Troubleshooting Alternator and Charging System Problems

Updated on October 1, 2017
Dan Ferrell profile image

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

The alternator is at the heart of the charging system of your vehicle.
The alternator is at the heart of the charging system of your vehicle. | Source

Alternator problems and charging system problems can affect your vehicle in different ways. You may notice:

  • your ALT or CHARGE indicator light comes on,
  • your engine is hard to start and cranks slowly, or doesn't crank,
  • your battery demands lots of water,
  • your alternator makes noises,
  • or your headlights suddenly go dim.

These and other problems may be caused by a fault in the charging system.

Before you start replacing components with the hope of fixing the problem, you need to make sure what component or components are causing you trouble. Often, it is possible to diagnose and solve the problem on your own using a few simple tests and tools.

The sooner you find out what is causing trouble, the better. You'll prevent a faulty component from ruining other parts in the system, making your repair more expensive.

This convenient troubleshooting guide will help you do find out what you need to know, using a few simple checks you can do at home using a test light or digital multimeter (DMM) and possibly a few other common tools.

It is a good idea to have on hand the vehicle repair manual (or a good aftermarket manual) for your particular car make and model. It will help you identify wires, specific components, and possibly suggest some specific tests recommended by your manufacturer.

If you want to test the condition of the system as a diagnostic starting point, do the tests described in the following section, "General Charging System Check."

However, if you need help for a specific problem with your charging system, for example an over-charge or under-charge condition, discharging battery, or noisy system, then skip over to the "Charging System Problems and Potential Causes" section.

I. Common Symptoms of a Bad Alternator or Charging System
II. General Charging System Checks
III. Charging System Problems and Potential Causes
IV. Troubleshooting for Unusual Voltage System Drops
The battery indicator should turn off when your engine is running.
The battery indicator should turn off when your engine is running. | Source

I. Common Symptoms of a Bad Alternator or Charging System

Indicator Light On

Usually, the first sign that your charging system is in trouble is a "battery," "ALT," or "CHARGE" warning light illuminating on your instrument panel while driving. It means that for some reason, the alternator has stopped charging the battery and your car is running on battery power. If the light illuminates only intermittently, it usually indicates worn-out or bad carbon brushes in the alternator. But remember that the light doesn't always mean there is a bad part; the light may be triggered by a sensor giving a wrong message.

Engine Cranks Slowly or Not at All

When the battery is undercharged, the engine cranks slowly or doesn't crank at all. Go over the next section on "General Charging System Checks," and, if necessary, check the section "IV. Troubleshooting for Unusual Charging System Voltage Drops." further down in this article.

Battery Overheating and Using Too Much Water

An overcharged battery causes the battery to overheat. You find yourself adding water to the battery frequently. If the problem is not taken care of on time, it can ruin your battery. Besides going through the next section, make sure to check section IV below on "Troubleshooting for Unusual Charging System Voltage Drops"

Noises Under the Hood

Squealing, buzzing or grinding noises coming from the engine may be caused by bad alternator components, a loose or worn-out serpentine belt, or a faulty belt tensioner. See the "Checking for Alternator Noises" subsection further down in this article.

Dimming Lights

If you notice the headlights, instrument panel, or interior lights dimming, either your battery has a low charge, or there's one or more faulty diodes inside the alternator. Check your battery and see the "Alternator AC Voltage Leak Check" subsection further down in this article.

Check the drive belt for adjustment and tensioner for proper operation.
Check the drive belt for adjustment and tensioner for proper operation. | Source

II. Six Tests for the Charging System in General

These series of charging system checks help diagnose the general condition of your charging system. They can help you confirm that you actually have a problem in your charging system and locate the source of the problem.

1. Make a Visual Inspection of the System

With the engine off, pop the hood open and visually inspect the different components of the charging system. Look for a worn out or loose drive belt, alternator connection problems, loose alternator mounting bolts, and corrosion and dirt around the battery case and cables.

When inspecting the drive belt, check for adjustment. If necessary, adjust the belt with the help of your car owner's manual or repair manual. Inspect the belt for signs of cracks, cuts, shiny spots, deterioration or other signs of wear or damage. Change the belt and belt tensioner at the manufacturer's recommended interval.

2. Check for Wiring Problems

Inspect the wires and connections at the back of the alternator, at the remote voltage regulator (if applicable), and at the battery. Look for corrosion at the connectors, damaged wires, and burned and missing insulation that might suggest an electrical short.

3. Do This if the Indicator Light Stays on, or Goes on and off

Follow the next steps:

  • Start the engine and apply the emergency brakes. Set the transmission to Neutral (manual) or Park (automatic).
  • Connect a voltmeter across the battery terminals.
  • While watching the voltmeter display, start wiggling wires at the back of the alternator, the battery, and the remote voltage regulator (if there is one).
  • If the voltage reading on the meter display goes up while wiggling one of the wires, you've found a bad electrical wire or connection.

Instead of using a voltmeter, you can have an assistant watch the indicator light on the instrument panel to see if it turns off as you wiggle various wires and connectors. Once the light turns off, you've found the problem connector or wire.

4. Check for Alternator Noises

Bad alternator bearings, rectifiers, rotor shaft, stator winding, slip rings, brushes and other parts inside the alternator with mechanical or electrical problems can become noisy.

Here's a test you can do: Use a length of vacuum hose to listen for alternator noises. The hose will amplify the noise if it's coming from the alternator. But be very careful around moving parts while you do this test.

  • Start the engine.
  • Place one end of the hose against your ear and move the other end around different points of the alternator body.
  • Listen for whining noises (this can indicate a bad diode or an over-charging condition), grinding (bad bearing), squealing, or other abnormal noise. If necessary, have your alternator checked.

5. Check for Under-Charging and Over-Charging

The next three tests are best done using a load tester, but you still can use your digital voltmeter. You do this by measuring system voltage while loading the system.

5a. First, measure battery base voltage to make sure you have a fully charged battery.

  • Turn on the high beams for 10 seconds and then turn them off.
  • Wait for two minutes
  • Measure battery voltage across the battery posts with your DMM. You should get between 12.4 and 12.6V. This means your battery is fully charged. If you get a reading below 12.4V, charge the battery before continuing.

5b. Measure the Charging System's No-Load Voltage

  • Ask an assistant to start the engine and hold engine speed at about 1500 RPM.
  • Measure voltage across the battery with your DMM. You should get 0.5 to 2 volts higher than base voltage. If you are getting more than 2 volts above base voltage, most likely your alternator is over-charging the battery or the battery is faulty. Other potential problems are a faulty voltage regulator or a problem in the charging system wiring. As part of your wiring checks, see the section "Troubleshooting for Unusual Charging System Voltage Drops" below.

5c. Measure charging system load voltage with a high-current-condition system measurement.

  • Ask an assistant to start the engine and hold engine speed at about 2000 RPM. Turn on all electrical accessories like A/C, blower motor, headlights, defroster, wipers. But don't turn on the heated windshield if your vehicle has one; alternator voltage may increase to over 100V and this can be unsafe.
  • Take a voltage reading across the battery posts. Your reading should be at least 0.5 volts above base voltage for your system to keep up with electrical system demands. Otherwise, the charging system can't meet the demand and charge your battery. This fault could point to a faulty alternator or voltage regulator. Check the section "Troubleshooting for Unusual Charging System Voltage Drops" below to check for wiring problems that can lead to this condition.
  • NOTE: these measurements correspond to an ambient temperature of about 70o F. The charging voltage will increase as temperature drops, and charging voltage will decrease as temperature goes up. So keep this in mind when making your measurements.

6. Check for Alternator AC Voltage Leak

Alternators use diodes to rectify alternating current produced by the alternator into direct current. When one or more diodes go bad, the alternator can cause all kinds of problems. AC voltage leak can cause your lights to dim and drain power from your battery, for example. Usually, you can detect this leak by measuring AC voltage across the battery posts using a digital multimeter.

  • Start and let the engine idle.
  • Set the parking brake and your transmission to Park (automatic) or Neutral (manual).
  • Set your meter to a low AC voltage range and take your measurement.
  • If you detect even a small amount of AC voltage, replace the alternator.

Once you've determined the condition of the system, you can go over the next section to zero in on the potential problem(s) that may be causing the condition of the charging system.

A single bad battery cell can prevent your charging system from working properly.
A single bad battery cell can prevent your charging system from working properly. | Source

III. Charging System Problems and Potential Causes

To speed up your diagnosis and repair, look up the system condition that most resembles your problem and go over the potential causes described under that condition. Some checks or tests may be suggested as appropriate.

1. If the Battery Seems to Stay Undercharged

You have already charged your battery a couple of times and you just found out the battery is undercharged again. Before you blame your battery, or the alternator, here are some preliminary checks you may want to do.

a) Check the drive belt or serpentine belt, especially if you haven't replaced it in the last five years. Make sure the belt has the proper tension. Look for signs of wear or damage like cuts, shiny spots, missing chunks. Today serpentine belts don't show signs of wear, even if they need to be replaced. Use a belt wear indicator or check your maintenance schedule for the replacement interval suggested by your car manufacturer. You may find the schedule in your car owner's manual or your repair manual.

b) Along with the drive belt, check the belt tensioner as well. Make sure it turns freely and is noise-free. Car manufacturers recommend replacing the tensioner at the same time you replace the drive belt.

c) Make sure your battery connections are tight and clean. However, just because the terminals look clean, it doesn't mean they are tight and in good condition. Look for damage to the cable and the terminal itself that may prevent proper electrical current flow.

d) Check your battery yourself, or take it to your local auto parts store. Many of these stores will test your battery for free.

e) There's a parasitic battery drain stealing power from your battery.

  • A quick test is to connect a test light in series between the positive battery cable and the positive battery post. If the test light comes on, there is an electrical drain in one of the systems.
  • First, unplug the alternator electrical connector. If the test light goes out, the alternator is causing the drain. If not, locate the parasitic drain.

f) Don't overlook the starter motor: a failing starter motor may draw excessive current during operation, draining battery power. If necessary, have your starter motor or starting system tested.

2. If the System Seems to Be Over-Charging

Besides test results, another potential sign that your battery is being overcharged is that your battery terminals keep accumulating corrosion.

Here are some checks you want to do:

  • Make sure that all the connections to the alternator, remote voltage regulator (if applicable) and battery are clean, tight and in good condition.
  • Check your battery or have it tested at your local auto parts store. A bad battery cell can cause the alternator to over-charge the good battery cells.
  • Check for a bad alternator voltage regulator and circuit. Consult your vehicle repair manual, if necessary.
  • Make sure you have good ground connections at the alternator (alternator case) and voltage regulator, especially if your vehicle uses a remote voltage regulator. Grounds should be free of rust, and the alternator and remote voltage regulator mounting bolts should be clean and tight.
  • Check the alternator rectifier or have your alternator checked at the auto parts store.
  • Also, conduct the tests described in the Troubleshooting for Unusual Charging System Voltage Drops below.

3. If the Alternator Is Not Charging the Battery

When your tests point to an under-charge or no-charging condition:

  • Make sure the drive belt is not loose or worn out.
  • If necessary, borrow a good battery just to operate the engine and confirm that your drive belt and belt tensioner are operating properly.
  • Manually check the belt tensioner for proper operation. Make sure the tensioner pulley turns freely and without noise. Check it for damage.
  • Manually turn the alternator pulley and make sure it turns without problem. If one of the bearings has seized, it won't allow the alternator to turn freely.
  • Check that the connections at the battery, alternator, and remote voltage regulator are clean and tight.
  • Check for a blown fuse or fusible link. Consult your vehicle repair manual, if necessary, to locate the alternator fuse or fusible link.
  • Do the tests described in the section "Troubleshooting for Unusual Charging System Voltage Drops" below.

4. If the Engine Makes Noises

Noise can be a sign of alternator problems when it comes from the charging system. The next check points will help you isolate the source of the noise.

  • Check the drive belt or serpentine belt. Look for signs of wear or damage like cuts, shiny spots or missing chunks under the belt. A worn out belt can squeal during operation.
  • Also, check the belt tensioner for proper operation and damage. It should rotate freely.
  • Make sure the alternator is properly mounted. Loose mounting bolts can cause the alternator drive belt and drive pulley to become misaligned and noisy.
  • Check the alternator for noises. See the General Charging System Checks section for a simple procedure for this.

5. If Lights Dim During Engine Operation

Most alternators use diodes inside a rectifier to turn Alternating Current (AC) into Direct Current (DC) for use by the system. When one or more diodes go bad, AC can leak into the electrical system. Sometimes you can notice this when the headlights, instrument panel lights and other interior lights dim during engine operation. To test your alternator for AC current leaks, see the Alternator AC Voltage Leak Check subsection in the previous section.

Measure system voltage and drop voltages using a digital multimeter.
Measure system voltage and drop voltages using a digital multimeter. | Source

IV. Troubleshooting for Unusual Charging System Voltage Drops

A charging system voltage drop check can help you locate the source of an under-charge or over-charge condition due to problems in the wiring or connections between the battery and alternator.

1. Start the engine and let it idle. Apply the parking brake and set the transmission to Neutral (manual transmission) or Park (automatic transmission).

2. Turn on the high beams, AC, wipers, and other accessories to provide a system load. Have an assistant raise engine speed to about 1500 RPM. But if your vehicle is equipped with a heated windshield, don't turn it on; this can make alternator voltage go over 100 V. Since you'll be working around alternator connections, this could be dangerous.

3. With your voltmeter test leads, touch the positive battery post and the B+ terminal connector at the back of the alternator.

4. Check your meter display. You should get around 0.2V or less of voltage drop. Getting 0.3V or more can lead to an under-charge condition. Check the connections in that part of the circuit for a loose wire, corrosion at the connectors or wiring damage that may prevent proper current flow between the battery and alternator. Also, check the electrical connections at the back of the alternator and at the voltage regulator.

5. Now repeat the test, but this time, connect your meter leads between the battery negative post and the alternator case.

6. Check your meter display. Again, you should get a voltage reading around 0.05V. If you get 0.1V or more, there's something wrong. This can lead to an over-charging condition. Checking that part of the circuit, make sure the battery ground connection is good, clean and tight. Add a temporary ground connection from the battery to the chassis. If this removes the high voltage drop, check the engine to body grounds connections. They should be clean, tight and in good condition.

Watch the next video for a visual reference on voltage drop checks.

Alternator problems are not uncommon after a few years of operation. A typical alternator may last anywhere from 8 to 12 years. So don't be surprised to find your alternator going bad or the system developing problems after a few years of trouble-free operation, even if you have maintained your car well.

The key point here is to do the proper diagnostic as soon as possible, because a bad alternator can ruin an otherwise good battery, and other components as well, depending on the fault.

Once you determine that your alternator is bad, you have several options. You can replace it with an original one from your dealer, an aftermarket replacement, or a rebuilt unit. Aftermarket alternators are a good option and less expensive, and many of them are just as good as their OEM counterparts. And rebuilt alternators are not as bad either. So consider your options.

Questions & Answers


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    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      4 days ago

      remove the belt and try spinning the AC pulley by hand. It should spin freely - if there's noise or the pulley feels rough,noisy, you'll need to replace it.

    • profile image

      Douglas Anayabere 

      5 days ago

      My compressor belt is dragging when i switch the aircondition on. What is the problem?

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      2 weeks ago

      Hi Ron,

      Check the voltage regulator.

      Hope this helps.

    • profile image


      2 weeks ago

      Put new battery and altinator on 2001 f150 and its still over chargeing is it the pcm?

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      3 weeks ago

      Hi Aaron

      Check the battery cables for corrosion and make sure they are tight. Also check the connecitons to the alternator and wires. Also, make sure the alternator belt is not loose or worn. If all that turns out okay, check the battery. An auto parts store will check the battery for you, if necessary.

      Hope this helps

    • profile image


      4 weeks ago

      My corolla 2e battery light remain even when the alternator is charging. What could be the fault?

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      7 weeks ago

      Hi Keith,

      I don't have the diagram with me but probably this pic might help. I believe is the same model.

      Otherwise you can check the repair manual in your local library.

      Good luck

    • profile image

      Keith Irons 

      7 weeks ago

      Need to know what does the spade connecter go to for 1982 bmw 320i on the alternater

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      8 weeks ago

      Hi Enos,

      They should test the circuit (wires and connections), ground (engine) connections and make sure the correct output voltage is coming out of the alternator and getting to the battery.

      Good luck

    • profile image

      Enos Kaufman 

      8 weeks ago

      alternator was changed even the one changed out tested fine Napa still flatlined im lost now

    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      8 weeks ago

      You might get occasionally over 16V from the alternator. Have your battery checked at a local auto parts store.

    • profile image


      8 weeks ago

      How about of light is on, but have 24.3 volts at the battery while running, 14.4 volts at the alternator, no dimming of lights or starting complications? 2018 ford F-59

    • WheelScene profile image


      12 months ago from U.S.A.

      Awesome detailed post, thanks for sharing!


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