What Are The Symptoms Of A Bad Alternator?

All of us had problems with the car alternator at least once in a life. An alternator has a lifespan of 5-7 years or 80,000 miles. I cannot say for sure how long will your alternator lasts because it depends on various elements like, how much you drive your car, the temperature outside, do you have a garage, etc.

Symptoms of a bad alternator are dim lights, service engine light, odd noises, electrical issues, engine stalling and a dead battery. If your alternator is completely broken your car won’t start because the alternator will drain your car battery and therefore you cannot even start your car.

I had issues with my alternator and changed it twice, so I can say that I am an expert when it comes to bad alternators. Allow me to get into details and explain the symptoms of a bad alternator:

How To Diagnose A Bad Alternator?

I’m going to show you how to test your alternator to make sure it’s working right. Some symptoms you might have with a bad battery or bad alternator is:
  • Your battery light might be on on the dash,
  • The engine cranking slowly or won’t crank at all,
  • The battery seems weak,
  • Interior lights or headlights seem weak,
  • Rumbling or squeaking noises coming from the alternator which indicates that there could be a bad bearing.

These are symptoms of a bad alternator. In order to test the alternator, you will need a multimeter (alternator testing tool). So,t you want to set your multimeter to DC v, which is DC volts, and you want to set it above 15 or 20. You want to make sure your positive and negative battery terminals are clean so that you’ll get a reading when you do this test. Put the black plug on your negative and red on your positive terminal.

You should get a reading of around 12,6. It could be a little bit lower, or a little bit higher. That’s not a big deal, but 12.6 is the magic number to shoot.

Next, start the car and ideally what should happen is the reading should go between 14.2 and 14.7 volts. That means your alternator is running correctly and charging the battery up correctly. If you’re getting over 14.7 volts, that means that your alternator is overcharging the battery and that could cause damage to the battery, so you don’t want that.

If it’s under 14.2, depending on how much under, let’s just say 13.2, that means your alternator isn’t strongly charging the battery. It’ll still charge it because it’s still above the 12.6 volts, but it’s not going to be enough to charge it when you run accessories such as your headlights or radio.

Next, while your car is running put on a load to the battery and alternator. Turn on your headlights, radio, off-road lights ( if you have ). The readings shouldn’t drop too much. We want to make sure the voltage doesn’t go below 13 volts. If it doesn’t go below 13 volts, your alternator is in great condition.

If Something Doesn’t Check Out While Doing These Tests, Check Out A Couple Of Things:

  1. First off you want to check out the connections on the terminals. If there is dirt on your terminals, sand them and get a really good connection. Sometimes the connection isn’t good, and that’ll cause the alternator to have a hard time charging the battery.
  2. Another spot to look at is the back of the alternator. Make sure all the wires are plugged in and secured.
  3. Check for any corrosion or damage to the wires, any frayed wires, any kinked wires. All that stuff could cause a problem.
  4. Also, your alternator might have an external voltage regulator. Check the wires to the external voltage regulator. Make sure that it’s going there. Make sure it’s clipped in and make sure the external voltage regulator is working.
  5. And finally, as the car is running, make sure that the alternator is spinning. Make sure that the belt is tight. You want to make sure your belt is tight on the alternator because that’s how you get your power.

Voltage Drop Test – Negative Terminal

This test is for the negative and positive sides of the battery. First, test the voltage drop on the negative side of the battery. Here is how to perform a voltage drop test:
  1. Start the car and turn on a bunch of accessories. You want your headlights on, you want your blower fan on, your radio on.
  2. Then connect the black lead to the negative side of the battery.
  3. Then touch the red lead to the alternator case. Make sure you have a good connection.
  4. Raise your RPMs to about 1500.

You should see a reading around 0.5 volts. You don’t want to see a reading that is 1.0 or 2.0 because then you have a problem. If you are getting a 1.0 to 2.0 reading, what you want to do next is take the red lead and touch it to the bracket, and if you see a substantial voltage drop that means the connection between the alternator and alternator bracket needs to be clean because the ground isn’t good. Now if you don’t find your problem between the connection of the alternator and the alternator bracket, then you should try the engine block. You want to be careful because the fan is running. If you see a substantial decrease in voltage from your alternator case to this then that would be where your problem is. At that point, I would clean the connection between the mounting bracket and the engine block. You can also check your chassis ground.

Remember to bring the RPMs up to about 1500 when you do all these tests. The idea is that keep testing the ground connections until you can find where the connection is bad. Once you find the bad connection sand it and get it clean so that the connection is good and then hopefully that will fix your problem.

Voltage Drop Test – Positive Terminal

Now we’re gonna test the voltage drop across the alternator and the positive side of the battery. To do this:
  1. Turn on the accessories on the car.
  2. Bring it up to 1500 RPMs.
  3. Connect the red lead to the positive battery terminal and the black lead to the B+ post on the alternator. The B+ post has a nut and a post, so make a connection. Here you should get a reading below 3.0.
  4. The next place you’re gonna want to check is the lead that’s coming from the alternator into the wire. There’s a metal piece between the nut and the red there on the alternator. So just touch it in there and then see the readings.
Keep testing all the connections until you can find where the connection is bad. Once you find that connection just sand it so it’s clean and good and you’re good to go.

How Much Does It Cost To Replace An Alternator?

Alternators typically last around 50,000-60,000 miles. So, if you had your car for many years, it is most likely that you will have to change your alternator at some point. So, when the time comes, you want to be informed about the prices of the replacement. A remanufactured alternator for a domestic car costs about $300-$500. Parts and labor are included. A new alternator is almost double the price $500-$1000.

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Can You Drive A Car With A Bad Alternator?

This is a great question. It depends on how bad your alternator is. It is certain that you cannot drive very long with a bad alternator. Once you start your car, you can drive your normal distance. The problem occurs when you turn off your car. Let’s say you stopped to refill your gas tank. If your alternator is bad, you won’t be able to start your car. The reason is that a bad alternator is not recharging your battery while driving, meaning that you will drain your battery while driving and when you stop your battery won’t have enough energy to start the car.

Why Is My Alternator Not Charging?

There are 5 reasons why your alternator isn’t charging your battery:

  1. Computer error: Most of us drive cars made in the last 20 years. Our cars have a central computer system. This system manages the alternator. All it takes is a small computer error and you will end up with a malfunctioning alternator.
  2. Broken belt: The belt produces the mechanical power that the alternator converts to electrical energy. This belt can easily break or get damaged.
  3. Blown fuse: Some cars have alternators with fuses. These fuses can break from power surge or old age. Check your owner’s manual to see if you have an alternator with a blown fuse.
  4. Wiring issues: There are many wiring components in the alternator. One disconnected wire can cause no power to the vehicle.
  5. Bad alternator: All products have a lifespan. An alternator has a 2-5 year lifespan depending on how much you drive your car.

I hope this article will help you to test your alternator and see if it needs replacement or just some good cleaning.

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Car Care Hacks

Hi there. I am a certified Automotive Service Excellence (ASE) mechanic since 2018 and a car detailer for 10 years.

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