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Why Is My Alternator Not Charging?

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

why-is-my-alternator-not-charging

An alternator not charging problem may be traced to:

  • Corroded, damaged, or loose wire or connector
  • Bad battery
  • Faulty alternator

Most car owners can diagnose this type of charging issue by visually inspecting and testing the system at home using a digital multimeter.

It is a good idea to have on hand the vehicle repair manual for your particular model. If you don't have this manual yet, you can buy a relatively inexpensive copy through Amazon.

Haynes manuals include:

  • Photos and images
  • Description of the different systems
  • Step-by-step procedures
  • Location of parts
  • Troubleshooting guides
  • Electrical diagrams
  • Maintenance schedule
  • Parts specification and torque values

Having this manual can also save you a lot of money just by keeping up with your vehicle's maintenance schedule to avoid unnecessary breakdowns. So you'll recoup your small investment in a short period of time.

Following a logical order, the next sections describe the systems or components you might want to check to find the cause behind a no-charge condition.

In This Article

1. Before You Start
2. Charging Voltage Ouput Test
3. Checking the Fusible Link
-- VIDEO: Locating the Fusible Link or Maxi-Fuse
-- VIDEO: How to Test an Alternator Fuse
4. How to Measure Your Alternator's AC Ripple
5. Checking for a Loose Drive Belt
6. Checking for Voltage Drop
-- 6.1 Check Power Side Voltage Drop
-- 6.2 Check Ground Side Voltage Drop
7. Resources

Battery connections should be clean and tight.

Battery connections should be clean and tight.

1. Before You Start

The most common reasons for charging system problems:

  • Loose battery or alternator connections
  • Corroded or damaged battery or alternator connections
  • A faulty battery

Always start your diagnostic with the most simple things.

  • Closely examine the battery connections and make sure they are clean and tight.
  • Unplug or disconnect the wires on the alternator. Examine the connectors, plugs, and wires for rust, corrosion or damage.
  • Check that the drive belt or serpentine belt is not too loose or shows signs of damage. Consult your repair manual for the belt's service schedule.
  • Take your battery to a local auto parts store and have them test the battery.

This pre-diagnostic procedure can save you a lot of time and headaches. You may be tempted to blame the alternator when the culprit could be hiding behind a loose or corroded wire or connector, or a bad battery.

The following sections will help you test the charging circuit to give you a more precise idea of where the problem is located.

Work With a Charged Battery

Before doing any tests on the alternator or charging system, make sure the battery you are using has at least an open volt charge of 12.4 volts. This will help you get more accurate results.

Measure open circuit voltage.

Measure open circuit voltage.

2. Charging Voltage Output Test

You can use a digital multimeter to determine if the charging system is providing a good charging voltage to the battery.

If you are working on a Honda or Acura model, you might want to turn on the headlights or some other accessory while conducting this test. Most of these models use a load detection circuit that activates the charging field circuit to charge the battery; otherwise, you'll only get battery open voltage (around 12.4 V) when conducting this test. If necessary, consult your repair manual.

  1. Set the transmission to Neutral (manual) or Park (automatic).
  2. Engage the emergency brakes.
  3. Open the hood.
  4. Measure open circuit voltage across the battery posts by setting your voltmeter to DC volts. You should get around 12.4 volts; otherwise, charge the battery or have it checked at your local auto parts store.
  5. Ask an assistant to start the engine and set engine speed to about 1500 RPM.
  6. Measure voltage across the battery posts.
    • Voltage should be about 2 volts higher than open circuit voltage (step 4).
    • If voltage is less than 13V immediately after starting the engine, there's a charging problem.
  7. Ask your assistant to raise engine speed to about 2000 RPM and turn on the headlights, AC, and rear window defrost or some other accessories.
  8. Measure voltage across battery posts.
    • Voltage output should be 0.5 volts higher than open circuit voltage (step 4); otherwise, there's a charging system issue.

Consult the vehicle repair manual for your charging system specifications.

Continue with the tests in the following sections.

Check the charging system diagram for a possible fusible link or max-fuse.

Check the charging system diagram for a possible fusible link or max-fuse.

Charging systems protect the power side of the circuit with a fusible link or maxi-fuse, located inline between the battery positive post and the alternator's output terminal. The maxi-fuse might be located in the engine's power box.

A faulty or blown fusible link will prevent the alternator from charging the battery.

Check the condition of this fusible link using a test light.

  1. Connect your test light alligator clip to a good ground on the engine (a metal bracket or bolt) or to the negative battery clamp on the battery negative post.
  2. Touch the test light to the output terminal on the alternator.

The test light should illuminate brightly; otherwise, check the fusible link or maxi-fuse and battery cable connections for corrosion and damage.

The following two videos show you where to locate the fusible link or fuse on a couple of vehicles. If necessary, check your repair manual to locate the fuse link or maxi-fuse in your vehicle.

How to Test an Alternator Fuse

A charging system produces a small amount of AC voltage, called ripple.

A charging system produces a small amount of AC voltage, called ripple.

4. How to Measure Your Alternator's AC Ripple

It's common for an alternator to produce a bit of AC voltage, but an excess amount indicates a serious problem with the alternator diodes or stator windings.

Check your alternator's AC voltage using a digital multimeter that can measure AC Voltage.

Measuring your alternator's AC ripple voltage:

  1. Set your transmission to Neutral (manual) or Park (automatic).
  2. Engage the emergency brakes.
  3. Open the hood.
  4. Ask an assistant to start the engine and raise engine speed to about 2000 RPM.
  5. Ask your assistant to turn on the headlights.
  6. Set your multimeter to AC Volts.
  7. Turn on your multimeter and connect the red lead, if possible, to the output ('battery') terminal on the back of the alternator for a more accurate reading; if this is not possible, connect the red lead to the battery positive terminal.
  8. Connect the multimeter black lead to the battery negative post.
  9. The AC ripple should be less than 400mV (0.4 volts) AC.
  10. If you are getting more than 500mV (0.5 volts) AC, there's something wrong with the alternator diodes or the stator.
Check your serpentine belt for wear and tightness.

Check your serpentine belt for wear and tightness.

5. Checking for a Loose Drive Belt

Loose or worn-out drive belts or serpentine belts are one of the main causes of charging system problems.

Belts stretch out or become loose, causing accessories to work intermittently or inefficiently.

Modern serpentine belts don't show signs of wear as readily as old types. Still, make a visual inspection of the belt.

Closely inspect the belt's ribs. If there are chunks of ribs missing, replace the belt.

Make sure the belt is sufficiently tight, and check the belt tensioner and pulleys the belt runs on for proper operation.

Follow the instructions, service procedures, and maintenance schedule in your repair manual for the belt. Also, you can find some help in the Resources section at the bottom of this post.

Diagnosing a noisy belt:

A noisy belt is usually a sign of a worn-out belt or problems with one or more belt pulleys. However, it's easy to confuse belt noise with a faulty bearing.

Here's a quick way to differentiate belt noise from bearing noise.

  1. Pop the hood open.
  2. Set your transmission to Neutral (manual) or Park (automatic).
  3. Engage the emergency brakes.
  4. Start the engine.
  5. Spray some water on the belt.

If the noise goes away momentarily, the problem is with the belt.

  • Make sure the belt has the proper tension (consult your manual).
  • Check for a worn or loose belt.
  • Check the tensioner belt and pulleys.

Consult the service schedule for the belt in your repair manual.

Check voltage drop between the alternator's battery terminal and the battery positive post.

Check voltage drop between the alternator's battery terminal and the battery positive post.

6. Checking for Voltage Drop

Although tight and clean connections between the battery and vehicle accessories should provide the required voltage, it's normal to expect some voltage drop in a circuit due to wire and connector resistances.

However, it's excessive voltage drop (resistance) that creates problems. This may cause one or more issues:

  • Discharged battery
  • Low alternator output
  • Voltage regulator failure
  • Slow cranking

The next procedure will help you determine charging system voltage drop—between your battery and alternator—using a digital multimeter.

Checking voltage drop on the power side of the circuit.

Checking voltage drop on the power side of the circuit.

6.1 Check Power Side Voltage Drop

  1. Set your transmission to Neutral (manual) or Park (automatic).
  2. Engage the emergency brakes.
  3. Open the hood.
  4. Ask an assistant to start the engine and raise engine speed to about 2000 RPM.
  5. Ask your assistant to turn on the headlights.
  6. Set your multimeter to DC volts.
  7. Connect your multimeter positive, red lead to the output (battery) terminal of the alternator.
  8. Connect your multimeter negative, black lead to the positive post of the battery.
    • If your multimeter reading is less than 400mV (0.4 volts), there's a good connection between the battery and the alternator.
    • If your multimeter reading is more than 400 mV (0.4 volts), there's too much resistance on the power side of the circuit.
    • If your multimeter reads battery voltage (about 12.4), you have an open circuit between both ends of the power circuit.

Depending on your particular results, check for:

  • Corroded or damaged wire or connections
  • Loose connections
Checking voltage drop on the ground side of the circuit.

Checking voltage drop on the ground side of the circuit.

6.2 Check Ground Side Voltage Drop

  1. Ask your assistance to maintain engine speed at about 2000 RPM.
  2. Keep the headlights on.
  3. Keep your multimeter set to DC Voltage.
  4. Connect your multimeter positive, red lead to the alternator case.
  5. Connect your multimeter negative, black lead to the negative battery post.
    • If your multimeter reading is less than 200 mV (0.2 volts), the alternator is properly grounded.
    • If your multimeter reading is more than 200 mV (0.2 volts), there's too much resistance between the alternator case or mounting bracket and the engine.
      • Clean the alternator mounting bracket and surface where the alternator bracket attaches to the engine.
      • Tighten alternator mounting bolts and bracket.

Consult your repair manual for charging system voltage drop specifications for your particular model.

You'll find more help in the Resources section at the end of this post.

Resources

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.

© 2022 Dan Ferrell