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Signs of a Bad Alternator

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

Signs of a bad alternator may manifest in different ways.

Signs of a bad alternator may manifest in different ways.

Signs of a bad alternator may appear in different forms:

  • warning light on your dashboard
  • noises under the hood
  • starting problems
  • battery issues

Any of these may indicate a fault in the alternator, a charging system component, or a related system.

The following sections can help you identify the potential problem with your car alternator, related component or system. Each section suggests what to check for, and some even suggest a test that you may be able to perform at home to diagnose a potential issue.

Furthermore, if you want to know how to test an alternator, troubleshoot the charging system, your battery, or starting system, the Resources section at the bottom of this post provides some links to specific posts dealing with troubleshooting procedures to help you isolate and fix the problem, if necessary.

Also, it's a good idea to have your vehicle repair manual on hand to locate and identify components and wires. If you don't have this manual yet, you can get an inexpensive copy at most auto part stores and online through Amazon. Haynes manuals come with step-by-step procedures for many parts replacement, troubleshooting and maintenance projects. So you may recoup your small investment in a short time.

Index

1. Battery Light On

2. Battery Light is Off During Test

3. Dimmed, Bright or Flickering Lights

4. Accessories Don't Work Right

5. Vehicle is Hard to Start or Won't Start

6. Battery Loses the Charge

7. Battery is Overcharged or Undercharged

8. Ammeter Gauge Indicates Low Charge

- Testing current output

9. Alternator is Noisy

10. Burned Out Lights or Fuses

11. VIDEO: Signs of a Bad Alternator

12. Resources

An illuminated battery light on your dashboard is a common sign of charging system or battery problems.

An illuminated battery light on your dashboard is a common sign of charging system or battery problems.

1. Battery Light On

This is perhaps the most common of the bad alternator signs or charging system problems.

A battery light, ALT, or CHARGE message appears on your dashboard while the engine is running. This may indicate one or more of the following issues:

  • Alternator drive belt worn or loose.
  • Battery or charging system wires or connectors loose.
  • Faulty regulator.
  • Bad alternator stator or diodes.
  • Alternator light or indicator has a short.

Check these components whether the battery light flickers, comes on intermittently or remains on.

Note: Keep in mind that, if your battery light or charging system indicator is On, chances are you are running on battery power. With a good battery, you may have about 30 minutes to get home or to a car shop before your battery is depleted. So fix the problem as soon as possible.

If the battery light doesn't come on when turning the ignition key to On, check the light bulb and its circuit.

If the battery light doesn't come on when turning the ignition key to On, check the light bulb and its circuit.

2. Battery Light is Off During the Test

When you turn the ignition key to the On position, before you actually start the engine, you see all the warning lights illuminate on your instrument panel.

This is a brief test to let you know whether your vehicle warning lights and their respective systems are working properly.

However, if you don't see the battery light, ALT, or CHARGE indicator illuminate, it could be one or more of the following problems:

  • Bad instrument panel light bulb or socket.
  • Blown circuit fuse.
  • Alternator problems.
  • Wiring with a short.
  • Faulty rectifier bridge.
Check headlights brightness.

Check headlights brightness.

3. Dimmed, Bright or Flickering Lights

You may notice your headlights or dashboard lights suddenly loose brightness, headlights glowing too bright or flickering.

The alternator may be producing too little power (dim lights) or too much power (lights too bright).

When you only drive your car from time to time, making short trips, and use your lights, radio, wipers or some other accessories that put a load on your alternator, you are shortening the life of the alternator diodes significantly.

Eventually, you may notice the headlights dimming, the lights flickering when listening to the radio or using another accessory that demands electrical power from your alternator. Or you may notice your headlights shining too bright.

This could be a sign of an alternator loosing its capacity to produce the required power or feeding too much voltage to the different electrical systems.

If a diode in the alternator is faulty, it'll drain battery power when your engine is not running, eventually damaging your battery.

Other potential problems:

However, problems in related components can also produce the same symptoms, like a faulty battery, or loose, corroded or damaged connections or wires.

Make sure to check the battery terminals, charging system and engine grounds for loose or corroded connections before blaming and replacing the alternator.

The posts listed in the Resources section at the bottom of this post can help you troubleshoot the alternator and appropriate circuits and other related systems.

To check the charging circuit's voltage drop, refer to the Resources section at the bottom.

To check the charging circuit's voltage drop, refer to the Resources section at the bottom.

4. Accessories Don't Work Right

You may have noticed your radio, wipers, power windows, or lights acting up lately.

This could be a sign that your alternator is not able to produce the required power to keep up with the different electrical demands.

However, this could also be a sign of a loose or faulty connector, worn or loose drive belt.

  • Have the alternator checked.
  • Make a visual inspection of charging system connections.
  • Check the battery terminal connections.
  • Check charging system voltage drop (see the Resources section).
  • Check engine grounds voltage drop (see the Resources section).
Check related components like the starting and ignition systems.

Check related components like the starting and ignition systems.

5. Vehicle is Hard to Start or Won't Start

This could be a sign of a problem with the charging system, starting or ignition system, or the battery.

Check for:

  • Faulty battery
  • Bad alternator
  • Loose or worn alternator belt
  • Loose, corroded battery connections
  • Charging system connection problems
  • Faulty ignition circuit or switch
Use your digital multimeter (DMM) to check battery's open circuit voltage and charging voltage with engine at idle.

Use your digital multimeter (DMM) to check battery's open circuit voltage and charging voltage with engine at idle.

6. Battery Loses Its Charge

Usually, this indicates a problem with the battery itself or the charging system:

  • Have the battery checked.
  • Have the alternator checked.
  • Loose or worn drive belt.
  • Loose or corroded battery terminals.
  • Loose or corroded charging system connections.

Also, if you've recently added one or more accessories to your vehicle, you may have exceeded your alternator's capacity.

A discharge indication from your ammeter gauge may point to a problem with the charging system, a related component or the gauge itself.

Buildup around battery posts may be a sign of battery overcharging.

Buildup around battery posts may be a sign of battery overcharging.

7. Battery is Overcharged or Undercharged

An indication that your battery is being overcharged is that you often have to add water to the cells or need to remove buildup from battery terminals frequently.

But the opposite may happen as well. If alternator output has decreased, it can fail to properly charge the battery and electrical accessories in your car may run erratically.

Check for:

  • Dirty or loose battery terminals
  • Faulty battery
  • Bad alternator
  • Loose drive-belt
Check alternator's alternating current (AC) output.

Check alternator's alternating current (AC) output.

8. Ammeter Gauge Indicates Low Charge

A discharge indication from your ammeter gauge may point to a problem with the charging system, a related component or the gauge itself.

Check for:

  • Loose or worn drive belt
  • Loose, corroded or bad charging system wire(s) or connections
  • Faulty alternator or regulator
  • Bad ammeter gauge or circuit

Testing current output:

You can check current output by connecting an ammeter in series with the alternator output wire, which connects to the battery positive (red) cable.

For the next procedure, you'll need a digital multimeter (DMM) to check voltage and current. If necessary, consult your vehicle repair manual to identify wires.

  1. Check the belt tension and make sure battery and alternator connections are clean and tight.
  2. Make sure the battery has a voltage of 12.4 volts or more before starting the engine; otherwise, charge the battery.
  3. Check your alternator's amperage output rating. This is usually stamped on the alternator housing. Or look up the rating on the internet by searching for your alternator's ID number (located somewhere on the alternator housing) and, if necessary, brand.
  4. Connect your ammeter in series between the alternator's B+ connector and its wire. If you have a clamp-on ammeter, place your meter around the B+ wire without disconnecting the wire. Consult your vehicle repair manual, if necessary.
  5. Before starting the engine, turn on the headlights for about 3 minutes to reduce the battery charge slightly.
  6. Start the engine with the lights still on.
  7. Your ammeter may read up to 10% of your alternator maximum output rating. For example, if your alternator is rated at 75 Amps, current output may be around 30 or 40 Amps. The higher the engine speed (RPM), the higher the alternator output.
  8. Now, increase engine speed to 2000 RPM.
  9. Set your headlights to high and heater blower to high.
  10. Ammeter readout should increase now; supply voltage at 12.5 volts or higher (measure across battery terminals).

As a reference, a modern, high-output alternator may register 20 to 50 Amps at idle, and possibly 120 to 150 Amps at about 2500 RPM.

If necessary, check the circuit's voltage drop and the charging system (see the Resources section). There should be about 0.5V of voltage drop maximum; otherwise check your connections.

Check alternator bearings and bushings for wear.

Check alternator bearings and bushings for wear.

9. Alternator is Noisy

Worn alternator bearings may get noisy as they struggle to rotate. Usually, you notice the noise getting louder when turning on the radio, headlights or some other accessory that adds a load to the electrical system.

Pop the hood open and verify the noise is coming from your alternator using a length of rubber hose. Be careful with rotating components:

  • Place one end of the hose against your ear.
  • Place the other end of the hose against the rear of the alternator.
  • Check around the alternator housing as well.

If the noise gets louder while checking around the rear of the alternator, most likely a bushing or bearing is worn.

Also, check the following:

  • Loose or worn drive belt
  • Loose or worn belt tensioner pulley
  • Faulty alternator diodes or stator
If light bulbs or fuses keep blowing out, check the alternator and charging circuit.

If light bulbs or fuses keep blowing out, check the alternator and charging circuit.

10. Burned-Out Lights or Fuses

A faulty charging circuit or alternator may supply more current than necessary, blowing fuses and burning out lights.

  • Have the battery checked
  • Have the alternator regulator checked
  • Have the charging system wiring checked

The following video gives some visual clues on those symptoms we talked about.

11. Signs of a Bad Alternator

If necessary, use the electrical diagrams in your vehicle repair manual to check other related systems.

If necessary, use the electrical diagrams in your vehicle repair manual to check other related systems.

12. Resources

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2021 Dan Ferrell