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How to Test an Alternator for Problems

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

How to Test Alternator

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 lose brightness, dimming dashboard lights, or a battery that loses power, may also indicate charging system problems. Still, you need to confirm the alternator has failed before you replace it.

These next tests will help you check the charging system operation.

1. Alternator Output Test

One of the most common problems with an aging alternator arises from worn-out brushes. But a charging circuit blown fuse, a broken fusible link, a failed diode in the alternator's rectifier assembly, or 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 the 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 ask 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 the 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 levels 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, especially 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 the proper operation of the belt adjuster and idler pulley as well.

A Useful Skill

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.