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A faulty alternator is unlikely to cause a rough idle, but it can in one specific scenario.

There is one exception, though.

While the engine is running, the alternator is in charge of powering electrical components. It is not in charge of the engine’s firing sequence.

As you’ll see, there is very little correlation between a bad alternator and your car running rough.

Nevertheless, if your car is idling rough and you’ve exhausted all the usual possible reasons, here are a few things that get your car running smoothly again.

Your Alternator Needs To Be Grounded

It is conceivable that the alternator is only occasionally grounded. Occasionally is the important word here.

If it weren’t grounded at all, it would be far more obvious that there was a grounding problem. 

One method of ensuring it isn’t the alternator is disconnecting it and seeing if that fixes the issue. Ensure your battery has enough juice to keep your car going for ten minutes without being charged by the alternator.

Turn off all lights, the car stereo, and the air con. If the idle becomes smoother when the alternator is disconnected, the issue is with the alternator – or its grounding.

Here’s why.

The alternator provides power to the crank position sensor. If the ECU gets a bad signal, it may fail to fire the coil at the right time. When this happens, it alters the sequence of the cylinders firing slightly. When this happens, it causes misfires and makes the car sound rough.

Disconnect Battery

Another way to find out what’s wrong is to disconnect your battery while your engine runs and see if the issue continues. Also, ensure that your battery’s voltage levels are stable – this also applies to the overall voltage in the system.

Connect your vehicle to an OBD reader and check for the error code P0562. This is a code that indicates a faulty alternator.

Have your O2 sensors and ECU inspected by a mechanic if you don’t have a reader. Even if the alternator is in good working order, an issue with either of these parts can create issues with the engine’s air-fuel mixture. If all other electronics are working properly, this could likely be a cause.

Finally, double-check your wire harness. This is in charge of delivering electricity to the systems that require it; a malfunction can cause problems with the ignition system, sensors, or the ECU.

Other More Likely Causes of A Rough Idle

A Fuel Injector Clogged Up

When an injector becomes clogged, the correct amount of fuel does not enter the combustion chamber.

Running lean means that air fills the greater proportion of the volume in that cylinder.

Vacuum Seal Defective

This can result in chamber decompression, which can disrupt the fuel-burning process.

A misfire can be caused by a loss of pressure or extra air caused by such a leak.

A faulty exhaust-gas circulation system could also be the cause. If the exhaust gases do not exit the system properly, or if their flow is hampered by a clogged exhaust pipe/catalytic converter, they can cause problems with the car’s sensors.

As a result, there is a rough idle or even a stall.

Faulty spark plugs

The average spark plug’s life has increased dramatically, led by the advancement of electronic ignition systems. What used to be a 12,000-mile service life has since increased to 100,000 miles or over.

How well the other parts in your car are maintained affects the wear and tear. Bad maintenance can speed up carbon deposits that will stop them from firing at full capacity, — resulting in an erratic idle.

The typical cost of the entire set replacement is $300 – $500, depending on the make and model of your car.

However, other parts play a role in making a spark happen before your car’s spark plugs do their job.

Coils, onboard computer sensors, and extensive related circuitry are examples of these.

When the engine is turned off, the battery is responsible for powering the electrical systems; when the engine is turned on, the alternator takes over and recharges the battery.

The alternator may affect your spark plugs and other electronics, resulting in it idling erratically, but it isn’t very likely to be the main cause.

Common Symptoms of a Faulty Alternator

Dimming and Brightening of the Headlights

This could be caused by fluctuating voltage as the alternator attempts to keep a constant rate.

Engine Stalling or Difficulty Starting

These are common problems caused by a faulty alternator, with the former being the more likely.

This is because your alternator isn’t providing enough power to the spark plugs and coils.

Alternatively, it could be that it is not charging the battery well enough.

Burning Smell

The odor of burning rubber or power cables could signal that components inside the alternator are wearing out.

Because the alternator’s drive belt is subjected to the engine’s heat, its rubber may begin to smoke.


It is unusual for the alternator to cause a rough idle. However, it’s worth investigating the possibility if you’ve tried to address and debug all of the more likely causes.

The most obvious check is to ensure that the alternator is grounded to the engine block. If it is not grounded, you’ve found the cause of the problem. Reconnect, and you’ve fixed it for no cost.

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