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Home >   How To >   How To Test an Alternator >   Symptoms of a Damaged or Failing Alternator

Symptoms of a Damaged or Failing Alternator

The alternator supplies power to a vehicle’s electrical components like the headlights. The alternator tends to fail gradually, and its warning signs can signify future breakdown long before it occurs. Diagnosing a problematic alternator early can save you from a world of trouble. Avoid ignoring the tell-tale signs, and get the problem fixed before you’re stuck with an inoperable vehicle. Here we have explained some of the prominent symptoms and causes of worn-out alternators. If the alternator is beyond repair, view the best car alternators for sale here.



Symptoms of a Faulty Alternator

Difficulty Starting/Runs Rough


It requires a good deal of energy to crank the car’s motor, and if a battery is not replenishing its power properly due to a faulty alternator, it will become drained and ineffective. You will hear a clicking noise while attempting to start the car, and the engine will have difficulty turning over. The initial course of action is to check your battery, be sure to check your alternator for problems, as well.


Degrading Performance of Electrical Components


This warning sign is less obvious, but will progress in frequency. Over time, it will become evident that the alternator needs attention. Electrical components in the car will slowly start showing signs of failure: the headlights and dashboard lights will dim or the power windows will roll slower than usual. Eventually, these electrical components will cease to operate completely.


Growling or Whining Noise


The alternator is driven by a belt in conjunction with the crankshaft pulley. The alternator pulley typically spins about two times faster than the crankshaft pulley to produce the necessary power for the vehicle at lower engine speeds, especially while idling. If the pulley is not in correct alignment with the belt, or if the bearings are worn out, the alternator will make a growling noise. Be sure to perform a visual preventative check of the belt for cracking, excessive wear and other age-related issues.


Smell of Burning Rubber


The scent of burning rubber or scorched wires sometimes indicate an alternator failure. Friction between a misaligned belt and the pulley will emit a smell of burning rubber. The burning wire smell can be caused by an overheated alternator.


Warning Light


When your alternator does not perform up to its usual standard, the ALT or GEN light in your automobile's instrument panel will light up. If the electrical components of your vehicle do not seem to have their original potential, it is recommended that you check your alternator along with them.



Causes of a Bad Alternator

  • One of the most common problems of a faulty alternator is a loosened belt.

  • The diodes of the alternator can go bad, failing to transmit the required voltage.

  • The bearings on which the alternator rotates can also be at fault and hinder the alternator from providing an optimal performance.

  • If there is any problem in the field voltage received by the alternator, it will not function up to its optimum potential.


    How to Test Your Vehicle’s Alternator

    Most times, you might be confused as to whether the problem is with your car's battery or alternator, as they work hand-in-hand with one another.


    The following tests can help you ascertain the actual defective part:


  • Allow the engine to run and remove the negative cable of your vehicle's battery. If you notice the automobile stalling, the problem is with the alternator. Otherwise, the problem lies with the battery.

  • Start your vehicle and have someone check the headlights. With the vehicle in “park” mode, press the accelerator pedal. If the headlights dim in intensity or turn off altogether, the problem is with the alternator. If not, the alternator is probably doing its job fine.


    How to Test Your Charging System Using a Multimeter

    Along with your battery, your alternator is part of your vehicle’s charging system. Failing alternators are often the cause of discharged or even failing automotive batteries.


    If you suspect a failing alternator has caused your battery to discharge, start by attempting to charge your battery back up to 100% capacity. Most local auto parts stores will charge and then test your battery for free. You may be able to charge your battery at home using a Battery Tender or another battery charging tool; however, you will need access to a load tester to determine if your battery is still good. Once you determine your battery is still in good shape, install it back into your vehicle. Then follow these steps to determine if your alternator is the source of your vehicle’s electrical problems.


  • Start the vehicle. There’s no need to let it warm up.

  • Use a multimeter to check the battery voltage while the car is running.

  • Make sure the vehicle is in park or neutral.

  • Now rev the engine a little bit. It doesn’t take much.

  • At the same time, watch the reading on your multimeter. If you’re not sure how to rev your engine from the engine bay, have a friend help from the driver’s seat.

  • If the voltage goes up, your alternator is in good shape and does not need to be replaced.

  • If the voltage goes down quickly, stays steady, or slowly drops, your alternator likely needs to be replaced.

  • The voltage should never exceed 14.7V or drop below about 12.7V. If it exceeds 15V or drops below 12.7V when revved, you likely need a new alternator.


    Most large automotive batteries can be discharged and recharged at least 10 times; however, the smaller the battery, the more susceptible it is to damage from repeated discharging and recharging.


    Note that your battery needs to be in good shape for this test to reveal anything meaningful.




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