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Can a Bad Alternator Cause Rough Idle? (Explained)

Batteries are a significant part of an automobile as it provides power to the headlamps, car radio, and other electrical components. An alternator is an electric generator used in modern vehicles to charge the battery. 

Since modern cars demand a lot of electrical energy, alternators are used to recharge the batteries instead of the traditional dynamo.

Can a bad alternator cause rough idle?

A bad alternator can cause rough idle. Since the defective alternator will affect the battery, the vehicle will malfunction. Therefore, it will cause rough idle along with poor acceleration, hesitation. Furthermore, the engine may also stall because of the lack of electrical energy.

Modern cars require a lot of electrical power to keep all of their electrical components running smoothly. As a result, the battery must recharge continuously to not run out of energy. For recharging a battery, a device called an alternator is used.

So, a battery relies on an alternator to keep it charged always. When an alternator starts to lose performance, the battery is heavily affected. Poor performance of the alternator can result in the expiring of the car battery and stalling of the engine.

Moreover, the electrical components of the car will also fail to function. You will see that the electric clock, power windows, power seats, and infotainment systems are malfunctioning. These devices will either completely malfunction or function significantly slower than usual.

The car battery will be affected the most. As a result, the battery will not be fully charged and will not have sufficient power to supply to the connected electrical components. The battery will soon expire, and the electrical components will fail to operate as usual.

Since cars require a continuous supply of power from the battery, interrupted power may cause severe damage to the vehicle engine. You will observe the vehicle to be rough while idle, and the car may stall due to insufficient starting power from the battery. 

Overall, the engine will provide poor performance, and you will experience poor acceleration and hesitation.

Therefore, a faulty alternator will affect both the battery and the car’s engine. It will also affect the rest of the electronic components that draw power directly from the car battery.

What are the warning signs of a bad alternator?

Several warning signs can help you understand that you have a defective alternator. Some of the common warning signs are:

The smell of burning plastic:

As alternators are used for some time, the drive belt of an alternator may wear out due to friction and heat. The wearing of the serpentine belt will release a rubber-burning smell. 

Also, if the alternator is defective, it will push out more current than usual. As a result, it will overheat the wires and melt the insulation. Melting of the insulation will produce a burning smell similar to the smell of electrical fires.

Battery warning light on the dashboard:

If you see that the battery warning light is flashing, you should check the battery. If the battery is charged insufficiently, then the alternator is faulty. 

An insufficiently charged battery may indicate that the alternator cannot charge the battery completely.

Frequent stalling:

Insufficient power from the battery may cause the engine to stop working suddenly. If your car is frequently stalling, the battery is probably not charged fully. The spark plugs receive insufficient power causing the engine to stall.

Malfunctioning accessories:

The most noticeable warning sign of a faulty alternator is the malfunctioning electrical components of the car. You will see that the power windows are working slower than usual. 

Also, you will notice your car headlights being unusually bright or dim. These symptoms can show that the alternator needs repairing.

These are the common symptoms of a car with a faulty alternator and an insufficiently charged battery.

What problems does a bad alternator cause?

The following are the consequences of having a faulty alternator in a car:

Whining engine noises:

When a car engine does not receive sufficient power to operate smoothly, it produces a whining noise while working. So, if the car alternator is faulty, the battery will be charged insufficiently. 

As a result, the engine will not receive enough power to operate smoothly and make whining sounds while running.

Unusual headlamps brightness:

If you notice that your headlights are unusually bright or dim, it may indicate a problem with the car battery. The car battery may be insufficiently charged or not recharging at all. 

If you see that your headlamps have an unusual brightness, the alternator is probably defective.

Malfunctioning battery:

The alternator is responsible for charging the car battery. If your car battery has passed and you see that the electrical devices and car engine are not working, the alternator is faulty. 

If the alternator is defective, consider getting the alternator repaired or replaced by a professional.

Engine stalling or not starting:

If the spark plugs do not receive sufficient power from the battery, the car engine does not work continuously. Moreover, you will experience the engine stalling more frequently than usual due to insufficient power to the spark plugs.

These are the consequences that you might have to face if your car has a faulty alternator.

How do I check my car alternator?

If you are concerned about your car alternator and want to check its health, follow the steps below to check the car alternator health yourself:


Before you check the alternator, it is necessary to be prepared. You will need a voltmeter and some gloves. You can use any cheap voltmeters to check the voltage.

Since you will be working with electric components, the gloves will protect you from electrical surges and shocks. It will act as insulation for you.

Check the battery voltage:

Before proceeding with this step, make sure that the engine and other electrical devices are off. Then, using the voltmeter, check the voltage of the car battery. If the voltage is between 12.5V and 12.8V, the battery is fine.

Start the engine and check the battery voltage:

Next, start the car engine and let the engine and alternator run for a few minutes. Then, check the voltage of the car battery again using the voltmeter. If the voltage reading has increased, then the alternator is working fine.

How to fix a bad alternator causing rough idle?

If you are sure that your alternator has become faulty and needs repairing or replacement, you can follow the steps below to fix the defective alternator causing rough idle:


It is necessary to gather all the required materials before you start. You will need gloves, a new alternator to replace the faulty unit and some wrenches of the correct sizes. Also, make sure that the car engine is turned off.

Remove the drive belt:

Start by removing the drive belt from the alternator. You can make a serpentine belt diagram to make it easier to put it back on later.

Unbolt and disconnect the alternator:

Using the wrench, unbolt the bolts and disconnect the wiring. Then, take out the defective alternator from the car.

Install the new alternator:

Put the replacement unit of the alternator inside the car. Then, use the wrench to connect the wiring to the alternator again.

Finishing up:

After the replacement unit is installed, put the serpentine belt back onto the alternator as it was before you removed it. Start the car engine and check the car battery voltage after a few minutes to ensure the alternator is working as intended.

How does a car act when the alternator is going out? Can you start a car with a bad alternator?

When the alternator is going out, the car starts to malfunction. You will notice that most electrical devices drawing power from the battery are malfunctioning. Moreover, you will hear the engine making whining noises while working and have frequent engine stalls.

Furthermore, the car might not start with a defective alternator. Since the spark plugs will not have sufficient power from the battery, the engine will not start. Instead, you will hear a clicking sound when you press the start button or turn the keys to start the car.

Final Thoughts

A defective alternator can cause rough idle, slow acceleration, and hesitation. If the alternator remains unrepaired for a significant period, the battery will expire. The engine and other electrical components of the car will follow and require frequent repairs and replacement.