Driver's Guide

How to Diagnose Alternator Problems & Test if Bad

You want to know how to diagnose alternator problems as this could get you stranded at the most inconvenient place. Not only that, but you also need to fix the problem, perhaps by yourself or with professional assistance such as a mechanic. Usually, when the alternator is failing, you’d have a dead battery, the light will be dim, and you’ll have difficulty starting, among others.

How to diagnose alternator problems

How to diagnose alternator problems

It’s usually recommended to have a professional handle vehicle diagnosis. Nevertheless, it’s okay to test your problem-solving skills if you have the experience. Below are some of the ways to diagnose alternator problems:

  1. Difficulty starting or regular stalling

Most cars are typically unique, and starting your car or keeping the car running may be one of the ways to diagnose alternator problems. However, numerous other mechanical issues can cause stalling or difficult starts. To determine the source of the issue, it’s important to examine all other aspects of your car.

If the difficult starts are the result of alternator problems, the car starts by drawing power from the battery. Also, if the alternator is not properly charging the battery, the battery will not be able to reliably and smoothly start the car.

If the car continues to stall even after it has been running, the spark plugs may not be receiving sufficient power from the alternator to keep the engine running. In other words, your battery is not yet dead, but it is close to.

  1. Slow or malfunctioning accessories

If your car’s alternator isn’t supplying sufficient power to its electronic components, you will experience sluggish or non-functioning accessories. If your windows take longer than usual to roll up or down, if your seat warmers feel “off,” or if your speedometer and other instruments begin to behave erratically, you may need to diagnose alternator problems.

Many modern cars are also equipped with a priority list of equipment that instructs the onboard computer where to cut power first if the alternator is unable to provide sufficient power. Thus, if your alternator fails while you’re driving, your radio (or other non-essential accessories) will lose power.

  1. Strange smells from the vehicle

Another helpful way to diagnose alternator problems in your car is when you perceive strange smells. This simply indicates that your alternator is working excessively hard and is close to overheating.

An overworked alternator can emit a similar smell. An overheated alternator that pushes too much power through the rotor and stator can cause a hot wire smell.

You may get a burning rubber smell; this is due to the rubber belt that drives the alternator and the rubber sheathing on the wires that connect everything in your electrical system, this is a distinct possibility. Incorrectly aligned or obstructed pulleys will increase belt friction, resulting in increased heat and the smell of burning rubber. If they become too hot, your nose will likely let you know.

Not all bad smells indicate potential alternator problems. The smell is distinct from other notable car smells, such as the smoky sting of burning oil, the smell of overflowing coolant, and the sizzle of overheating brakes.

None of these smells are desirable, so if you detect any of them, you should visit a professional mechanic. Remember that the absence of a foul smell does not necessarily indicate that everything is fine. In other words, a foul electrical smell will help you narrow down the source of your problem, but it is possible that the alternator could be failing even without these smells.

  1. Growling or whining noise

Cars emit a plethora of peculiar noises, some of which are harmless while others may indicate the need to diagnose alternator problems. If you ever hear growling or whining noises emanating from under the hood, you may have alternator problems that require immediate professional attention.

When the belt that turns the alternator’s pulley becomes misaligned or rubs against the pulley’s side, this growling or whining noise occurs. This noise may also be heard if the bearings that spin the rotor shaft are failing.

  1. Bad belts

When opening the hood, mechanics are accustomed to discovering a missing belt, a belt hanging from the engine block, or a belt that is so loose that it is flapping around the engine compartment.

Since a broken or loose belt is fairly obvious, check under the hood to see if your car has any of these problems, even if you are unable to repair them yourself.

If you’re familiar with basic maintenance and your car’s components, you’re in a good position to detect a problem before it leaves you stranded. A quick visual inspection of the belt for cracks, excessive wear, and other age-related issues can indicate an impending problem.

Remember that the belt must have the correct tension for the alternator to function properly; too much tension is just as bad as too little. Typically, a quick examination of the belt tension is sufficient to determine if a problem exists. Ensure that the engine has cooled before you begin working underneath the hood.

  1. Dead battery

A dead battery is a more common way to diagnose alternator problems, which may indicate that your battery may have reached the end of its useful life, or you may have left the headlights on overnight by accident. On occasion, a dead battery may indicate that you have alternator problems.

If your car is experiencing alternator problems, your car will not charge the battery sufficiently while the engine is running, causing the charge to deplete more rapidly than usual.

Jump-starting the car is one way to determine whether the problem is with the battery or alternator. If you jumpstart your car and it continues to run, you may need to replace your battery soon. However, if the car dies again shortly after being jump-started, the alternator may not be providing sufficient power to the battery.

  1. Dim or overly bright lights

When an alternator begins to fail, it supplies erratic voltage to your electronic devices. Typically, this manifests as equipment that under or over-performs, such as headlights that are either too dim or too bright. You may also encounter flickering lights or lights that fluctuate erratically between bright and dim.

  1. Flickering lights

On the previous number, you learned that dim or overly bright lights are typical to diagnose alternator problems. If the lights are not actually dim, they may be flickering. 

Usually, but not always, these two symptoms occur together. If your lights are bright, then dim, then bright again (either on a regular beat or seemingly at random), you have a serious problem with your electrical system, and the alternator is most likely to blame.

The logic is the same as if the lights were dim — the alternator isn’t producing enough power to keep the lights on at full strength, and instead of dimming all of the lights in response, the car is attempting and failing to maintain the proper lighting output.

If you desire to be certain that your car is experiencing alternator problems and not something else in the electrical system, all you have to do is search for a pattern such as determining why the lights flicker when you use other electrical components. You can check components like the radio, climate control, or power windows.

  1. Battery warning light on the dash

When the battery warning light illuminates on the dashboard, it is frequently misinterpreted as a battery-specific problem. However, the battery warning light indicates that there may be an issue with your car’s electrical system, including the alternator.

Alternators are typically designed to operate at a voltage between 13.5 and 14.5 volts. If your alternator is malfunctioning, its voltage may drop below capacity, triggering the battery warning light to illuminate. Similarly, the battery light will illuminate if the alternator exceeds its voltage limit, depending on its level of stress.

Depending on the electrical load from your car’s accessories (headlights, wipers, radio, etc.), the alternator may fluctuate in and out of its intended voltage capacity, causing the battery warning light to flicker. While this may seem like a minor inconvenience, it is preferable to have your alternator inspected than to become stranded on the side of the road.

How do I test to see if my alternator is bad?

The steps below describe how you can test to see if your alternator is bad with a multimeter.

  • Get a multimeter.
  • Adjust your multimeter’s DCV (DC Volts) above 15 volts.
  • Clean the positive and negative terminals of your alternator.
  • Connect the black cable to the multimeter’s negative terminal and the red cable to its positive terminal.
  • Look for an alternator reading of approximately 12.6.
  • Start the vehicle and check the gauge for reading between 14.2 and 14.7.
  • A reading greater than 14.7 indicates that the battery is being overcharged, while a reading less than 14.2 indicates that the alternator is undercharging the battery.
  • Turn on the lights, radio, fog lights, fan, and other electronic components while ensuring that the voltage reading does not fall below 13.
  • Make sure the reading is above 12.6 before you turn off the car.
  • If any of these readings are off, your alternator may be malfunctioning and you may need to visit a professional mechanic.


Can you diagnose an alternator without removing it?

You can diagnose an alternator without removal. Use a multimeter to test the battery, listen for a squealing sound from the engine, or examine the radio for a fuzzy sound.

What happens if the alternator fails while driving?

If the alternator fails while the car is being driven, the car will gradually lose power. The headlights will begin to dim just before the car loses power and dies.

What can cause alternator problems?

Several factors can cause an alternator to fail:

  • Incorrect use of jumper cables; overburdening the charging system with too many accessories.
  • Leakage of fluids, or;
  • A tight belt may cause bearing damage.

How does a bad alternator sound?

A grinding or whining sound indicates a faulty alternator. Even a minor rattling noise emanating from the engine may indicate a bad alternator. The majority of alternator noises are caused by a loose bearing.


As mentioned earlier, the diagnoses of alternator problems to check for include: no-starting and trouble starting, dimming lights, and problems with the stereo system’s output. Also, if your car starts but stalls while driving, the battery is likely not being recharged due to a bad alternator.

Latest posts by Bernard Juchli (see all)

Bernard Juchli

Bernard Juchli is an experienced racer, mechanic and team owner who trusts Avon Tyres.Bernard is the lead driver and force behind his Big Dog Garage Race Team. He is the General Manager and Chief Mechanic of Jay Leno’s Garage. Bernard and his crew of seven are responsible for all repairs, restoration and fabrication of Jay’s incredible automobile and motorcycle collection.

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button