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How to Diagnose & Fix Charging System Failure


If you are experiencing dimming lights that brighten at higher RPM or constantly blowing light bulbs, it is very possible that you have a problem with the charging system of your vehicle. Some diagnostic procedures are not difficult, but others require professional assistance.

The vehicle’s charging system is made up of:

  • Alternator
  • Battery
  • Wiring
  • ECU (Electronic Control Unit)

A vehicle charging system is responsible for running your lights and other electrical devices. If any of the above components fail, you will not be able to use your car. The most common problems with charging systems are:

  • When your battery is no longer holding charge.
  • Your alternator is not producing enough current or is shorted and producing too much. This could be a problem not necessarily with the alternator, but with bad connections and wiring.
  • A voltage regulator is malfunctioning and not doing its intended job. Once again, the problem could be in wiring and connections as well.
  • If your serpentine drive belt is worn out, too loose, or is slipping, you will have a problem with the alternator producing adequate voltage.
  • If the circuit that your alternator is on is broken, you will have no power delivered to your accessories and a battery.
  • Improper positioning of a pulley on a shaft and angular misalignment of pulleys are common problems that can destroy a belt.
  • Bearings allow the rotor to spin freely inside the stator. If bearings fail, the rotor won’t spin and your alternator will stop working.
  • If Powertrain Control Module (or PCM) in your vehicle controls voltage regulation (instead of the voltage regulator attached to your alternator), it needs to be checked as a vital part of your charging system.

What are the first two things you need to do when you are suspecting a charging system problem?

  1. Test your battery. It needs to be fully charged (12.4 – 12.7 volts) for your car to even start. If it’s not charged, take it to the local AutoZone and they will test and charge it for free (this is not an affiliate, just a good service).
  2. Check your dashboard. There is a lightbulb (also known as an “idiot light”) that acts as a fuse between an alternator and a battery. After you start the car, if the “battery light” does not come up, your battery will NOT be charging until you replace that light bulb!

This light bulb does two things: (1) it kicks the alternator into functionality and (2) displays a battery light on the cluster panel. It is an easy fix if you diagnose it before doing more complicated tests and disassembling it.

All you have to do is pull out the instrument cluster and change a light bulb. If you find that your lightbulb is in a good shape, then it’s a good idea to check the wiring as well.

Battery (or Alternator) light

The first warning sign that you have a problem with the electrical system is an illuminated “battery light” on your instrument panel that does not go away after you start the car. It could also come ON in the middle of driving.

It could mean anything from an alternator producing less than 12 volts of power (under-voltage) to overvoltage (15+ volts) or a notification that you are just running on a battery alone and the alternator has stopped working! Of course, there could be other reasons for this warning light (like a problem with a circuit or sensor), but the above three are the most common.

If you get this light illuminated in the middle of driving, the first thing you may want to do is decrease the number of power accessories that you are using! Regardless of the cause, you need to take care of this problem ASAP before getting stuck on the road.

Here are some variations of the “Battery light” problem and the possible causes behind it:

#1. The battery light comes ON and stays ON

If this light does not disappear after you start the engine, you could possibly have the following problems:

  1. Indicator light circuit problem.
  2. The connector is loose or unplugged. Plug it back or tighten it.
  3. The voltage regulator is faulty. Change regulator or alternator.
  4. Bad battery. Replace battery.
  5. Loose or corroded battery connections. Clean and tighten connections.
  6. Damaged battery cable. Change cable.
  7. Alternator problem (diodes or stator). Get it rebuilt or replace it.
  8. Loose, worn, or broken serpentine belt. Replace belt.

#2. The battery light does not come ON at the start

If the light does not come ON when you turn the ignition key (before starting the engine), the problem here could be:

  1. Open light indicator circuit.
  2. Burnt light bulb. Replace it.
  3. One of the fuses is blown. Check all the fuses and replace them, if needed.
  4. Loose wiring connections. Tighten them.
  5. Defective relay. Check for continuity and replace if necessary.
  6. Defective voltage regulator. A professional can replace the voltage regulator for you or you need to get a new alternator.
  7. Alternator wiring short. Get professional help.

#3. The battery light comes ON intermittently (while the engine is running)

After you start your car, your alternator light could come ON and OFF. The problem here could be:

  1. Loose or corroded connections. Clean them and tighten them.
  2. Alternator problems. Take the alternator for a checkup.
  3. Voltage Regulator problems. Replace the alternator or regulator.
  4. The drive belt could be worn out, damaged, or loose. Replace belt.

** Important! If you are not professional and are not comfortable working with electricity, do not do any testing yourself!

Under the hood

Now, let’s look under the hood. First thing you want to find there is a Fuse box:

With fuses will look like this:

Make sure there are no blown fuses there. If there are, replace them with the same exact size fuses.

Some fuses are embedded in wires and are called: Fusible links. This is what fusible links look like:

If the fusible link is blown, you will notice it right away because the wire will be melted. Your fusible link could also be damaged (even if there is no visual of that) and you can check it by setting your multimeter to Ohms.

Either pierce the wire jacket or disconnect the wire and connect both edges with probes. The reading should be 0.00. If you pierce the wire, then don’t forget to seal the hole with a waterproof sealant.

Check the battery cables and connections

Corrosive battery cables should be cleaned and reconnected. When cleaning battery cables, the negative terminal should be disconnected first and connected back last.

Frayed, damaged, or worn cables should be replaced.

Check the alternator cables and connections

Connectors to the alternator should be tight and secure. Any worn-out cables should be replaced.

Visual alternator and belt inspection

Check if there is dust, oil, or water contamination in your alternator. If the battery had reversed polarity, there will be spark marks on the housing. Check if the pulley makes audible noise.

A worn-out or damaged belt could cause a problem with your charging system. Cracks or glazing on the belt should indicate that it needs to be replaced. If the belt wobbles or is too tight, the belt tension needs to be adjusted.

Now, let’s look at the common charging system problems:

Overcharging problem

It’s a good idea to take care of the overcharging problem in an urgent manner before your important (and expensive) electrical components get fried! The light bulbs in headlights and taillights will usually go first.

If you change light bulbs frequently, this could indicate an overcharging problem and that includes:

  1. The alternator is overcharging.
  2. There is a problem with the alternator’s electrical circuit.
  3. Your battery is bad.

If your alternator is overcharging your battery, you will probably notice an acidic smell along with a leak from your battery due to boiling. If your battery’s electrolyte is depleted in a short period of time, it means that your battery is constantly overcharged.

You can test for an overvoltage problem by measuring the voltage at the battery side (while your car is running). Your battery should put out around 12.6 volts when the car is not running and NO MORE than 15 volts when you press the gas pedal.

If your battery’s voltage is above 15 volts, then you have an overcharging problem! If the alternator is producing higher voltage than it should, then the problem could be bad ground on the voltage regulator, bad connections, or a problem with the alternator.

Reasons for battery getting overcharged and possible solutions are:

  1. Bad voltage regulator. Overvoltage is commonly caused by a malfunctioning voltage regulator. The voltage regulator could be replaced (and it will be cheaper if it is external), but sometimes it makes sense just to replace the alternator.
  2. Corrosive connections in voltage regulator. Clean the corrosion.
  3. Poor contact at the voltage detection terminal of the alternator. You need to make sure that the contact area is clean and corrosion-free.
  4. Loose wiring between alternator and voltage regulator. Tighten it.
  5. Battery itself. Check the battery and replace it if needed.

If voltage regulation in your car is being handled by your vehicle’s computer, you will need to check for Diagnostic Trouble Codes. Your vehicle repair manual should give you more information.

What to do if you have an overcharging problem? Get your car to a mechanic ASAP before you start damaging devices and bulbs!

Undercharging problem

Your alternator is supposed to output at least 13.8 volts of power, otherwise, you will have a problem with your electrical system. Undercharging or under-voltage is usually noticed when your headlights become brighter as your start accelerating and dimmer when your hit your brakes.

The lights inside the car may start dimming or flickering as well and your instrument cluster panel can behave erratically. Power windows and power seats could start being too slow or not work at all.

It also becomes noticeable when the battery gets weak or dies completely, to the point that you are unable to start your car. All of this is usually due to problems with an alternator or a voltage regulator that is attached to it, but there are other cases as well.

The most common Diagnostic Trouble Code for alternator problems on OBD-II is:

P0562 – System Voltage Low

This type of problem is usually produced by:

#1. Blown fuse

The alternator fuse can be found in a fuse box. The owner’s manual should be able to show how to find the fuse. Check to see if the fuse is blown and replace it if necessary.

#2. Blown fusible link

It’s a small device that is placed on a wire between the alternator and a battery to break the continuity upon an overvoltage detection. Here is an interesting video on how checking for blown fusible links simplifies the alternator troubleshooting:

Check and replace if needed.

#3. Defective wiring

You can perform a voltage drop test:

  • Output circuit (positive side). Attach the negative lead to the positive side of the battery and the positive lead to the output stud of the alternator. Turn on ALL your electrical devices and rev your engine to 2000 RPM. Reading should be less than 0.2 volts.
  • Ground circuit (negative side). Attach the positive lead to the negative side of the battery and attach the negative lead to the case of the alternator. With everything ON and the engine at 2000 RPM, your meter should read less than 0.2 volts.

If the voltage reading is greater than 0.2 volts on the positive side, make sure all the wiring and connectors are corrosion-free and intact. If the voltage reading is greater than 0.2 volts on the negative side, make sure ground connections and contacts are clean and there are no broken/missing ground points between the engine and chassis.

#4. Defective battery

Perform a visual inspection of the battery to make sure that nothing is broken or leaking. Cables must be damage-free and properly attached.

Check that your battery is:

  1. Free of cracks.
  2. Has corrosion-free terminals.
  3. Has proper electrolyte level.
  4. Fully charged (12.6 volts)

If you notice any of the above problems and cannot fix them, replace your battery.

#5. Bad battery connections

Check that the battery terminals are clean and free from corrosion. Here are some tips on how to do that:

#6. Bad alternator

The alternator itself could be faulty and not produce enough voltage.  Problems with the alternator may include:

  1. Loose or worn-out belt.
  2. Loose or corroded battery connections.
  3. Ground problem.
  4. Bad alternator diode.

You can also take your alternator to AutoZone for a free checkup. There should be many places around that can test it for you as well (for a fee).

Here is how a car alternator bench test is done:

#7. Failed diodes

Diodes inside the alternator rectifier are responsible for converting AC (Alternating Current) into DC (Direct Current) that your battery and power accessories need for working. If one of the diodes goes bad, you will experience dimming and flickering headlights, instrument panel lights, and other displays.

In the worst-case scenario, your battery will be drained completely. Here is how you test your alternator for a bad diode:

  • Step 1. Run your engine to approximately 1500 RPM.
  • Step 2. Turn the blower and headlights ON.
  • Step 3. Put your multimeter in the low-setting voltmeter mode, AC (Alternating Current).
  • Step 4. Touch battery terminals with the probes (while the engine is running) – positive to positive, negative to negative.

If AC measurement gives you more than 0.1 volts (50-100mV), then one or more diodes are failing and the alternator’s rectifier or the alternator itself should be replaced.

#8. Excessive load

Your alternator can only handle so much electricity demand. If your needs require more power, you may want to upgrade your alternator.

This also applies if the internal components of your car (that demand electricity) are getting old and starting to demand more power!

#9. Parasitic draw

What is the parasitic draw? Your alternator may create a parasitic drain caused by a bad diode inside. If you have a good battery, but it goes dead overnight, it’s time to start looking at the parasitic power draw.

Take your alternator to the nearest shop to check it out. If you want to test it yourself with a multimeter, here is what you should do:

  • Step #1. Put your multimeter in the ammeter mode.
  • Step #2. Disconnect the negative battery cable.
  • Step #3. Connect one of the probes to the battery cable terminal.
  • Step #4. Connect the second probe to the battery post.

If the current is more than 50 mA, disconnect the alternator and check again. If after disconnection the current levels become normal, you have a problem with a parasitic draw.

Also, check any aftermarket installs that could be wired improperly and a simple radio (that doesn’t shut off), can also drain your battery.

#10. A faulty PCM

This is a rare case. If an onboard computer called “Powertrain Control Module” (or PCM) gets damaged, it can be a serious safety issue and needs to be taken care of ASAP.

It can be repaired or replaced.

Intermittent charging problem

If your “battery light” comes ON and OFF, especially when electrical demand increases, take your car to the shop ASAP for a checkup. This generally means that your alternator is losing its charging ability or there is a problem with the wiring.

Here are some points that you can check when you have this problem:

#1. Belt tension

If it is inadequate (meaning you have a worn-out or loose drive belt), you will need to adjust or replace it.

#2. Battery connections

Your battery connections need to be clean and corrosion-free. Check the battery side and the alternator. Clean if necessary.

#3. Alternator ground

You need to check if your alternator is grounded properly and fix it, if not.

#4. The alternator itself

There are many things that could go wear out inside the alternator:

  1. Diodes. If diodes are open or shorted, you will need to replace an alternator (or rectifier assembly).
  2. Stator windings. If the stator windings are shorted or open, an alternator needs to be replaced.
  3. Voltage regulator. Replace it or replace the alternator.

Once again, take your alternator to the parts store and let them check it. They can run a test under load to confirm the alternator failure.

#5. Got new accessories?

Did you recently get installed new aftermarket power accessories to “play with”? If so, your alternator may not be able to handle this extra load due to its inadequate capacity!

But don’t worry, you can still have your “toys” as long as you get a larger alternator.

Unusual noise problem

There are two types of abnormal noises that could come you can from a faulty charging system:

  1. Mechanical noises
  2. Electrical noises

Mechanical noises

Grinding noises could come from a loose or worn-out bearing that gets louder as you step on the gas. Worn out or a loose serpentine belt will produce wining, squealing, or squeaking sound.

A growling sound is produced by a misaligned or loose drive pulley. A loose mounting bracket or a broken bolt that holds the alternator in place produces a knocking sound.

Mechanical noises can also come from alternator slip rings and bad brushes. So, what do we do?

  1. Loose or worn-out serpentine belt. The reason for this problem could be binding, contamination, or simply age and use. Adjust tension or replace the belt.
  2. Defective or worn bearings. This could be caused by water or too-tight belt adjustments. Replace alternator.
  3. Did you recently replace your alternator? It could be installed improperly. Inspect how everything is installed and make necessary repairs.

Electrical noises

The whirring noise is usually coming from a faulty diode. Other electrical noises could come from a shorted rectifier or stator winding.

If one of the diodes in your alternator is defective, it could be due to excessive vibration, jump start or reverse polarity. You will need to replace your rectifier assembly unit or the alternator itself.

When the alternator is replaced, it is a good idea to replace the battery as well (unless it is less than two years old). Over time, batteries lose their ability to absorb and damp voltage spikes that easily damage your new alternator.

Hope this helps and once again, you should not perform any electrical work if you don’t know what you are doing. Here is a map of local mechanics near you:

Attention! This article is for informational purposes ONLY and is NOT a replacement for professional advice! ALWAYS consult your local specialist for an appropriate solution to your problem. All statements, prices, contact information, recommendations, and reviews contained herein came from sources that we believe to be reliable, but the accuracy or completeness thereof is not guaranteed. Please contact the service provider for complete details and updates.

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