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Troubleshooting Alternator and Charging System Problems

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

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.

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 determine which 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, which would make your repair more expensive.

This convenient troubleshooting guide will help you 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 for your particular car make and model. You may buy a relatively cheap copy through Amazon.

Haynes manual include:

  • Step-by-step procedures
  • Photographs
  • Systems descriptions
  • Troubleshooting
  • Parts location
  • Maintenance schedule

The manual helps identify wires and 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 with 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.

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 the alternator voltage output is below 12 volts, or over its limit or has stopped charging the battery and your car is running on battery power.

If the light illuminates only intermittently, it usually indicates a loose or worn drive belt or serpentine belt, 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 or there's something wrong with the circuit itself.

Engine Cranks Slowly, Doesn't Crank or Stalls

When the battery is undercharged, you may have difficulty starting the vehicle, the engine cranks slowly or doesn't crank at all. Also, the engine may stall if the alternator can't produce enough power to run the ignition system. 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, loose alternator pulley or mounting bolts, worn bearings, or faulty diodes. 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 or flickering, power windows running slow, radio or gauges don't operate as they should, 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.

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 or a stator winding fails, 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 at the alternator.

  • 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.
  • Ask an assistant to increase engine RPM to 2000 rpm.
  • Touch the red meter lead to the alternator battery terminal B+.
  • Touch the black meter lead to the alternator case (ground).
  • If your DMM reads 0.4 AC volts or less, the diodes are good.
  • If your DMM reads 0.5 AC volts or more, the diodes or stator is faulty.

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.

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, and 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.

g) If you recently added an electrical accessory to the vehicle, you may have overpassed your alternator's capacity.

h) There could be an alternator wiring problem.

i) Have the alternator and battery checked.

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 a 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 checkpoints 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.

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 to provide a system load and have an assistant raise engine speed to about 1500 RPM. 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 0.4V or less of voltage drop. If your voltage drop is higher, it 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 of around 0.2V or less. If your voltage drop is higher, 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.

A Bad Alternator May Lead To More Problems

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.

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.

Questions & Answers

Question: I have a 2012 Honda Civic LX, and I have put three new remained alternators in the last month. My lights would start to fluctuate, so I went in and had the store test it, and it would show a failure on the voltage regulator. But when I turn on the headlights, it would pass everything. Is there something else that could be causing this failure?

Answer: I don't have the manual for this model with me, but I believe the ECM controls the output. There could be a fault in either the control circuit, one of the sensors the ECM uses to compute the output, or the ECM itself.

Question: I have a 1961 Corvette. I changed the generator too a one wire alternator. Why do the diodes keep blowing out of them; meaning 5 different alternators, under warranty. What's happening?

Answer: Make sure you got the correct unit for your application. Excessive heat is usually the cause for faulty diodes. Make sure alternator has adequate air flow (for example, AC condenser or radiator clogged with bugs and dirt), engine heat shield (if any) in place. Another source of heat could be too much resistance in the charging circuit. Check voltage drops on the charging system and engine grounds as well. These other two posts may help you:

Question: I replaced the original alternator on my 2015 Kia Forte. The replacement alternator shows that it went bad and that I replaced it with the second one, now it went down. What is the problem? I have also noticed the positive battery post on my battery gets hot. Do you have any idea what it could be?

Answer: There could be too much resistance on that side of the circuit. Check the cable for damage, or a problem at the connectors. If everything seems okay, have your battery tested.

Question: I have a 2006 Volvo XC90 where the alternator first quit working, then started working again. The battery light came on, and it started running off the battery. I brought it to the shop, and the alternator started working again. They couldn't get it to kick off again, so we left it. A week later the alternator went out again and stayed out. I just put a new alternator in and the battery is still not being charged; the battery light remains on. Any ideas of what my problem could be?

Answer: Check the charging circuit, you may need the diagram for your particular model. Also, make sure all the engine grounds are tight and free of corrosion.

Question: I have an 03 Dakota. It needed a new battery, however, the battery gauge goes to 0, and the idiot light comes on after I drive it for a minute. It starts fine and the lights are fine; just that gauge quits working. What causes that?

Answer: Check the alternator, perhaps a bad voltage regulator.

Make sure the drive belt is not loose or the idler bad, causing the belt to slip. If the battery charge is not affected and everything else seems to work just fine, there could be a loose wire or connector in the circuit to the gauge.

Question: I have a 1990 Ford F-150 and the alternator stays hot and won’t charge the battery, any suggestions on what is wrong?

Answer: First, make sure the belt is properly installed, not slipping, and accessory pulleys are running OK. Check the charging system wires for loose or corroded connections or damaged wires. This other post may help here:

If necessary, have the alternator checked (worn, bad voltage regulator) and battery. Hope this helps.

Question: What would cause a charging system to have a good amperage and voltage output at the alternator terminal and not see the amperage at the battery? I have done voltage drop tests on the cables and tried with new as well; the amperage is being fed to the cabin circuits.

Answer: Try connecting the meter in series between the ground side of the circuit - black probe to battery ground and red probe to the ground side of the circuit (car's body) using the battery wire that goes from battery to the body. You should be able to read the amperage.

Question: I have a 1999 Honda Accord when I want to turn the engine on, the lights turn on, but when I switch the key to the end, the lights turn off immediately, and the battery dies. I checked the alternator and the battery. Both work very well, but I don't know why the battery immediately dies after switching the key. What do you think about this problem?

Answer: Seems like a battery problem. It doesn't have the amperage to start the car. Have the battery load tested.

Question: When I touch the alternator, it's hot enough to burn my hand, and it was only running for a minute or two. Why is that?

Answer: Possibly excess of power going through the stator and rotor. This could be an internal alternator problem or a bad electrical ground. Check voltage drops for the alternator circuits, especially the ground side. These posts might help:

Question: I have a 2002 Ford Mustang with a 3.8L engine. I was wondering if there is a fuse that could be blown to make it not charge?

Answer: Check the power distribution box, there should be a fuse that connects to alternator power and a fusible link. The box lid may help you find the fuse. If the alternator is not charging, check the belt’s tension and pulleys, the connections between the alternator and battery, as well as the battery itself and alternator. Checking the circuit’s voltage drop may help you test the connections. For this, go over this post, so you have an idea how to do it:

Basically test between the alternator power post and battery power; then ground and battery ground.

Question: Will there be any signs in a Chrysler car battery terminal, if you remove the terminal that there is a problem with the alternator and charging system?

Answer: If the alternator is overcharging, there could be signs of corrosion; but the terminal may be cracked or loose. But you may find problems while the engine is running, but don't remove any connections while the engine is running. It can affect sensitive electronics. If you are trying to find a fault in the charging circuit connections, it's better to do a voltage drop test. This will tell you the condition of the connections. These other two posts may help you locate this type of issues with the charging system and engine grounds:

Question: I have a ‘06 Chevy Cobalt that has been running fine up until two weeks ago when one morning the car would not start. Long story short, I had the alternator replaced. And now it is still dying once in a while. Apparently, it's overcharging! And there is a short. How do I find the short?

Answer: The overcharging may come from a bad voltage regulator in the alternator. You should get between 12 and 15 volts with the engine idling at 2500 rpm, headlights on and fan blower on high.

If you think you have a short, this other post may help:

Question: I have replaced the alternator because the battery wasn't being charged. After the change, the same thing was happening. The engine starts, the battery light on the dash goes off. If you remove the negative terminal while running the engine, it shuts off (I know it’s not a good practice). What could other issues I can check for? I haven't checked the wires for proper connections as yet.

Answer: There could be some high resistance in the charging circuit preventing the alternator from charging the battery: corroded cables or wiring, incorrect wires or cable size; bad crimped connectors or loose connections.

This video shows a simple voltage drop test on the charging system that may help you see if there are circuit problems:

Question: I have a 2001 Mazda 626. I was having problems with the battery holding a charge. I replaced the battery and alternator, but the battery still won’t hold a charge. I constantly have to put it on charger to be able to drive anywhere close. What could be causing the problem now?

Answer: There could be a parasitic drain on the battery

Also, you may want to check voltage drops in the circuit, make sure connections are clean and tight

Question: When doing a battery test on my BMW 323i, the battery number on the left goes up above 14, then down below 11, and the battery light comes on. What could be the problem?

Answer: Have the alternator checked. There could be a problem with the voltage regulator. Also, if necessary, do a voltage drop test on the charging circuit. This other post may help:

Question: I have a 2001 Toyota Solara with 158,000 miles. I've gone through 5 alternators in about 6 yrs. Even the Toyota oem. Two reliable mechanics can’t figure it out. I’m taking it to a shop that specializes in electrical problems. Do you have any ideas?

Answer: Knowing how the previous alternators failed can give a clue about the type of issue you are dealing with (electrical, mechanical). Usually when alternators failed prematurely is because of a bad battery (electrical), voltage regulator issues, high electric loads (e.g. running AC fan at high) or a drive belt that is putting unusual stress on the alternator pulley (mechanical)(make sure the belt is not too tight). Alternator quality can be another factor of course.

Check for signs of fluid leakage around the alternator that might affect the stator or rotor or both.

Question: I have a good alternator, but my battery runs down. It passed tests performed at two auto parts stores. When this problem occurred, I was able to get home by starting the vehicle (2000 GMC Yukon, 5.3L) by disconnecting the battery. Voltage when up close to 14 according to the gauge. When I reconnected the batter at home, it was still dead. Got a new battery and have the same problem with battery drain. Do you have any ideas?

Answer: If you have some power amplifier, or some other power accessories, this could be running down the battery if the alternator can’t keep up with the demand. Other than that, you may want to have the alternator checked in a shop, and see where the charge is going while the engine is running.

You can check for a parasitic drain that you may not be aware of. This post can help you:

Question: I installed a new 105-amp alternator in my 1982 Chevrolet C10. When I start the engine, it indicates 14.6 volts on the voltmeter connected to the battery. When I start the AC fan and the AC compressor, the voltage increases to 15.8 volts. After a while, the alternator stops charging; I think it might be defective. I changed two alternators, but I still have the same problem. What do you think?

Answer: Check the voltage regulator and the battery. An auto parts store may check the battery for you. Also, may sure the circuit connections for the charging system are clean and tight and check the engine body grounds.

Question: I have a 2004 Taurus, and I've changed the battery and alternator but the charging system light is still on. Any ideas why?

Answer: Check the fusible link, you can test it with your multimeter.

Question: I got a 2012 Nissan Xterra. I just installed a new alternator and new battery. It starts up good and idles good now as well, but when I give it full throttle and get close to 3,000 RPMs, my battery light and park brake lights comes on and flashes at me. And the battery volt gauge goes down a whole lot. What could be the problem?

Answer: Check the alternator belt and tensioner. Belt should have the proper pressure. If it's loose, it won't charge properly during hard accelerations. Check voltage at the battery when accelerating (at idle and emergency brakes applied). You should get around 13.5 or so. Otherwise, there could be a problem with the voltage regulator. You may need to replace the alternator. If not, check the wires and connections between the battery and alternator.

Question: I have a 2005 Ford Taurus one day it wouldn’t start. It finally started and flashing lights to check the brake system in the charging system, but then it wouldn’t start once it was jumped. It kept stalling, so we had it checked at an auto parts store. They agreed alternator replaced the alternator and it is still doing the same thing do you have any idea what else could be wrong?

Answer: Check all the grounds -- battery to body and body to engine. They should be clean and tight. Bad grounds can eventually, damage the alternator. Test for voltage drop on the grounds, battery and charging system.

Question: I had my alternator rebuilt. Now I am having a problem getting it to charge at idle, I get 14.5 volts at 1500 rpm, but as soon as I release the throttle it drops down to battery voltage, I've returned the alternator once already. He checked it again, replaced the regulator and said it's putting out full voltage, but on the car, it's doing exactly as I mentioned before, no voltage at idle, full voltage when revving the engine. Could this be a PCM issue?

Answer: Check the belt and belt tensioner for the correct tension. Take a look at the connections at the back of the alternator, especially the B+ connector. On early models the indicator light on the dashboard should come on when the ignition key is in the On position, otherwise, you'll have problems at idle. There could also be a problem in the charging circuit, a loose, corroded or damaged wire can also prevent full voltage at low rpm. Take a look at the grounds and the output wiring to the battery. You may need the diagram for this. These other posts may help:

Question: Battery tested fine, but alternator voltage would go up and down. Occasionally, a BATTERY SYSTEM FAILURE light will come on. Could it be a sensor/fuse problem since it doesn’t stay on all the time?

Answer: The alternator could be faulty. But you may want to run a voltage drop test on the circuit first. Some models, like GM, have a fusible link in the circuit. Make sure that it’s good. This other post has a voltage drop section that may help:

Question: My voltage is fine when I start the vehicle, but after driving around for about 10-15 minutes, my voltage goes from 14v to 15.5-16v. I have an aftermarket voltage regulator, and a high output 320 amp alternator and my battery dies if I don’t unhook the wire from my battery to the cars fuse box. Do you have any ideas?

Answer: You need to make sure the alternator is suited for your application. These alternators need the right pulley ratio and pulley diameter to work correctly. Also check that it has the right size battery cables hook to the battery.

Question: Our engine will take too much time to start. How can I check the alternator?

Answer: The problem might be with the battery, starting system, ignition or fuel system. Check the batter condition, check for spark and fuel pressure.

Question: I have a Toyota Corolla VVTI model 2004, I have changed the alternator but the battery indicator is still present. I have checked the fuses and connections, but the problem continues. What could be the issue?

Answer: Have the battery tested, and check the connections in the charging system. Make sure they're clean and tight. Also, the check the engine grounds.

Question: What are the most common faults likely to occur in a charging system? More than 6 points at least.

Answer: There can be a problem with the circuit, voltage regulator, ignition switch, corroded battery cables, faulty battery, worn or loose drive belt, bad car computer.

Question: I put my OBDll reader on my car and it says my charging system output is high. What does that mean exactly?

Answer: This basically means the alternator is overcharging the battery. Usually the problem here is the voltage regulator. Before replacing the alternator, you can take the alternator to a local auto parts store and have them check it for you. Make sure the alternator is the problem.

Question: I have a 2003 VW Eurovan. Recently, the ABS light went on, then, after a few minutes the regular red brake light lit and the car stopped. What is going on?

Answer: You need to download the trouble codes. There could be a number of things that triggered the ABS light - problems with the tone ring, wheel bearing, speed sensor (and wiring).

Question: I have a 06 Mustang GT. I replaced the alternator because it was causing a camshaft position sensor code. After replacing it and checking my work, the battery light with a warning "check charging system" comes on. It comes on for a minute or so then goes out for a few minutes. The battery seems to stay around 13.7 volts what could be causing the light to come on randomly? I have not driven it yet.

Answer: Check the circuit connections. This could be a loose or bad wire/connector. You may want to try a voltage drop test. This post gives you an idea of the test:

Question: I have a 2014 Nissan Pathfinder. I changed my alternator in July 2018. Alternator stopped working again on 2/6/2019 and replaced it on 2/7/2019. Back in the shop on 2/11/2019 with bad alternator again. This time the alternator is overcharging. The alternators were bought from O'Reilly's auto parts. Just found out they are rebuilt alternators, and not new. Still, the price was $400 for the part. Could there be something causing the part to go bad so quick or could it just happen to be bad parts?

Answer: Make sure you are using the correct battery for your application. On some newer models using the incorrect battery can affect the alternator, especially those that interact with the ECU. Also, check the belt and pulleys. Make sure the pulleys are correctly aligned, and the alternator is properly mounted without rust accumulation, especially the ground connections, including engine and battery grounds. Also, check the wiring for signs of electrical faults. And yes, some models are very sensitive to using parts other than OEM, but first, make the previous checks.

Question: I have a 1998 Ford Mustang. I’ve changed the battery and the alternator. I’m still losing voltage from the alternator to the battery. It is just running on the battery. What can cause this problem?

Answer: You might want to make a voltage drop test on the charging circuit. There could be a bad connection or wire that is not too evident. This other post will give you an idea on how to go about the test:

Question: I have an 04 Lincoln aviator. I changed the battery, alternator and I am still getting a check charging system message. What do I do next?

Answer: Make sure you actually have a charging problem. Your local auto parts store can check the alternator and battery for you. If there's a problem, it could be in the circuit. If not, the circuit for the warning light might be the issue.

Question: What causes voltage to fluctuate?

Answer: There could be several causes behind the issue:

bad alternator, bad voltage regulator, bad connections between the battery and alternator.

First, make sure the connections are clean and tight, and the cables are good. Especially, check the grounds between the engine and body. Then, have the alternator tested in your local auto parts store.

Question: What would cause my alternator to be charging where it supposed to be, then cause it to drop all of the sudden? You can barely tap it and it goes back up.

Answer: Probably you are dealing with an alternator with worn brushes. Tapping the alternator shake the brushes a bit and help them make better contact or the copper wires connected to the brushes are damaged.

Question: I have a 1966 Ford Mustang and have swapped out the alternator 3 times. It will work fine at times, then at other times the voltage steadily drops, especially if I turn the lights on. The belt is good, connections are good. The battery is good. Any ideas as to what the problem could be?

Answer: Make sure you have clean and tight connections to the voltage regulator. If necessary, have the regulator checked. it might not be sensing the voltage drop at the alternator when load increases. And the battery doesn't get the required charge. Hope this helps.

Question: On my way home, the alternator light came on but made it home fine. The next day, the truck was hard to crank and would only stay running if my foot remained on the gas. The next day, I charged the battery and put it back in. Thought it was fixed but now the battery is dead and won’t recharge. What’s the problem now?

Answer: Have the battery and alternator checked. If the alternator or charging system was faulty, it could've damaged the battery. Also checking the charging system voltage drop may help in the diagnosis.

Question: I have a 2012 Town and Country with 110k miles. Last year at around a 100k miles I replaced the alternator due to regulator failure. This year I installed a new battery. The problem is the charging light comes on randomly. I drive around with a voltmeter to run these tests the moment it happens, but everything checks out. Why does the light come on and not go out?

Answer: Check the belt. A worn belt may slip intermittently, affecting the charging system. The problem with newer belts is they look good even if they are worn. Check the belt for tension and wear (use a belt wear gauge), tensioner, pulleys, and grounds, and especially engine grounds.

Question: I have a 2007 Cadillac CTS. I replaced the battery, cranked up and the negative connection got hot. What would cause my Cadillac's battery to heat up?

Answer: If the negative cable gets too hot just cranking the engine, there's too much resistance in that part of the circuit. If this happened right after installing the new battery, probably the negative cable is not well connected or is damaged or there's some corrosion. You can also do a voltage drop test on that part of the circuit. These posts may help:

Question: I have a GM 3500 6.0 with an overcharging problem. I ran new wires to the PCM and fuse box for the connector. I have a brand new alternator, PCM, and ACDelco battery. The power wire for the alternator to the battery reads correctly as well. But it is still charging to 15.6-16.0. What could possibly be going wrong with my car's alternator?

Answer: The problem could be on the ground side of the circuit. Increased resistance (loose, corroded grounds) can cause this type of problem. Of course, you want to make sure you got the right alternator for your application. This other post may help:

Also, make sure to watch this other video:

Question: When my engine is at idle the voltage is around 13V, but when I turn on the headlights and AC, it drops to 11V to 11.8V. What seems to be the problem?

Answer: My guess is the AC pulley may be putting a strain on the crankshaft pulley, affecting alternator output. Or the AC pulley is affecting the belt. Check how the three operate when the AC is on.

Question: I have a 2003 VW beetle. I have changed the alternator more than 8 times. I finally got the right one through the VW parts, but it’s still not charging back my battery, and I bought the battery a year and a half ago. I have been charging it and it still holding battery, so it’s driving me crazy as to why is not working. Can you help?

Answer: The problem could be in the circuit. If your particular model has a fuse (check your repair manual) it may be blown. Less likely, but it could happen, a fault in the computer can prevent the alternator from charging the battery.

Question: I have a 2011 Mazda 3, 2.5. The battery light turned on when I was driving at 120 km/h and would turn off if I slowed down. The light now starts at 90 km/h and will turn off if I slow down (maybe rpm related, not sure). Battery, alternator and start engine were tested at the dealership but seem to be OK. The battery seems to slowly drain within a few weeks as the speed decreases for the battery light to turn on. What could it be?

Answer: This may be an alternator issue with worn brushes, slowly losing the ability to charge the battery. A common test is to replicate engine rpm at which the light comes on. Watch the engine rpm when the battery light comes on. At home, with the transmission in Neutral or Park, increase engine speed to the same rpm and see if the light comes on. If it does, the alternator needs new brushes.

Question: 2002 Mitsubishi Magna. Battery 12.43v, drops to 11.98v at idle. 11.95v from alternator. There seems to be no AC leakage. Fuses all good. Wiring seems all good. Voltage to alternator from battery is correct (connector tested). Is the alternator just producing low voltage because the alternator is faulty? I should mention it is absolutely covered in oil from an oil leak; wiring is dirty but working. Could oil inside the alternator be the cause?

Answer: Oil can definitely affect the alternator. If you have fixed the oil leak, check the alternator circuit voltage drop to make sure it is not interfering with current. This other post may help:

Question: I have a 2000 Civic EX 2 door coupe. The alternator went bad, and I replaced it along with the battery. The car runs fine. It seems to charge the battery, but then the battery light comes on. I have driven the car, and I haven't had any problems running the AC, lights or radio. I can unplug the alternator, plug it back, and the light will go out. But as soon as I plug it in, the light comes on again. Doesn't seem to change the engine running or idle when I unplug the alternator. What could be the problem?

Answer: If there are any DTCs, clear them from the computer memory. Also, check the fusible links. Make sure they are still well connected and good.

Question: Bought a remanufactured alternator for my 2004 Civic today. I cannot get the wiring connector to snap back into the receptacle on the alternator. Can you give me some pointers, please?

Answer: Make sure the harness clip or connector is not damaged or broken. If the connector is OK, make sure you have the correct alternator for your particular model.

Question: What would cause a new alternator to whine and have the pulley heat up excessively? Starting at ambient air and shooting up to 275+ degrees. But when the battery cables are disconnected while the engine is running, the alternator pulley runs smooth and there is no excessive heat build up. The battery is also brand new.

Answer: This is usually caused by a faulty diode(s), making alternator work extra-hard. Check voltage output; also, check the charge circuit voltage drop:

Question: I have a ‘03 Chevy Silverado alternator that whines and displays a check engine code, mainly knock sensor. Occasionally cylinder 7 and 8 are too lean. I suspect a bad ground. Do you know where I can find a visual of where all the grounds are on this truck?

Answer: If you don’t have the repair manual for your specific model, check your local public library. They may have the manual in the reference section. Alternator whining sounds may come bad bearings, bushing or misaligned, worn belt, even a bad diode on some models.

Question: I have a new but cheap alternator. What could be causing such a large fluctuation in voltage?

Answer: The diodes could be leaking or shorted. Have the alternator checked before replacing it.

Question: I have a 2004 Chevy truck and the gauge will stay at 14 volts but sometimes it’ll drop down to 12 volts. Is this normal?

Answer: Check for a loose connection, wire or cable between the battery and the alternator. Wiggle the wires and see if the voltage changes. Also have the alternator checked, if necessary.

© 2017 Dan Ferrell