Troubleshooting Alternator and Charging System Problems
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 (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
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.
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.
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.
5b. Measure the Charging System's No-Load Voltage
- 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.
- 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.
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 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.
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.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
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?
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.Helpful 1
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?
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.Helpful 3
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?
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.Helpful 19
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?
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:Helpful 11
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?
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.Helpful 9