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How to Test an Alternator for Problems

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Alternator problems manifest themselves in various ways, depending upon the particular failure. Still, using a few simple tests at home that show you how to test an alternator, how to inspect a battery, if necessary, and check a drive belt will help you pinpoint the cause of the problem.

These tests only take a few minutes, and you don't have to remove the alternator from your car.

For the main tests, you'll only need a digital multimeter. However, if you have to test the battery, you'll need a hydrometer tool as well.

It's a good idea to buy the service manual for your particular vehicle model so that you have the electrical specifications and particular procedures that apply to your car model. The manual will pay for itself if you plan on doing some maintenance and simple repairs yourself.

You can get an inexpensive aftermarket manual at your local auto parts store. Also, Amazon offers many of these repair manuals at low prices as well.

NOTE: Some auto parts stores will charge your battery and check the operation of your alternator free of charge. If you don't have a digital multimeter or a hydrometer and can't buy the tools now, you can take advantage of this to establish the condition of both system items at once.

What We'll Cover

Here is an outline of the content you'll find in this guide. Skip to a particular section if you want to check or test it first.

I. Alternator Problems Troubleshooting
1. Alternator Output Test

  • Measuring Battery Base Voltage
  • Measuring Charging System No-Load Voltage
  • Measuring Charging System Load Voltage
  • Checking the Voltage Regulator
  • Replacing a Voltage Regulator
  • Checking for a Bad Diode

II. Battery Testing
1. Inspecting the Battery

  • Performing a Hydrometer Check

III. Checking the Drive Belt

Common Causes of Charging System Problems

  • Bad charging and starter system's connectors and wires.
  • Corroded or loose battery terminals.
  • Low battery electrolyte level.
  • Faulty battery.
  • Blown charging circuit fuses or fusible links.
  • Worn out or loose drive belt.
  • Loose alternator mounting bolts.
  • Worn out alternator bearing or other internal components.

Alternator Problems Troubleshooting

A red-battery or ALT warning light on your dashboard coming on warns you of alternator problems. But headlights that suddenly loose brightness, dimming dashboard lights, or a battery that looses power, may also indicate charging system problems. Still, you need to confirm the alternator has filed before you replace it.

These next tests will help you check charging system operation.

1. Alternator Output Test

One of the most common problems on an aging alternator arise from worn out brushes. But a charging circuit blown fuse, a broken fusible link, a failed diode in the alternator's rectifier assembly, even a worn out bearing will affect charging system operation.

So, using a digital multimeter, let's start by checking how much voltage your charging system is delivering.

1.1 Measuring Battery Base Voltage

NOTE: If your battery is discharged, try to borrow a good battery for these tests. That way you'll quickly check whether the charging system is the one causing trouble. Otherwise, you may want to concentrate on checking your battery first. So go to the battery tests outlined below.

Set your voltmeter to a setting of about 20 on the DC voltage scale, and connect the probes across the battery posts following the proper polarity. If your battery is fully charged, you'll read about 12.6 volts. Make a note of your reading. You'll use this voltage as a reference point for the next two tests.

1.2 Measuring Charging System No-Load Voltage

* Start the engine and let it idle.
* Ask an assistant to keep engine speed at about 1500 rpm.
* Turn off all vehicle accessories.
* Connect your voltmeter across the battery terminals.
* Make a note of the voltage reading.

You should get between 0.5 and 2 volts above your base voltage. Any voltage higher than this will indicate an overcharging condition. The problem may come from the alternator or the electrical circuit.

However, if your voltage reading remains the same or slightly lower than base voltage, the charging system isn't working. You may have a poor connection in the charging system (check that connectors and wires are in good condition and making good contact), a failing alternator or voltage regulator, which usually you'll find mounted on the alternator's case in modern vehicles.

Visually inspect wires and connectors, and refer to the "Checking the Voltage Regulator" section below, if necessary.

1.3 Measuring Charging System Load Voltage

If the charging system passed the previous test, you need to see if the charging system produces enough power for your battery and the different electrical systems.

* This time, ask your assistant to start the engine, and increase and hold engine speed to about 2000 rpm.
* Now, turn on all accessories like radio, air conditioning, headlights, and wipers.
* Connect your voltmeter across the battery terminals.
* Make a note of your voltmeter reading.
* Subtract the base voltage from your load voltage reading — this is your charging voltage.

Your charging voltage should be 0.5 higher than your base voltage. Otherwise, you may have poor circuit connections (inspect wires and connectors), or a faulty regulator. Continue with the section below.

2. Checking the Voltage Regulator

If the charging system didn't pass the No-Load or Load test, check your alternator's voltage regulator. Charging systems come under different designs. So consult your vehicle service manual to test the voltage regulator.

These tests include a "full-fielding" test, which is nothing more than bypassing the voltage regulator, and it's not as hard as it may sound. Your manual may also asks you to check a fusible link for continuity and inspect fuses related to the charging system and tell you their locations.

NOTE: On some modern vehicles, the powertrain control module (PCM) — the engine's and transmission's computer — regulates voltage rate fed through the alternator. With this type of system in place, your service or repair manual will tell you what type of tests you can do in your own garage.

2.1 Replacing a Voltage Regulator

If your tests determined that you have a bad voltage regulator, check your local auto parts stores or online for availability. In many alternators the voltage regulator mounts on the outside of the case and you'll replace it in a matter of minutes. Keep an eye on small nuts, screws and springs. They are very easy to lose.

However, if your alternator is four years old or more, replace the whole unit. The different components in your alternator will soon start to show signs of wear. You'll save more on a new or rebuilt unit now than having to deal with more alternator problems later on.

If you have the luxury of waiting a couple of days or more before replacing a bad alternator, check for prices online, you may find a brand new, quality alternator at a lower price than a good, rebuilt unit from your local auto parts store.

3. Checking for a Bad Diode

Usually, a bad alternator diode will cause your headlights or instrument panel lights to flicker or dim and, sometimes, drain battery power overnight, or in minutes.

* To check for a possible bad alternator diode, switch your voltmeter to a low setting on the AC (alternating current) voltage scale.
* With the engine running, touch the meter probes to the battery terminals.
* Your voltmeter should read 0 AC volts.

Any amount of AC voltage would indicate a bad diode, so you'll need to replace the alternator.

Battery Testing

In addition to alternators, batteries are also a common source of charging system problems for two main reasons. First, if your battery sees little use because you don't drive your car often, the battery sulfates. Although the battery still provides good voltage, it won't deliver the correct amperage. Second, if you use your car regularly, you may forget to service your battery regularly, which may result in low electrolyte level or corrosion buildup, leading to poor electrical flow.

1. Inspecting the Battery

Check the battery terminals. If they are corroded, this might be your problem. Corrosion prevents proper electrical flow. If necessary, disconnect the terminals and mix a tablespoon of baking soda in 8 ounces of warm water. Apply the solution with a soft brush to the terminals. Clean the battery posts as well.

If your battery has filler caps, clean the battery top and carefully remove the caps using a screwdriver. Check the electrolyte level. Add distilled water to the cells to bring up the electrolyte to the correct level and recharge the battery if necessary.

If you need to recharge the battery, and you are going to recharge it at home, make sure to follow safety precautions. A charging battery produces hydrogen gas, which is highly explosive. Charge the battery in a well-ventilated area away from appliances with open flames like dryers and water heaters. Also, follow the instructions that come with your battery charger, specially when connecting and disconnecting the battery.

2. Performing a hydrometer check

The hydrometer test will let you know whether your battery is fully charged, needs a charge, or has failed. A hydrometer is an inexpensive tool. Buy one at your local auto parts store, and then check this article on troubleshooting a bad car battery using a hydrometer.

Basically, a reading below the 1.265 mark usually means your battery needs charging. A difference of 25 to 50 points between one or more cell readings means your battery is defective.

Checking the Drive Belt

Can a drive belt cause charging system problems? Yes, if the belt has become loose, or you haven't changed the belt at your manufacturer recommended interval.

Check the belt's tension and condition. The belt will fail to operate the alternator if it's too loose, or it's stretched due to years of service, causing it to slip over the pulleys. A slipping belt will usually produce a squealing noise.

Visually inspect the belt for cuts, tears and missing chunks.

Also, check the alternator pulley and the rest of the pulleys the belt rides on. If one of them is misaligned or not turning freely, it'll damage the belt as well.

Your service manual will offer you a good procedure to check not only your belt, but proper operation of the belt adjuster and idler pulley as well.

You'll deal with alternator problems more effectively if you know how to test alternator output voltage and conduct other related tests. Still, don't overlook the obvious. Sometimes, there's where you need to start troubleshooting. For example, when doing a quick visual inspection, you may notice corrosion on the battery terminals (preventing power in and out of the battery) a wobbling drive belt while the engine is running (worn out or damaged belt) or a noisy alternator (failing pulley). And keep in mind that, more often than not, charging system — and other systems — problems arise from lack of adequate maintenance. Using your best judgment and the tests outlined here will help you pinpoint the cause of your charging system problems faster.

Test Your knowledge of Vehicle Charging Systems

For each question, choose the best answer. The answer key is below.

  1. Which of These Conditions is Not a Potential Charging System Problem
    • Coil low voltage
    • Noisy Drive Belt
    • Low battery power
    • Flickering Headlights

Answer Key

  1. Coil low voltage

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question: Is .043 AC volts a high voltage when doing the diode test?

Answer: Test between the battery terminal on the alternator and a good engine ground. Any AC voltage above .5 will point to bad diodes.

Question: When I drive for a long time with AC and all lights on in my Toyota VVT-i, the engine stalls. It will not restart unless a kick start is given from an external battery. After the kick start, all is normal, and the battery is fully charged. What could be the cause?

Answer: If you’ve seen this pattern – AC on – engine stalls after some driving, there could be a problem with the charging system not able to handle the demands from the AC; there could be a problem with the AC system itself, or an engine performance issue is causing the fault. You may want to download trouble codes from the computer, even if the engine light is not coming on. Pending trouble codes won’t trigger the light but you can still retrieve them.

Question: Can a faulty alternator cause a P0601 code in PCM?

Answer: If the battery is not properly charged, it can have an impact on computer performance. Have the alternator and battery checked.

The computer sets this code after conducting a self-check and finds an error in the module itself. The error could affect a memory module used to operate an engine system. This is considered a critical error and usually, the car's computer needs to be replaced.

This is a rare condition. Usually, this happens because the computer is not receiving a good, consistent voltage signal. This may be because of a faulty power wire (loose, corroded, faulty wire or connector), or a faulty computer ground (loose, corroded, faulty wire or connector), or one of the output devices in a system is faulty (shorted). Have the computer power and ground wiring checked. And make sure output devices are working properly. If everything is OK, the computer has to be replaced.

A faulty alternator would affect other systems as well. Hope this helps.

Question: My battery is charged but my car won’t start, what could be the problem?

Answer: Usually, the problem is with the ignition, fuel system or a sensor. Download trouble codes and see if there are any pending ones. This other post may help too:


Question: I have a Marelli alternator on a Massey Ferguson tractor, and it has one slip worn away to the plastic. A new slip ring is no longer available. Can the slip rings be rebuilt?

Answer: You may be able to rebuild the rings. This video is helpful:


Question: When driving, my battery light doesn't come on. All the dials just seem to stop working. And when I release the gas pedal, it works again. What could the problem be with my car's dashboard?

Answer: There could be a bad ground. Check under the dashboard panel, by the accelerator pedal. A wire or harness connection may move every time you depress the pedal. Hope this works.


Dan Ferrell (author) on June 29, 2020:

If this happens only when turning on accessories, may be a loose, damaged, or corroded connection at a battery cable or a bad fusible link can cause the engine to shut off. That could be the reason why you don't hear a click when trying to restart the enigne. Also, a bad ignition switch can also cause this type of fault. Check the connections at the battery, trace the battery cables to the other end and check those connections as well. Check the ground connection to the frame and engine grounds as well. If necesssary, chekc the alternator too.

These other posts may help:



james on June 29, 2020:

hi dan i have a nissan navara when i jump start my car then go for a drive then turn heater fan on and lights and other things turned on then car stalls then wont click over

Dan Ferrell (author) on November 07, 2019:

There might be a problem in the circuit. These other posts may help. Check engine grounds and charging circuit:



SunnY on November 07, 2019:

I've changed 3 alternators and 1 battery in month. Still the technician can't seem to figure out what the problem is. I need help please. Thanks!

Dan Ferrell (author) on July 05, 2019:

Have the battery checked, probably it's faulty and not able to hold the charge any more.

Charles Ajoba on July 05, 2019:

My car battery dies in two seconds

Dan Ferrell (author) on August 22, 2018:

If the charger said to reverse the cables, most likely the cables were hooked up to the wrong polarity. See if you didn't blew a main fuse. Hopefully damage didn't go beyond that.

Bennie Sweat on August 22, 2018:

My battery would not hold a charge, I replaced the alternator & voltage regulator. I installed a new battery. Soon the battery went dead. I returned to the auto parts house, they checked it and said it had a full charge on it, I asked how that could be, he said it was charged up in the wrong direction. I remember connecting a batter charger, and it had a message on it, "Reverse The Cables." what went wrong or what did I do wrong?

Dan Ferrell (author) on August 22, 2017:

Hi PW,

Probably it'll be worth to go to a local auto parts and have it checked for free, just in case.

Good luck.

Paul Wismann on August 22, 2017:

I fixed my alternator by tapping on it lightly with hammer. Pretty sure water had splashed up to it from very wet roads and the brush was stuck. Original problem was CHARGE LIGHT was on.

Dan Ferrell (author) on February 17, 2017:

Hi Greeny,

There can be many reasons for a lack of charge: alternator failure (voltage regulator, stator problems, slip rings worn out, etc), but make also sure your battery still can take the charge, check the alternator belt. You may want to take the alternator to a local auto parts store to get tested.

Good luck

Greeny on February 16, 2017:

I have a not very old bosch alternator and it has lost its charge? with my multimeter on the back of it and direct to negative it still has nothing, this was a running driving car and it just stopped charging??

Dan Ferrell (author) on August 04, 2016:

Hi Alvim

That is a tricky fault to diagnose. The best way to go about it is with a scanner that can access the sensor and check for voltage values. But before going that route you might want to have your alternator tested at Autozone first and make sure you don't have a faulty unit.

Good luck.

Marco Alvim on August 04, 2016:

Good day. I have a faulty transmission... it has a solenoid problem except the solenoids keep om changing which one is the faul... ive replaced the solenoids in the valve body but still same problem... i have checked and replaced wiring looms inside the gearbox and checked the plugs... all is 100% except that fault of it changing its problem solenoid... I have a sneaky suspicion that it is the voltage regulator sending incorrect voltages to the TCU as it sits in the valve body... Can this be an alternator problem? i am also getting voltage in the radiator which tends to be the voltage regulator.

The car is an R51 Nissan Pathfinder 4.0 V6

Dan Ferrell (author) on April 27, 2016:

Hi Holden,

You might want to check the belt for wear, tightness and oil or fluid leaks that may be causing the belt to slip. Also, check the pulleys the belt rides on for problems.

good luck.

Holdenhsvgtsmonaro on April 23, 2016:

I have problem with my drive belt when stop the car i can hear a noise coming from the belt please help me thanks...

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