a permanent reduction of output of the battery.

When does a bad alternator completely kill a battery and when does a battery survive a bad alternator? It depends on

  • How far the battery was discharged after the alternator failed. How long did you drive on battery power only? The longer it run the higher the chances it will die.
  • The age of the battery. If it is a fairly new battery, you can get away with it once or twice.
  • How long the alternator has been ailing. If it had been ailing for a long time, it means it had probably suffered improper charging for some time. The day the alternator dies completely and the battery is deeply discharged, it may be too weak to survive the strain.

Tips on Dealing with a Bad Alternator

Can you jumpstart a car with a bad alternator?

Yes you can jumpstart your car even if the alternator is dead. Chances are the battery is dead because the alternator didn’t charge it enough. Even then, rarely would a battery be completely dead. It doesn’t have enough power to crank the engine (which is a lot) but it can be charged and provide enough power to get you to the repair shop.

If you decide to do this, perhaps to save on towing costs, make sure the person who helped you jumpstart your battery follows you to the mechanic especially if it is more than a few miles away.

Does a bad alternator affect the air conditioning system?

Yes, the air conditioning system will not work if the alternator is faulty.   Air conditioning systems are among several components such as the radio, power windows, lights and engine which are powered by the alternator.

How to differentiate between battery and alternator problems

For many drivers, when the car won’t start it is assumed that there is a problem with the battery. While battery failures are common, it could also be caused by a bad alternator. How can you determine which of the two is to blame?

The answer takes a good understanding of how the two work together, which part does what. After that it is pretty much a game of elimination.

First get your hands on a set of jumper cables and a car with a good battery and jump start the car. Once the car is running, loosen the positive battery cable and remove it. If the car suddenly stops running you know you have bad alternator. If it keeps running, you have a bad battery.

Alternatively, check the voltage of the battery using a voltmeter. With the car running, check to see if voltage rises as you increase engine speed. If voltage remains the same, you have a bad alternator.

 How long does an alternator last?

If your alternator starts giving you trouble after a couple of years of use, it may just be that it has served for its expected lifespan. Alternators are designed to last between 100,000 and 150,000 miles, which translates to about 7 years.

However, most drivers don’t get to use alternators for that long. Mechanics say they often see alternator failures after about 80,000 miles. Why the difference between design and actual experiences? It depends on the workload you place on your alternator. If you ordinarily have many electric components running in your car, it reduces the overall lifespan of the alternator.


When you hop into your car, turn the key but the engine won’t start, your fist suspicion will be the battery. However, it could very well be a problem with the alternator. Alternators could fail as a result of old age, a blown fuse or damages wiring.

Check both the battery and alternator at least once a month to make sure they are both in good condition and to catch any problems before they cause any problems.


Some people’s alternators last longer than others. What can I do to make mine lasts as long as it can?

There are some measures you could take to prolong the life of your alternator. First check posts and cables leading to the battery are free of corrosion. It ensures charge travels freely and easily. Secondly, make a habit of checking the drive belt which loops around the rotor shaft.  This way you will catch signs of deterioration early and prevent more extensive damage.

How can I replace an alternator fuse?

Actual replacement of an alternator fuse is very easy. The challenge is in finding the one which needs to be replaced. A typical car fuse box can have up to 30 fuses for different components making it difficult to identify the one for the alternator. Use the owner’s manual as a guide.

Actual replacement simply takes sliding the old one out of place and sliding the new one into position.

What should you do if the alternator dies while driving?

If the alternator dies while you are driving, the first thing you will notice is that headlights and dash lights go dim and then go off. The best thing to do is turn off all components such as the radio and air conditioning and drive to the nearest repair center. If you can’t get that far, pull over safely and call for towing assistance.

Can I replace the alternator myself?

If you have some basic mechanical know-how you can save yourself a few hundred dollars and replace the alternator yourself. You will need basic tools and something to help you remove the belt.


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