Can A Bad Alternator Cause Rough Idle [Explained A-Z]

A faulty alternator can cause many issues with your car. Significantly, the electrical system. If you’re facing a rough idling issue and have a damaged alternator, you may suspect that the alternator is causing the problem. So, can a bad alternator cause rough idle?

As the alternator is mostly responsible for powering the electrical components in the car when the engine is running, there’s little chance of it causing rough idling. But that doesn’t mean that it can’t happen.

A bad alternator can cause rough idling when it produces low voltage, has a bad diode, depletes the battery, has an intermittent grounding, or is misaligned.

can a bad alternator cause rough idle

Keep on reading this guide to find out all about it in detail and the other reasons behind the rough idling issue in an engine. Let’s dive right in!

Can A Bad Alternator Cause Rough Idle [Explained A-Z]

When you start the car, it usually relies on power from the battery. Once the engine turns over, the alternator takes over the role of powering the electrical components. The battery also gets charged during this time.

car bad alternator

Unless there’s a high demand for electric power, the battery no longer produces any output. Only when the required demand surpasses the capacity of the alternator, the battery gets engaged once again during driving.

The alternator produces this energy by converting mechanical energy into electrical energy. The magnets inside it engage in a spinning motion to power the electrical components in your car including the headlights, AC, radio, and even the windshield wipers.

As you may have realized by now, the alternator doesn’t do much with the firing sequence of an engine. But it can still cause a rough idling issue in your car when it goes bad. Here are the most common scenarios in which a faulty alternator can cause this problem:

Scenario 1 – Not Producing Enough Voltage

Your car won’t be running as it should if the alternator fails to provide the required juice. The spark plugs in most vehicles are dependent on the alternator to get the power needed to generate the spark.

car alternator not producing enough voltage

When the alternator produces 13 volts or fewer volts instead of 14 volts which it’s supposed to produce, your engine begins to misfire. The more spark plugs that are affected due to the poor performance of the alternator, the more severe the rough idling issue will be.

If the alternator affects only a single spark plug, you may get away with it and be able to drive your vehicle with poor fuel economy. But if multiple spark plugs are impacted by it, you will get rough idling from your car and may also have trouble moving it at all.

Scenario 2 – Bad Diode

The alternator initially produces Alternating Current (AC). It’s the wrong format of electrical power required for your car. It needs Direct Current (DC) to run the electrical components and charge the battery. The AC is converted into DC using a device called a rectifier or diode.

car bad diode

The alternator diode ensures that the current flows in a specific direction. When the diode goes bad, it can produce inconsistent voltage to the electrical components. The main issue that occurs due to a bad diode is that it creates a ripple effect that makes it interferes with the other sensors.

The camshaft position sensor is responsible for telling the engine when to fire the spark plugs. Due to the ripple effect of a faulty alternator diode, the signals can get mixed up and cause the spark plugs to fire at the wrong time.

It will inevitably lead to issues like the engine misfiring and rough idling. Even an inspection of your car with an OBD2 scanner may throw an error code that points to a bad camshaft sensor. But in reality, it’s the alternator diode that’s causing the issue here.

Scenario 3 – Depleted Battery

You must know by now that besides supplying power to the electrical parts in your car, the alternator also charges the battery. When the alternator fails to charge the battery, it no longer has the ability to give the initial burst of power required to start the engine.

car depleted battery

You may end up with a dead battery altogether if you have been driving with a bad alternator for a while now. A faulty voltage regulator in the alternator can also make the battery undercharged or overcharged.

When the battery is undercharged, it will run out of power sooner than expected. When the battery is overcharged, it will swell up and would also start to malfunction. It can lead to inconsistent performance both during high and low RPMS and cause the rough idling issue.

Even though it may seem like the battery is the main culprit here, upon closer inspection, you could find that the alternator is what destroyed the battery. You’d have to replace both the battery and the alternator if you end up with a dead battery and a faulty alternator at the same time.

Scenario 4 – Alternator Intermittent Grounding Issue

Besides the battery going bad, there may also be an intermittent grounding issue with your alternator. If the battery has enough juice, run your car for a while with just the battery after disconnecting the alternator.

car alternator intermittent grounding issue

If the car runs fine only on battery power when you remove the connection to the alternator, you can isolate the problem to an alternator grounding issue. Fixing it should get rid of the bad idling issue in your vehicle.

Scenario 5 – Misaligned Alternator

When the alternator isn’t aligned properly, it won’t be able to do a perfect job of converting mechanical energy into electrical energy. The alternator pulley needs to be in the right position compared to the serpentine belt to generate the right amount of electricity.

A misaligned alternator will not produce enough current to power the electrical components. It will also lack the power to charge the battery. All these issues can lead to rough idling as well as overheating of the battery and issues with the electrical system.

Moreover, any loose components like an engine mount will also cause both the alternator and the engine to vibrate vigorously and lead to bad idling. Check if this is the issue with your alternator and get it fixed if it’s not aligned right.

Note: You can also read our guide on can a bad alternator cause overheating.

What Are More Common Reasons Behind Rough Idling

If you’ve read this far, it should be obvious to you by now that a bad alternator causing rough idle is a rarity. There are plenty of other reasons that can cause the bad idling issue in your vehicle. Here they are:

1. Engine Misfiring

When one or more of the cylinders in your car fails to fire the engine, that condition is known as engine misfiring. It is the number one reason behind rough idling in a vehicle.

There are many issues that can lead to an engine misfire in a vehicle. The major reasons behind engine misfires are:

Worn Out Spark Plugs – If any of the spark plugs are fouled and fail to fire when needed, it causes an engine misfire. You’ll have to replace the spark plugs if they have gone bad.

Clogged Or Dirty Fuel Injectors – If the fuel injectors are clogged up, they won’t be able to send enough fuel to create proper combustion. It can cause an engine to misfire. You can solve this issue by cleaning the fuel injectors.

Incorrect Ignition Timing – The spark plugs need to fire at the right time to create proper combustion. When the ignition timing is messed up and they don’t fire at the right time, it causes an engine misfire.

Vacuum Leaks – The engine needs the right amount of both air and fuel to work properly. When there are vacuum leaks, the amount of air in the engine gets messed up and that also causes an engine to misfire. You can solve this issue by removing the source of the leak.

Bad Alternator –  Ironically, a bad alternator can also cause an engine misfire issue though it’s also a rare occasion. Check out our detailed guide on can a bad alternator cause a misfire to learn all about it.

2. Clogged Air Filter

The air filter in your car has an important role in preventing the dust present in the air from entering the engine. But with time, the air filter gets clogged with dust particles. When that happens, the required amount of air can’t enter the engine.

clogged air filter in the car

The engine always needs the right amount of air and fuel to function properly. The absence of sufficient air messes up the optimum air-fuel ratio. It causes the engine to run rich due to a higher percentage of fuel compared to air.

When the situation gets too bad, the car can also idle rough. Fortunately, solving this issue is extremely easy and cheap. You can get the air filter replaced for as little as $10-$20 and this problem will go away.

3. Faulty Oxygen Sensor

The oxygen sensor calculates the amount of oxygen present in the exhaust gas that’s being released via the exhaust pipes. It passes this information to the Electrical Control Unit.

faulty oxygen sensor in the car

The ECU acts as the brain of the car and it uses this information to maintain the right level of oxygen in the engine at all times. The oxygen sensor can go bad when it’s exposed to high temperatures. The build-up of carbon deposits or simply wearing out with time can also cause it to malfunction.

When it goes bad, it sends inaccurate information to the ECU. So, the engine runs either runs too rich or too lean and that causes bad idling. You’ll also get poor fuel economy when the engine runs rich as it’ll be burning more fuel.

If you have an OBD2 scanner, you can test the oxygen sensor. If error codes like P0131, P0134, or P0137 pop up on the reader, it means you have a bad oxygen sensor and you’ll have to replace it.

4. Dirty Fuel Filter

Just as the air filter keeps the dirty contaminants present in the air from entering the engine, the fuel filter keeps the dirt present in the fuel from getting into the engine. The end result is that the engine only gets high-quality fuel that won’t damage the engine components.

dirty fuel filter in car

As time passes, the dirt begins to pile up, the fuel filter gets clogged, and the fuel can no longer pass through it. When that happens, the engine starts to run lean as there’s more air compared to fuel in the mixture. It causes rough idling as well.

You can solve this issue easily by changing the fuel filter in your car. As a general rule, you should change the fuel filter every 50,000 – 100,000 miles to avoid this problem.

5. Faulty PCV Valve

The main job of the Positive Crankcase Ventilation valve (PCV valve) is to direct the unburnt gases that escape from the cylinder to the crankcase back into the combustion chamber to get burned.

faulty PCV valve in car

The PCV valve is exposed to extreme conditions and that’s why they often get filled with dirt, debris, and sludge. When that happens, it can get stuck open. The end result will be that too much air will go into the combustion chamber.

The engine will start running lean and it’ll lead to bad idling as well. Periodic review and maintenance of the PCV valve can help you stay on top of things and avoid this issue. You can try cleaning it if it is clogged. If that doesn’t work, you’ll have to replace it.

6. Defective Fuel Pump

The fuel pump collects the fuel from the fuel tank and supplies it to the fuel injectors. If the fuel pump gets clogged, it won’t be able to supply the necessary fuel to the injectors.

defective fuel pump in car

The fuel pump can also go bad at times and it won’t supply any fuel at all to the injectors at times. You already know what happens when the engine doesn’t get enough fuel. It runs lean and that causes rough idling.

If the fuel pump is just clogged, you can get away with just cleaning it using a fuel system cleaner. But if it has gone bad and is starting to malfunction, you have to replace it with a new one.

7. Faulty EGR Valve

The EGR or Exhaust Gas Recirculation valve does as its name suggests. It reduces the number of nitrogen emissions by sending some amount of exhaust gas to the combustion chamber to get burned again.

faulty EGR valve in car

Doing that not only reduces the emissions but also improves the fuel economy of the car. The EGR valve should ideally be closed when you’re idling your vehicle. But due to the build-up of carbon deposits in it, it can get stuck open. When it happens, your car will idle rough.

Sometimes, the Check Engine Light will come on if you have a bad EGR valve. You can also check if the EGR valve is working properly or not using a vacuum pump. If it turns out to be defective, you have to replace it.

8. Faulty TPS

The TPS or Throttle Position Switch is usually mounted on the throttle body. It measures the movement of the throttle plate and sends that information to the ECU. The ECU uses that to adjust the mixture of air and fuel and also achieve the correct ignition timing.

throttle position switch in the car

When the TPS goes bad, it can pass the wrong information to the ECU and that can throw off the right ignition timing. It causes rough idling and can also lead to more drivability issues.

When you suspect having a defective Throttle Position Switch, it’s a good idea to have it tested out by a mechanic. If it has gone bad, it needs to be replaced with a new one.

9. Dirty IAC Valve

The IAC or Idle Air Control valve regulates the amount of air getting into the engine to control the idle speed of the vehicle. But as we saw with many other components, carbon deposits can make the IAC valve malfunction.

dirty IAC valve in the car

A dirty IAC valve will restrict the airflow and the engine will have less air than required. The engine will run rich and it’ll cause the engine to idle roughly and even stall. If it has just gotten dirty, you can clean your IAC valve. But you’ll have to replace the IAC valve if it has gone bad.

10. Carburetor Problems

This issue can only be found in vehicles made before the 1990s. Modern vehicles have fuel injectors instead of carburetors. But if you have an older vehicle that has a carbureted system and you see black smoke from the exhaust, it’s an indication of carburetor problems.

carburetor problems in the car

If this black smoke comes along with a rough idling issue, that warrants an inspection of the carburetor. You can use a carburetor cleaner to minimize the severity of the issue. The cleaner will remove the carbon contaminants and hopefully, decrease the state of rough idle in your car.

What Makes Alternator Go Bad

The first thing you should keep in mind here is that all of the components in your engine have a lifespan. None of them won’t work forever and it’s okay for them to malfunction when their time comes.

what makes the car alternator go bad

An alternator lasts for ten to twelve years on average if you’ve maintained it well and not put it under any unnecessary stress. In terms of mileage, you can expect it to last for 120,000- 150,000 miles if maintained properly. If your alternator goes bad faster, it could be due to fluid leaks into the alternator.

Engine oil, coolant, or steering fluid leaking into the alternator can cause it to malfunction. If you spend too much time idling your can with the electrical components turned on, it’ll also cause additional pressure on your alternator and make it wear out faster.

As the alternator is generally located at the bottom of the engine, exposure to salt and water can make it corrode and go out faster. If you don’t get the right alternator for your vehicle and get one that has lower power, it will also go bad faster than a normal alternator would.

Avoid making those mistakes and take good care of your alternator to make sure that you get the most out of it. The first sign of a bad alternator is that you’ll see the lights flickering or dimming down.

When that happens, there’s a high chance that your alternator is entering its last stage and will go bad soon. Be on the lookout for it and get ready to replace your alternator if you see these symptoms. That way, you can stay on top of things and won’t get stranded in the middle of the road.

How To Fix An Alternator

Normally, it’s a good idea to replace the alternator altogether if you have a bad alternator. It usually costs around $250 or more to replace the alternator depending on the vehicle you drive. But if you’re low on budget, you can get away with fixing the alternator instead of replacing it.

Keep in mind that you’ll need an understanding of all the parts in an alternator if you want to go ahead and fix it. Secondly, not all alternators will have the option to replace them as they won’t come with any repair kit. In that case, your only option is to replace the alternator.

You’ll know if it’s possible to fix your alternator using a repair kit after researching a little bit about the specific model of your alternator. If it works for your vehicle, you can follow these steps to fix your alternator:

Step 01 – Disconnect The Battery

Remember that safety is the most important thing when it comes to doing any car repair work. You don’t want to end up hurting yourself to save a few bucks. So, wait for the engine to cool down before you start working on the alternator.

disconnect the battery of the car

Once you’re sure that the engine is cool, start by disconnecting both terminals of the battery. Always disconnect the negative terminal first and the positive one later. When connecting the battery terminals, connect the positive one first and then, the negative one.

Keep the cables away from the alternator so that they don’t get entangled while you work on them. Unscrew the power cable to disconnect the power to the alternator and isolate it from the rest of the vehicle.

Step 02 – Remove The Serpentine Belt

If you need to take off any other leads to remove the alternator, mark their positions so that you can reattach them easily. Next, you’ll have to remove the serpentine belt to access the alternator. Loosen the tension belt to make the serpentine belt and then, you’ll be able to remove it.

serpentine belt in the car

Step 03 – Remove The Alternator

After removing the serpentine belt, there should be nothing else in the way of you removing the alternator from the vehicle. Loosen the bolts holding the alternator in place and make sure you pull it out from the socket gently without entangling or damaging any of the cables.

Step 04 – Remove The Rear Cover & Inspect The Alternator

You need to inspect the alternator to figure out what’s wrong with it. Remove the bolts at the rear of the alternator to get rid of the plastic cover. Once you do that, you’ll have access to the internal parts of the alternator.

remove the rear cover & inspect the alternator

Now, you’ll have to inspect the parts one by one. Start by checking the alternator bearings as they have a high likelihood of going bad. Make sure that you mark the wiring arrangement before disconnecting anything. You’ll be able to place it back together later on if you do that without getting lost.

If the bearings don’t make any noise when you spin them or they simply look disconnected, you’ll have to replace them. Install the new bearings.

Step 05 – Replace The Faulty Components

Just replacing the alternator bearings won’t be enough in most cases. You’ll most likely have to replace other vital components like the rectifier, brush assembly, and voltage regulator.

Pull out the leads of the defective rectifier and separate the rectifier from the alternator mountings. Then, put the new rectifier in place of the old one and attach it to the rectifier leads.

You’ll find the brushes next to the rectifier in your alternator. As they get older, they can develop significant debris. Remove each mounting of the brush assembly from its place.

alternator brushes assembly in the car

Once you do that, you’ll be able to pull out the old brush assembly. Next, clean the surrounding area and install the new brush assembly.

You have to disconnect the rear screw of the brush assembly to remove the voltage regulator. Take the ground lead screw out as well. When you do that, you can easily remove the old voltage regulator and install the new one.

Step 06- Connect The Plastic Cover & Put The Alternator Back Together

Now that you have replaced all the faulty components in question, it’s time to put the alternator back together. Start by putting the plastic cover back on and tighten the bolts to make sure it stays together.

putting the cover back of alternator in the car

Once you put the alternator back together, place it back in the car and connect all the electrical leads to it. Put the serpentine belt back in its place and make sure that it has the proper tension. It’s a good idea to install a new serpentine belt if the one you have is worn out.

Step 07 – Connect The Battery And Test The Alternator

Connect the battery terminals back together. Make sure you do it in the right order – the positive one first and then the negative one. The terminals of the battery should be clean at this stage.

If they’re dirty, take the time to clean them. The alternator should work fine now. Let the engine run for a few minutes at idle. After that, test it with a multimeter and see if it’s providing the required voltage.

If the voltage range is between 14.2 and 14.7 volts, it means you have fixed the alternator. But if it’s outside that range, that means it’s either generating too little or too much power. In that case, you’ll have to replace the alternator as there is no other choice.

In that case, watching the video given below will be helpful for you.


How to locate the alternator in your vehicle?

The alternator is located at the bottom part of the engine in most vehicles. It’s easy to locate as you can visually see it. If you can’t see it, follow your serpentine belt and you’d be able to easily find it.

Can a bad alternator cause sensor problems?

The ripple effect of a malfunctioning diode in an alternator can cause the illusion that your camshaft position sensor has gone bad. It can even trigger a false error code and replacing the sensor won’t solve the issue here.

When is the right time to replace an alternator?

If you see the warning signs like dim or flickering lights, test your alternator with a multimeter. If the test shows that it’s providing less voltage than required, you should replace it right away.

Can a bad alternator cause a car to die while driving?

If you have a bad alternator, you’ll be relying solely on the power generated by the battery to drive your vehicle. You can drive for 5-30 minutes depending on how much charge your battery has and the car will eventually die.

Why won’t my alternator charge my battery?

There are many potential reasons why an alternator won’t charge the battery. The main ones are the alternator itself is bad, the battery terminals are loose, the battery has gone bad, or the serpentine belt has gotten loose.

How to get your alternator and battery checked for free?

You can drive your car to the nearest Autozone from your location. They’ll be happy to check your battery, alternator, and starter. All these things are free of charge.

Can a bad alternator keep you from starting the car?

A bad alternator won’t be able to charge your battery properly. Eventually, the battery won’t be able to provide the initial power required to start the car. So, yes – a bad alternator can keep your car from starting.


Can a bad alternator cause rough idle? If you’ve read this alternator guide so far, you should clearly know the answer to that question. While the chances of it happening are once in a blue moon, you know why it happens.

It’s much more common for the other reasons we mentioned to be the reason behind the rough idling of a vehicle. Diagnose your car carefully and figure out what’s causing the issue. Let us know what caused the problem and how you fixed it by leaving a comment below.

About John M

John contributed as a technical head at an automobile company just 2 years after his post-graduation in Automobile Engineering. He loves to lead a free life, so he left his job & started blogging. Now, he does research on every automotive problem, part & product and seeks a better solution & best products & shares his findings with his readers to help them as well as to minimize their struggle.

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