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How to Do a Charging System Check Using Simple Tests

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

Old charging systems used very few electronic control modules.

Old charging systems used very few electronic control modules.

Checking Car Charging Systems

Charging system problems may come from one or more troubled system components:

  • A bad battery
  • A bad alternator
  • A bad voltage regulator
  • A worn or slipping drive belt
  • Circuit problems
  • A misaligned pulley
  • A bad bearing
  • Computer or control module issues

Problems with one or more of these components can manifest under different symptoms:

  • An undercharged battery
  • An overcharged battery
  • A battery, alternator or engine light indicator that illuminates when the engine is running
  • An ammeter or voltmeter showing troubling readings
  • Grinding or whining noises coming from the alternator or drive belt

Although some potential problems in modern automotive charging systems may be a little more difficult to diagnose than in older systems, there are still some common problems you can diagnose with the help of some simple tests.

Remember, a charging system still has to do two basic functions:

  • Meet the electrical needs of the different electrical systems.
  • Recharge the battery after the engine has started or while the engine is running, if necessary.

Even more, newer charging systems can trigger diagnostic trouble codes (DTCs) that you can retrieve with a scanner tool to help you troubleshoot potential problems.

Next, you'll find a section dealing with charging system symptoms and possible causes to help you narrow down potential problems. This is followed by a visual inspection and a charging system tests section. The tests require a digital multimeter (DMM) and, depending on your particular system, a few common tools.

This outline can help you speed up your diagnostic and repair the charging system in your car.


I. Symptoms of Charging System Problems

II. Make a Visual Inspection of the Charging System

III. Simple Charging System Test

Unusual voltmeter readings may indicate charging system problems.

Unusual voltmeter readings may indicate charging system problems.

I. Charging System Symptoms

It's easy to confuse problems in other areas, like the starting system, with problems in the charging system. The following list of symptoms, and their possible causes, can help you not only to confirm you actually have a charging system problem but to focus your diagnostic efforts on specific system components.

If one or more of the following symptoms matches your vehicle problems, continue with the Visual Inspection and System Tests sections that follow, paying special attention to the appropriate troubled subsystem or component defined within the indicated symptom.

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Common charging system problems and their potential causes:

  • The indicator light stays on after the engine has started: This could mean an electrical open in the indicator light circuit, a loose or unplugged connector; a bad voltage regulator, a bad alternator stator or bad diodes; or a loose, worn or broken drive belt. In some models, problems with the alternator light circuit may cause the charging system to fail as well. So check the circuit, if necessary.
  • The alternator light stays off when the ignition switch is turned to the On position: Check for a blown fuse, bad light bulb or socket; however, this may also be caused by problems with the alternator itself or a short in the alternator wiring.
  • The headlights shine brighter when accelerating: Your battery may be undercharged, or there are problems with the alternator or regulator.
  • The alternator light comes on intermittently when the engine is running: There could be connections loose or corroded, or problems with the alternator or regulator; also, check for a loose or worn out drive belt or serpentine belt.
  • Light bulbs frequently blow out: If you are frequently changing light bulbs, there might be an overcharging condition, or problems with the battery or the alternator electrical circuit.
  • The voltmeter or ammeter gauge reading indicates charging problems: Check first for problems with the drive belt (tension, wear, or looseness); then, check for problems with the alternator, voltage regulator or the charging circuit; if the system seems fine, verify that the gauge is working properly.
  • The battery loses its charge: Check for a worn or loose drive belt, and check the connections at the battery and alternator; this can also point to problems with the alternator or battery. If you recently installed new electrical accessories, your alternator may not have the capacity for the extra load and you may need to install a new unit with a larger capacity. If you recently replaced the alternator, make sure it has the capacity recommended by your manufacturer.
  • The battery is getting overcharged: Check for corrosion or loose wires in the voltage regulator connections, bad voltage regulator or battery.
  • The battery is not being properly charged: Usually, this points to problems with the alternator, regulator, circuit or battery. A different component can also be drawing too much current like the starter motor; however, if you recently installed new accessories, the alternator capacity may have been exceeded.
  • The battery is not getting charged: This can point to problems with the drive belt, a bad fusible link, alternator problems, bad battery, corroded or loose battery connections, bad circuit connections, bad voltage regulator, or loose alternator pulley.
  • The alternator is producing a higher than normal rate of charge: This could be caused by a bad ground on the voltage regulator, alternator problems, or loose connections.
  • The alternator is producing a lower than normal rate of charge: This can point to a loose or worn drive belt, problems with the alternator itself, corroded or loose battery terminals or the charging circuit. Loose or disconnected engine grounds can also cause this condition.
A worn or loose drive belt is a common issue in the chagrining system.

A worn or loose drive belt is a common issue in the chagrining system.

II. Make a Visual Inspection of the Charging System

Many common charging system problems can be diagnosed by making a simple visual inspection. So before you start removing or replacing components, pop the hood open and search for these signs of trouble that can lead you to the source of the problem.

1. Check the charging system for warning indicators:

Most vehicles come with one or more system warning indicators like an ammeter, a voltmeter, an alternator or a battery indicator light. Some come with a message center that can warn about problems in a particular system.

  • If the alternator, battery, or engine light indicator comes on while the engine is running, this could be an indication that the alternator is not providing enough current output for the present needs of the various systems, or the battery is not getting charged. Watch the next video to see how you can do a simple fusible link test, a common problem that can trigger the battery or alternator light.
  • If you have installed aftermarket accessories, probably the electrical system demand is too much for the alternator to handle; or this could mean alternator or regulator problems.
  • If the indicator light doesn't come on when the ignition key is turned to the On position (engine off), there could be problems in the indicator light circuit. On some models, this may cause problems with the charging system.
  • With the engine running, an ammeter on your instrument cluster should indicate a positive charge, and a voltmeter should indicate more than 12 volts; otherwise, there might be potential problems with the system.

2. Check the alternator drive belt or serpentine belt:

  • Look for proper adjustment. the belt should not feel loose. when you start the engine or accelerate, the belt should not squeal or flap. otherwise, this might be an indication your belt needs adjustment or that one or more of the pulleys are out of alignment. check the belt for wear or damage like cracks, glazing or contamination.

3. Check your battery and battery terminals:

  • Look for corrosion around the battery cables and make sure the terminals are tight. Corrosion and loose terminals can act as resistance against the electrical current, preventing the alternator to charge the battery, and eventually preventing your battery from starting the engine. if your battery is four years old or older, you make want to diagnose the battery and, if necessary, replace it.
  • Next, inspect the battery case and look for signs of damage like cracks and bulging spots.
  • If your battery has removable caps, clean the battery top and carefully remove the caps. Check the electrolyte level. Add distilled water to each cell as needed. If you have a maintenance-free battery, look at the charge indicator (hydrometer). Usually, a dark green dot means your battery is fully charged. A darkened indicator means your battery needs to be charged. A yellow indicator means your battery needs to be replaced.

4. Now check the wires:

Charging system wiring can also cause all sorts of problems.

  • First, make sure wiring connections are clean and tight. Wires with missing insulation can point to an electrical short. Pay special attention to the connections and wires on the back of the alternator, and the voltage regulator (if you have a remote type).
  • If your car computer handles voltage regulation for the charging system, make sure to scan for diagnostic trouble codes (DTCs) and check the wires that connect to the alternator as well. Consult your vehicle repair manual.
  • If you don't have the repair manual yet, get a relatively inexpensive copy from Amazon like a Haynes manual. This manual comes with step-by-step procedures to troubleshoot and repair many systems, and general maintenance procedures as well.

If you found no obvious problems with the wiring, your next step is to check the wiring with the engine running.

  • Start the engine and wiggle the wires in the charging system. You may need to have an assistant watching the indicator lights or the voltage gauge for any changes. Any change will indicate a problem with the particular wire you wiggled during your test.

You can do the previous test using a voltmeter connected across the battery terminals and the engine running at idle. Wiggle the wires while watching the meter output. Any abnormal voltage changes will indicate an electrical problem in that part of the circuit. Check the wire for insulation damage, bad connections or opens.

5. Check the alternator for unusual noises:

With the engine running, check the alternator for abnormal noises. You can use a mechanics stethoscope or a small length of hose. Be careful with moving components while doing this test.

  • Put one end of the hose against your ear and with the other end contact various points around the alternator including the backside. Mechanical or electrical problems can produce grinding or squealing noises:
    1. Mechanical noises may come from a loose drive belt or serpentine belt, worn bearings, alternator slip rings or bad brushes, a loose or misaligned drive pulley, loose mounting bracket or bolts or an accessory rubbing against the drive belt. You may need to remove a drive belt to manually spin and check pulleys.
    2. Electrical noises may come from a shorted rectifier or stator winding; the most common is a whirring noise coming from a bad diode.

If you haven't found anything unusual so far and you think your battery is not being properly charged, do a battery drain test to make sure the problem is not somewhere else. Usually, any current draw above 50 milliamps (.05 amps), with the engine and all accessories turned off, is considered a harmful drain on the battery. If necessary, consult your car owner's manual.

To check if the alternator is causing a parasitic draw on the battery, connect your ammeter in series with the battery.

  • Disconnect the negative battery cable and connect one of the meter probes to the battery post and the other probe to the battery cable terminal.
  • If the current draw exceeds 50 mA, unplug the electrical connector from the back of the alternator.
  • If the current draw drops to acceptable levels, there is a problem with the alternator.
Use your digital multimeter to do charging system tests.

Use your digital multimeter to do charging system tests.

III. Simple Charging System Tests

Before you start your test, make sure your battery is in good condition and properly charged. If possible, do a hydrometer test on your battery (those with removable caps) or take it to a shop for a diagnostic. Many charging problems come from an apparently good battery.

An ideal test tool for the charging system is a load tester, which can measure voltage and current while applying a load to the system at the same time. Still, you can use a digital multimeter to get an indication of the condition of the charging system by testing for charging output and an overall charging system condition.

Keep in mind that different manufacturers may use different electrical values specifications than those shown here because of the different charging system configurations in use today. Also, if you are testing a charging system in a modern vehicle where the computer makes 'decisions' about alternator voltage output depending on load, temperature, battery state of charge and several other factors, consult your vehicle repair manual so that you don't incorrectly blame the alternator for unusual readings. Consult your vehicle repair manual to get the specific values for your particular vehicle make and model.

1. Checking Base Voltage

  • Turn on the high beams for ten seconds to remove any surface charge.
  • Turn them off and wait for 2 minutes without turning any electrical accessory on.
  • Then, with the engine off and all accessories off, connect your voltmeter across the terminals of your battery.
  • Your battery voltage should be between 12.4 and 12.6 volts.

Here are some battery voltage values you can use as a reference to check the charge percentage on your battery:

  • 12.6 volts or higher- 100% charged
  • 12.4 volts or higher- 75% charged
  • 12.2 volts or higher- 50% charged
  • 12.0 volts or higher- 25% charged
  • less than 12.0 volts- discharged

These values may not apply to a modern charging system equipped with a modern battery.

2. Measuring Charging System No-Load Voltage

Next, measure system voltage output without current draw (all accessories turned off).

  1. Ask an assistant to start the engine and hold engine speed at about 1500 rpm. Make sure all accessories are turned off.
  2. This time, your voltage reading across the battery should be no more than 2 volts higher than your base voltage. When you start the engine, if the system voltage is below 13 volts, a charging problem exists. On the other hand, a voltage reading over 15 indicates an overcharging problem. Overcharging problems usually are caused by a bad voltage regulator or its control circuit.

If your no-load voltage reading is lower than 13.2 or 13.8 volts or lower than base voltage, measure the no-load voltage at the alternator as follows (no-load voltage specs may vary according to your particular alternator or application. If necessary, consult your vehicle repair manual):

  1. With the engine running, connect the meter red probe to terminal B+ on the back of the alternator and the black meter probe to the alternator case (ground).
  2. If your voltage is still the same, there's a problem with the charging system. The problem could be with your alternator or the voltage regulator. Consult your vehicle repair manual to check the system bypassing the voltage regulator.
  3. If the alternator voltage is equal to or higher than 13.2 or 13.8 volts, head over to the Voltage Drop test section.

If your no-load voltage is more than 2 to 3 volts higher than the base voltage, there is an overcharge condition. The problem might be with the voltage regulator or the wiring. NOTE: For this test, some manufacturers find an output voltage range between 13 and 15 volts at 2000 rpm as acceptable (this is also true for voltage at the alternator -- B+ and case). Compare your results to your manufacturer's specifications in the vehicle repair manual.

3. Measuring Charging System Load Voltage

This test will check if the charging system can service the different electrical systems in your vehicle while still being able to recharge the battery.

  1. Ask an assistant to start the engine and hold engine speed at around 2000 rpm.
  2. Turn on all the electrical accessories like wipers, air conditioning, blower motor, headlights, and radio.
  3. Measure voltage across the battery terminal with your multimeter.
    • Your voltage reading should be about 0.5 volts higher than your base voltage.
    • If the voltage reading is not about 0.5 volts higher than the base voltage, bypass the voltage regulator to check whether your voltage regulator or alternator is bad. Consult your vehicle repair manual, if necessary.
    • Another reason for an alternator to fail this test is the installation of aftermarket accessories that surpass the alternator capacity when run along other devices.

NOTE: If you diagnose a bad alternator, make sure to do a charging system voltage drop test as well. If there's too much resistance in the charging circuit, most likely it'll force the new alternator to overcharge to compensate for the voltage drop in the circuit. Eventually, the new alternator will overheat and fail. This also reduces the service life of the battery.

4. Testing for Alternator Current Leak

A charging system should only charge your battery with direct current. However, if one or more diodes in the rectifier go bad, alternating current (AC) can leak and cause problems.

You can use a DMM to measure the presence of AC current from the alternator output terminal. Don't turn on the engine during this test.

  • Temporarily disconnect the negative battery cable.
  • Disconnect the wire from the B+ connector on the back of the alternator.
  • Set your DMM to milliamps and connect it in series between the alternator B+ (output terminal) and the wire you just disconnected.
  • Connect the ground negative battery cable to the battery. Your meter should read no more than 0.5 milliamps. If so, continue with the next steps; otherwise, one or more diodes are bad.
  • Disconnect the battery negative cable from the battery and reconnect the output wire to the alternator B+ terminal.
  • Set your DMM to AC voltage and connect the meter black probe to chassis ground, and touch the positive meter terminal to the B+ terminal on the alternator. There should be no more than 0.5 AC volts; otherwise, there are one or more bad diodes.

5. Charging System Voltage Drop Test

A voltage drop test can help you locate a bad connection or bad wiring harness in a circuit caused by corrosion, loose, or damaged wires. These tests will help you check voltage drop on both sides of the charging circuit. Bad connections introduce unwanted resistance into the circuit and prevent a charging system from operating properly. For this test, you'll need a digital voltmeter.

Voltage drop test points on the positive side of the circuit.

Voltage drop test points on the positive side of the circuit.

Positive Side Voltage Drop Test:

  1. Set your voltmeter to a low setting on the VDC scale.
  2. Connect the meter's positive (red) probe to the B+ terminal connector on the back of the alternator.
  3. Connect the meter's ground (black) probe to the battery positive post.
  4. Start the engine. Then, increase and hold engine speed to about 2000 rpms.
  5. Turn on the headlights, wipers, radio and blower motor. You need the alternator to put out at least 20 amps for this test.
  6. Your meter read-out should indicate a voltage drop of 0.2 volts or less; otherwise, there's unwanted resistance in the circuit you need to clear. NOTE: some manufacturers may consider a voltage drop of 0.5 volts acceptable for this test, consult your vehicle repair manual for the specifications for your particular application.
Voltage drop test points on the negative side of the circuit.

Voltage drop test points on the negative side of the circuit.

Negative Side Voltage Drop Test:

Repeat the previous test on the negative side of the charging system circuit; however, meter connections should be as follows:

  1. Connect the meter's positive (red) probe to the battery negative (-) post, and the meter's ground (black) probe to the alternator case.
  2. Your meter should read 0.2 volts of voltage drop or less; otherwise, check that side of the circuit and remove the unwanted resistance.

Check Your Vehicle's Charging System Specifications

Doing a charging system check on your vehicle is not difficult if you know how to diagnose common problems that affect most charging systems. Many of these problems are not difficult to repair either and often require a few tools. However, depending on your particular system, the electrical value specifications may vary among vehicle makes and models. So make sure to consult your vehicle repair manual to get the specifications for your particular application.

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

Question: I am now having a charging system fail when I put it all back together without figuring out the problem to send it to a shop. Now it doesn't want to start. What could that mean?

Answer: Make sure the battery is fully charged. If necessary, have the battery tested. You may still need to have the charging system checked. This other post may help:

Question: What would cause the gauge in the dash to show undercharge when the alternator is good?

Answer: Sometimes these type of problems come from bad grounds. Check the battery, drive belt, pulleys and engine grounds. This other post may help:

Hope this helps.

Question: Why it is necessary to retest the charging system after the repair of the vehicle?

Answer: You want to make sure the problem has actually been fixed. Basically this prevents overlooking additional problems and makes sure the repair has been successful.

Question: How to carry out a ' current' output test on a charging system?

Answer: You can use a load tester for this. Connect the tester to the battery with the inductive (clip-on) around the negative battery cable. Set the controls as manufacturer recommendation and switch the key to run. Record your reading. Idle the engine and set the idle speed to 2000 rpm (recommended speed). Crank up the load control until the ammeter reads specified current output of your alternator (don't let voltage drop below, usually, 12 volts). Compare your reading to specs. Normally, reading more than 10 amps out of specs means there's something wrong with the alternator. Hope this helps.

© 2018 Dan Ferrell

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