8 Signs of a Bad Alternator | How to Test a Car Alternator?

The battery is one of the most important parts of the vehicle. It runs the stator motor to start the vehicle engine. An alternator uses to charge the battery. The alternator takes mechanical power from the engine and transforms it into electrical power to charge the battery. When the car alternator goes bad, it generates different signs to alert the driver. This article mainly explains the bad car alternator symptoms, causes, replacement cost, function, and how to test it.

What is an Alternator?

An alternator is an electrical generator that transforms mechanical power into electrical power in the form of AC (alternating current). The alternator uses a rectifier to transform the output alternating current into a direct current. The resulting DC is used to charge the battery of the vehicle.


The alternators are most commonly used in the latest vehicle models. The engine shaft is attached to the pulley via a drive belt in vehicles. The pulley is further connected to the alternator. As the engine shaft rotates, it turns the pulley, which further turns the shaft of the alternator.

When the shaft or rotor of the alternator turns, it produces alternating current (AC). The rectifier further converts AC into DC and transfers output DC to the battery. The battery further uses this DC to run the starter power.

The alternator is located at the engine front and is run by the engine crankshaft. The alternators are available in different designs.

The magneto alternator is one of the most famous alternators. It is equipped with a permanent magnet for the magnetic field. However, the main function of the alternator in all mechanical applications is to charge the electrical units of the system.

Therefore, alternators are used in nuclear power plants, hydel power plants, and thermal power plants to convert mechanical power into electrical power to deliver electricity to the power system.

What does an Alternator do?

The alternators take mechanical energy from the engine and convert it into electrical energy to charge the battery. An alternator performs the below-given functions:

  • The main function of the alternator is to charge the vehicle battery. The battery power further uses to run the starter and other electrical parts in the vehicle.
  • When the vehicle is in motion, the alternators produce energy to power the electrical systems and battery.

The alternators work in the same way as generators. When the engine turns the shaft of the alternator, it produces AC (alternating current) which further converts into (direct current).

Signs of a Bad Alternator

The car alternator plays a big role in the efficient working of the vehicle. As the car alternator goes bad, it doesn’t properly charge the vehicle battery; due to that, the vehicle engine doesn’t start. A bad or faulty alternator produces one or more of the below-given symptoms:

  • Dead battery
  • Dim lights
  • Malfunctioning or slow accessories
  • Burning rubber or wires smell
  • Whining noise
  • Stalling
  • Warning light
  • Engine misfiring

1) Dead Battery

A dead battery is one of the most common signs of a bad alternator. The battery has a specific service life. As it dies, it doesn’t work and doesn’t supply current to the electrical parts of the vehicle.  

car Battery, signs of bad alternator

During the operation of the engine, a faulty alternator cannot fully charge the battery and drains it faster than normal. One of the best methods to check if the problem is with the alternator or the battery is to jumpstart the vehicle.

Read More: How to Jumpstart Your Vehicle?

2) Dim Lights

As the alternator goes bad, it doesn’t properly supply voltage to the electrical parts of the battery. This often manifests the under-performing or over-performing devices, e.g., headlights that are either too weak or too bright.

Dim Lights due to bad alternator

The blinking light or lights that change unsteadily from dark to light and vice versa is also a sign of bad alternators.

3) Malfunctioning or Slow Accessories

The faulty alternators damage the electrical accessories of the vehicle. When the alternator doesn’t provide sufficient power to the vehicle’s battery and other electrical components, then they will ultimately slow down or fail.

Slow Accessories

For example, when you observe that the window rolls up and down process takes more time than usual, warmers feel “off,” the speedometer or other gauge starts to lose control, and there may be a problem with the alternator.

4) Burning Rubber or Wires Smell

An unpleasant smell of burnt rubber or wire may indicate that some components of the alternator have been worn out.

Burning smell, symptoms of a bad car alternator

When the vehicle alternator is overloaded, or cords are damaged or frayed, it may generate a burning smell like an electrical fire. The overloaded alternator attempts to transfer excess power through the cords due to that; the cords heat up in an unsafe state.

5) Whining Noises

The whining noise is one of the major signs of a bad alternator. As the alternator goes bad, your vehicle may start generating growling noise from under the hood.

whining Noise

This whining noise usually occurs as the belt that spins the alternator pulley is misaligned or rubbing against the pulley sides. You must immediately contact a specialist to fix the problem in such a condition. If you ignore the issue, it may lead to expansive repair.

6) Stalling

As mentioned above, the inability to start the engine might mean that the alternator is not producing enough power to charge the battery. This means that as you turn the ignition key, you will hear a clicking noise, not the roar of the engine.

However, if the vehicle stalls regularly while driving, this may indicate that the spark plug is not receiving adequate current from the alternator to continue the engine operation.

engine Stalling due to faulty alternator

However, your vehicle may start stalling due to many other reasons, such as a bad spark plug, low engine oil, bad head gasket, damaged cylinder block, or low coolant.

Read More: Engine Stalling Causes and Preventions

7) Warning Light

Your car dashboard contains multiple warning signs and symbols. The illuminated warning light on the vehicle dashboard is one of the clear symptoms of a bad alternator. In most cases, the light is shaped like a battery, though some show “GEN” or “ALT,” meaning generator or alternator, respectively.

ALT Warning Light

 This light is connected to the vehicle computer (usually PCM). It monitors the output data of the alternator through the vehicle computer. When your computer monitors an issue with the alternators, it triggers the warning light to alert the driver. 

Read More: Car Dashboard Warning Lights and Symbols 

8) Engine Misfiring

As we discussed above, the main function of your car’s alternator is to charge the battery. If it is not working efficiently, it will not properly charge the battery.

Engine Misfiring

If your battery is not charged or charged poorly, it will not deliver sufficient power to the spark plug to generate the spark. When your spark plugs don’t generate a sufficient spark to burn the air-fuel mixture, the air-fuel mixture doesn’t burn properly and leads to misfiring.

Read More: Bad Spark Plug Symptoms and Causes

Causes of Bad Car Alternator

The alternator of your car usually goes bad due to one or more of the below-given symptoms:

  • Age and wear
  • Faulty regulator
  • Faulty diodes
  • Overheating
  • Electrical shorts
  • Belt tension problems
  • Worn-out brushes

1) Age and Wear

Like any mechanical component, alternators have a restricted life cycle. Therefore, it may ultimately fail or wear out due to age, especially if it isn’t properly maintained.

2) Faulty Regulator

The damaged or faulty voltage regulator is one of the major causes of the bad alternator. It uses to regulate the output voltage of the alternator and ensures that it remains under safe limits.

A bad regulator may cause your alternator to overcharge the battery or deliver insufficient power to the electrical system.

3) Faulty Diodes

 The main function of the diodes is to transform the AC voltage generated by the alternator into DC voltage. This DC power is utilized to run the electrical parts of your vehicle. When any diode of your system fails, the alternator will be unable to generate sufficient power.

4) Overheating

The rotating parts of the alternator produce a large amount of heat. When these parts overheat, they may lead to the alternator malfunction.

Read More: Engine Overheating Symptoms and Causes

5) Electrical Shorts

A short circuit in the electrical wires of the alternators may lead to electrical problems such as low output voltage or no output at all.

6) Belt Tension Problems

A drive belt is used to run the alternator. This belt is attached to the crankshaft of your engine. When the belt is damaged or loose, your alternators can’t spin fast enough to generate adequate power.

7) Worn-out Brushes

The alternators of your car contain multiple brushes. These brushes have the responsibility of transmitting power from the rotating rotor to the stationary stator. These brushes may wear out with time which may cause electrical contact problems.

How to test an Alternator?

Before testing, you must have a voltmeter, a multimeter, and all other necessary tools. Follow the below-given steps to test an alternator using a multimeter:

  1. Turn off the engine: First of all, ensure your engine is turned off.
  2. Battery Check: You need to check the voltage of your car before testing the alternator. Use a voltmeter to check the battery voltage. The battery voltage must be about 12 volts. If the voltage is less than 12 volts, it means your battery is dead or weak. A weak battery may badly affect the performance of your alternator.
  3. Visual Inspection: Visually inspect your alternators for symptoms of fault or damage, such as damaged wires or corroded connections. Also, properly inspect the belt tension and make sure it is not too tight or too loose.
  4. Start the engine: After checking the battery voltage and visual inspection, it’s time to restart your engine and let it idle.
  5. Connect the Multimeter terminals: Use a multimeter and connect it to the battery. Connect the negative terminal of the multimeter to the battery’s negative terminal and connect the positive terminal of the multimeter to the battery’s positive terminal. Make sure that your multimeter is set in the DC volts.
  6. Note output voltage: After connecting the multimeter to the battery, note the voltage reading showing on the multimeter while the engine is running. The readings must be from 13.5 volts to 14.5 volts. If the voltage is less than this range, it means your alternator may be defective.
  7. Load test: If the reading noted in step 6 is between 13.5 volts to 14.5 volts, then further perform a load test. In this test, turn on the radio, headlights, car internal lights, and other electrical fixtures to put a load on the alternator. Now, recheck the voltage readings of the multimeter. If the voltage drops sharply, you may have a faulty alternator or charging system problems.
  8. Diode test: After performing all the above tests, now test the diodes of your alternators. For this test, set the multimeter to the diode test mode. Attach the multimeter’s negative lead to the alternator’s negative output terminal and the multimeter’s positive terminal to the positive output terminal of the alternator. Now, note the multimeter reading. A value of infinity or significantly higher than the other diodes can specify a defective diode.
  9. Interpretation of results: If the voltage output and diode tests are according to the acceptable measures, the alternator is probably operating properly. If the diode test fails or the voltage output is very low, it means your alternator may be faulty, and you need to repair or replace it.

How much does it cost to replace an alternator?

The replacement or repair costs of the alternator depend on your vehicle model, engine design, and your living area.

The average replacement cost of the alternator is usually from $600 to $798. In this cost, the labor costs are from $120 to $150, while the estimated costs of the parts are from $480 to $648.

FAQ Section

What is the difference between an Alternator and a Generator?

The main differences between the alternator and generator are given below:

  • A generator is a machine that converts the mechanical energy of the main engine into direct current or alternating current, while an alternator is a machine that converts the mechanical electricity of the main engine into alternating current.
  • The output power of the generator is not much stronger than the alternator’s output power.
  • The alternator contains a spinning magnetic field, while the generator contains a rotating magnetic field to produce a low-voltage constant magnetic field and a high-voltage magnetic field.
  • The alternator can’t charge a fully dead battery, while the generator has the ability to charge a fully dead battery.
  • In the generator, the rotor provides the input power, while in the alternators, the stator provides the input.
  • The output voltage of the alternators is variable, while the generator output voltage remains constant.
  • The generator armature is static but alternates within the generator.

What are the symptoms of a bad alternator?

  • Misfiring
  • Dead battery
  • Dim lights
  • Warning light
  • Malfunctioning or slow accessories
  • Whining noise
  • Burning rubber or wires smell
  • Engine stalling

How long does it take to replace an alternator?

Replacing the alternator usually takes approximately 2 hours, but this depends on the knowledge of the auto mechanic. When you drive your vehicle to the workshop to replace the alternators, sometimes, you may need to leave the vehicle for the rest of the day.

How do you know if your alternator is bad?

When your alternator goes bad, it produces different symptoms, such as engine stalling, hard starting, rough idle, misfiring, unpleasant smell, or warning lights. If you observe any of these signs, your alternator may be bad, and you need to replace it.

Can I drive my car with a bad alternator?

Yes, you can drive your car with bad alternators. It is not recommended to drive with bad alternators. A bad alternator may lead to different drivability issues, such as engine stalling, dead battery, or misfiring. If your battery becomes dead, you won’t restart your car.

What does a bad alternator sound like?

A whining or gurgling noise is a sign that your alternator is failing. Even a slight engine rattling noise may indicate the alternator problem. Most of the noise coming from the alternator is caused by loose bearings.

Is alternator AC or DC?

Alternators convert the mechanical power of the vehicle engine into electrical power through AC voltage. The rotor and stator in the alternator act like magnets and rotate to create AC voltage. It then converts alternating current (AC) to direct current (DC) to charge the battery.

What causes the alternator to fail?

  • Age and wear
  • Faulty regulator
  • Faulty diodes
  • Overheating
  • Electrical shorts
  • Belt tension problems
  • Worn-out brushes
Read More

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