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How Do I Know My Car Alternator is Going Bad?

How Long Can You Drive with A Bad Alternator

Your car’s alternator is an essential part. It keeps your car charged and running smooth. It also powers all your gadgets and entertainment like your radio, and GPS. So it is valuable to learn all you can about it, especially signs of a failing alternator, one that is about to go, and what it takes to change it. 

Auto Repairs Are EXPENSIVE


Signs Your Car Alternator is Starting to Go


Luckily, the alternator gives off many signs and symptoms it is failing or about to fail. For instance, if you go out at night and notice your headlights are unusually dim, that is a sign that you are having problems with your alternator. Another symptom is if your headlights flicker or if they are unusually bright. 


If you go to start your car and find your battery is dead, the first thing you will want to check is the age of the battery. If it should still have had life in it, check to ensure nothing was left on inadvertently. Still, if you have no answers, then it is probably your alternator. When you are driving with a defective alternator it drains the battery, leaving a dead battery at the end of the trip. 


Perhaps you are driving and the first thing you notice is a delayed reaction in  equipment including powered windows, seats, and it may seem as though your heated seats are off even though you turned them on. 


If you are having difficulty starting or keeping the car running, there are two aspects that the alternator affects. This would point to the alternator not charging the battery or not supplying the power needed for the spark plugs. 


If you hear a high-pitch whining sound coming from under the hood, it could be the alternator belt. 


If you smell something burning like rubber or wires it could be that your alternator’s drive belt is wearing away. 


You may notice a warning light in your instrument cluster, usually the battery light is lit up. Sometimes it will come on and off. Many believe it is a defective battery from this light. This light refers to the entire electrical system however. 


How can I tell if it’s the Battery or Alternator?


Signs of a Bad Battery


If your car is turning over but seems sluggish, it is most likely the battery. 

If the interior lights come on when you open the door, it is the battery. 

It is more difficult to get started on icy mornings but the lights still work. 

Corrosion on top or on the sides of a battery points to an issue with the battery. 

When jumping the car works then it is an issue with the battery. 


Signs of a Bad Alternator


The car may start but will then stall when you start to drive. 

Lights will dim and stereo will not work. Lights and instrument panels may flash on and off. 

There may be the smell of rubber burning. 

If the vehicle will not turn over but lights and stereo work, likely you are looking at a faulty starter. 


Can AutoZone Test Your Alternator?


As of the time of writing this article, most AutoZones were willing to test alternators at their location. This test will check to see if your vehicle is charging the system properly and if the battery is still sound. 


How Do You Test Alternators by Disconnecting the Battery?


First, park your car in an area free from trees and pets. Make sure the area is stable and level. 

Open up the hood and prop it up. 

Start the car which may be difficult to do if you are having problems with your alternator. 

Disconnect the battery from the terminals. 

If the car is still running, then it is not the alternator. 


How do I Test My Car's Alternator Using a Multimeter?


Step 1:  Open the hood and locate the alternator and belt. 


Step 2:  Turn the dial of the multimeter to 20 volts. 


Step 3:  Start the car's engine. 


Step 4:  Check the belt to make sure it is spinning correctly without slipping. 


Step 5:  Take the positive multimeter probe and touch it to the positive battery terminal. The reading with the engine hot should be between 13.7-14.4 volts. If it is higher than 15 volts, that means the voltage regulator which is on the alternator is starting to fail. 


How Far Should I Make it With a Charged Battery and a Failing Alternator?

This question all depends upon the car, the age of the battery, and how bad the alternator is. You should be able to make it at least three to four miles through. 


Is My Alternator Bad When My Lights are on, but the Car Won’t Start?


When your car won’t start but your lights come on, check the radio. If the radio works, then it is not the alternator. It usually points to a weak battery. If the battery is newer, check the battery connection, the terminals and the spark plugs. 


Still, no solution? Next, a check to make sure that no fuse has blown prevents the engine from starting. Another possible scenario is the ignition switch. Symptoms of a failing ignition switch include stalling and accessory issues. 


How Much Does it Cost to Replace an Alternator?


While the exact price depends upon your car make, model and engine type, we will provide a range. Replacing your alternator with a rebuilt one from the mechanic is typically in the $300 to $600 range. Getting a new alternator from a mechanic will cost you roughly $500 to $1,100. 


Should I Change My Alternator Myself?


Changing an alternator, as long as it is accessible, is one of the easier jobs in mechanics. 


Tools Needed


New or Rebuilt Alternator


Hand Tools include Wrench to fit bolts on alternator. 

New Serpentine Belt





First, as with most mechanic jobs, disconnect battery


Remove the Serpentine Belt by loosening it first. 


Remove the bolts and wiring harness from the alternator.


Take the previous alternator out of the car. 


Put the new alternator into the car. 


Re-attach the bolts and tighten them. 

Put the new serpentine belt on. 


Make sure it is tightened. 


Re-attach the battery


Start your car, allowing it to run for 15 minutes. 


Check battery and alternator by doing Voltmeter test. 


Do Alternators Fail Suddenly?


In some cars, the alternator will fail slowly with warning noises and flickering lights. In other cars, the alternator will fail suddenly, leaving you no warning time and limited time and miles to get home or to a repair shop. 


What is the Lifespan of an Alternator?


While it is impossible to give an exact time or mileage mark of when an alternator will fail, we can however provide a range. Knowing when you are in this range can alert you to replace it prior to its failing or to be more aware of symptoms that the car displays. 


The average time an alternator lasts is 5-10 years or 80,000 to 100,000. It depends upon your driving habits. You may drive only a few thousand miles but be on the road a great deal of time because you drive in the city, in which case it may only last 5 years or may not even get 80,000. At the other extreme, you may drive all highway miles and drive many miles in a short period, so for you it may reach 100,000. 


What Causes an Alternator to Fail?


While this can vary from car to car, we can discuss some common elements of what causes an alternator to fail. 


Bad Diodes


The alternator's AC currents are converted to DC by the diodes. The diodes are just one part of the rectifier system. Before hitting the battery and electrical system these electrical currents pass through 6 diodes. Short trips with stop and go traffic burn out the diodes as does driving at night with the lights and other accessories on. 


Bad Voltage Regulator


No matter what the cause, a bad voltage regulator requires a new alternator. Voltage regulators tend to fail when they get too hot. 


Bad Shaft Bearings


The shaft bearings are what allows the alternator to rotate inside. If the alternator is flooded with water or splashed, the shaft bearings may stop working. 


Faulty Wiring


There are two sets of wiring in an alternator system – one inside and one outside. The one inside helps induce current. The other set carries the current from the alternator to all the other electrical parts. Look at the wiring to see if you notice any fraying or cuts. New wiring can be replaced or resoldered. 


The best way to determine if there are problems with the writing is to do a voltage drop test. 


Voltage Drop Test for Alternator Problems


Why Should You Perform a Voltage Drop Test?


Before going out and getting a new alternator, starter, or battery, you should do a voltage drop test. This is because it is more likely to be an issue with the wire or wiring system than it is the starter or alternator. 


You may notice that your battery is dead or under charging. Maybe it is overcharging. When you try to start the car, it is slow to start, or there is nothing. Perhaps you have actually noticed issues with the wiring or observed the battery light on in the car. 


You’ve noticed loose or torn wiring in the electrical system. The battery connections are full of gunk and need to be cleaned. 


This test will help you find electrical problems fast. 


Performing the Voltage Test Drop


First you will need a voltmeter and the manual that came with your car to locate specific wires. 


Begin by using the emergency brake.

Start the car and leave it in the park for automatic transmissions and neutral for manual transmissions. 

Have your friend increased RPM to 2000 by depressing the gas pedal. 

Turn everything on in your car – headlights, radio, air conditioning, windshield wipers and anything else you can think of that draws electricity. 

Turn your dial on your voltmeter to 2 DC. 

Take the black lead from the voltmeter and connect it to the housing of the alternator. 

Take the red voltmeter lead and attach to the negative battery post. 

Ensure none of the leads are going to get tangled in any moving parts of the engine. 

Check to see what your voltmeter readout is. It should be between 0.3 and 0.5 Volts. 

If the reading is below 0.3, have the alternator checked out as it is not functioning as it should. 


Is it Dangerous to Drive with a Bad Alternator?


Yes, it is dangerous to drive with a failing alternator. This is because it can have complete failure at any time including when you are driving on a highway going 70 miles per hour in the fast lane. Your car will stall and that is a cause for a car accident. 


You will lose power, steering, brakes, and everything else electrical. 


Steps to Take if Your Car Dies While on a Highway


Try not to panic, you need to remain calm so that you can anticipate areas to pull off.

Turn your emergency flashing lights on, sometimes called four ways. 

Try to get your car off the road. Remember, likely you do not have steering or brakes. 

If you are in the fast lane, look for a median or someplace to pull into. 

Once you have identified a safe area, use the emergency brake to stop.

Restart the car if possible to get yourself to a repair shop. 

Put a white rag out your passenger or driver window, whichever sidecars going past will notice. 

Call someone for help. 


Does Water Damage an Alternator?


If you have driven through a deep puddle of water, enough to splash up into the alternator, then damage may be done. Damage may have occurred to the shaft bearings. Once they get wet, they may seize up and stop working altogether. 

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