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Alternator Not Charging – 6 Causes & Diagnostics

In Engine by 17 Comments

Many people assume that the car’s battery is the only thing that powers all electrical components in the car.

But this is not true. The alternator plays a vital role in supplying power to the car’s AC, recharging the battery and ignition.

When your alternator is faulty, the battery power is drained fast, and you will find yourself with a stalled car. But what can cause the alternator to go bad?

6 Causes of why your Alternator Not Charging

There are a few things that can cause an alternator to not charge anymore.

When your alternator is not charging, the most common causes are worn carbon brushes inside the alternator, a damaged alternator, a broken belt, a faulty fuse, or a wiring issue.

Here is a more detailed list of the 5 most common causes of an alternator that is not charging.

1. Worn out carbon brushes or damaged alternator

Alternator Car

The most common reason why your car is not charging is actually because of a worn-out or damaged alternator.

You can carefully tap it with a hammer while the car engine is running while checking it with a multimeter on the car battery to see if the voltage is changing.

If the voltage change and go back to normal when you tap it lightly with a hammer while the car is running – the Carbon brushes are worn out and needs to be replaced in the alternator, or the whole alternator.

Sometimes there is an electrical problem in the alternator, and even if the voltage does not change, it might be damaged.

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Another common cause is a bad diode plate or a voltage regulator. You might need some knowledge about alternators to replace these in most cases.

It was more common to replace parts inside the alternator like the carbon brushes, diode plate, or voltage regulator on older cars. Nowadays, alternators are quite cheap, and mostly it is more worth to replace the whole alternator.

2. Broken Serpentine belt

Serpentine Belt Components

A closer observation of the alternator will reveal a pulley and belt system that works to convert mechanical energy into electrical energy.

The serpentine belt powers the alternator, and if it fails, the alternator will stop working the moment your belt wears out and breaks, or the pulley becomes damaged.

This can also happen if the serpentine belt is not tightened correctly. Most cars have automatic tensioners, but these can fail, so it is better to double-check.

Some older cars have manual tensioners, and in this case, you might have to tighten the serpentine belt.

The serpentine belt and pulleys are often pretty easy and cheap to replace.

3. Faulty fuse

Fuse Box Car

There is often a huge fuse connected to the alternator’s big power cable. It is often an 80A fuse or more and is most often found in your car’s fuse box in the engine bay.

Fuses blow up due to a power surge, or they wear out. When this happens, the current will stop flowing from the alternator. The solution is to check your car’s manual for the particular fuse that controls the alternator and replace it.

In some cars, you might also find another small fuse to the alternator’s control—usually a 15A to 20A fuse.

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4. Wiring issues or connectors

Corrosion On Cable

An alternator does usually has 3 or 4 wires to it to function properly. You will find one big main cable together with two or three small ones.

All these wires are important for the alternator’s function, and if one breaks off – you might lose the charging function.

Check the big power cable connectors between the alternator and the car battery to ensure there is no corrosion there. You can usually find that the cable will get warm if there is a bad connection somewhere.

Check or measure these wires with a multimeter. Remember that measuring them is not always correct because you have to load test wires that are half broken or have a bad connection.

You should usually have 12 volts on one of these wires, and the other one goes to the battery light on the dashboard. If you have a 3rd one, it does often go to the engine control unit. To measure this properly, you need a wiring diagram of your specific car model.

5. Damaged car battery

Car Battery And Alternator

The alternator and the car’s battery work hand in hand. A really bad car battery might actually not take care of the charge from the alternator, which will cause the alternator not to charge at all.

In theory a car can run only with the alternator charge, but this can cause heavy voltage spikes and other strange symptoms so this means that a bad car battery can cause an alternator to not charge also.

6. Engine control module error

Engine Control Unit

Cars are increasingly coming with modern electronics. In this regard, the engine control unit (ECU) controls most of the car’s electrical components.

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Modern cars also control the alternator. In some rare cases, there might be a problem with the engine control module not controlling the alternator’s charging.

Check for any trouble codes with an OBD2 scanner to determine if anything is any other damaged part that prevents it from charging.

In some rare cases, there might actually be a faulty engine control unit. But always check all other possible causes first.

Diagnosing an alternator that won’t charge

There are some easy steps you can go through to check the function of your alternator.

  1. Tap the alternator carefully with a hammer while the engine is running; if the charging goes back to normal, the carbon brushes inside it are worn out and need a replacement.
  2. Check the large power cable to the alternator and the fuse, usually, a large 40-60 amp fuse near the battery.
  3. Check the ground cable between the engine and the body.
  4. Check the serpentine belt and make sure the alternator is spinning with the engine.
  5. Check the small power supply wire and the charging light wire. You can measure it with a multimeter, but you might need a wiring diagram and some car electronic skills to do this correctly.
  6. You can measure the diode assembly and the voltage regulator to make sure they are not damaged. You can replace these with some alternators, but it is often the same price to buy a new alternator nowadays. It is up to you what you think is more worth it.

17 thoughts on “ Alternator Not Charging – 6 Causes & Diagnostics ”

  1. Good day magnus your explanation on the alternator has been so helpful and I quite appreciate the in-depth knowledge on this singular topic. Well done.

  2. 1999. 6 magnum dodge Dakota. New battery . New alternator. But alternator want charge. Why??

  3. Hey magnus
    I’m using 24volts on my machine, I connected two 12volts batteries , which is 24 volts. My alternator charging does not exceed 25. 7 volts,, what could be the problem??

  4. For My KIA SEDONA , i installed a new battery and a new alternator after giving me problems 2,3 times. But it still keep not charging.

    1. The rectifier converts the AC electricity being produced by the alternator into DC so it can be stored in the battery. The battery’s DC power is what runs everything in your car, even your alternator.

  5. Didnt use bmw for a while and battery wouldn’t hold charge so replaced with new battery started first time then ran flat and wouldn’t start no lights etc….recharged battery and started first time again…could this be the alternator.?…thankyou Karen.

  6. My 87 Chey truck. My alternator charges when disconnected from battery. When I reconnect the battery the alternator stops charging and starts whining. One of the alternator wires reads as ground.

  7. Hi

    I have a 1989 Honda Prelude which I have just put in a brand new alternator and brand new battery.
    This after the battery light did not light up when ignition key was switched on.
    I can start car but as soon as I disconnect battery +ve terminal, the engine shuts down: a sign that alternator is not supplying power.

    where should I look next to fix the problem?

  8. I had my battery tested and it was fine they told me that it was my alternator in my 2001 Lincoln Town Car Signature Series as soon as I shut it off it doesn’t even click when I try to start it again I just now came into possession of it 3 days ago and then this has been my only issue with it I haven’t even been able to drive it since I got it so as far as I know that is the only issue I have I was told that you cannot drive with a bad alternator so what else could it be

  9. How can person fix generator that is ok but the inner light is off and on when will start the engine

  10. Your comment is quite informative. You helped me win an argument against a mechanic today.
    Thanks for offering us a peek into your vast knowledge and experience. I appreciate it. It helped me to correctly diagnose an issue today.

  11. I have a 07 ford escape, just replaced my alternator and battery and my dashboard indicators still came (except for the engine & battery) on while driving. Lost power to my heater, lights, windshield wipers came on. Could use some advice

  12. My 1999 Honda Civic battery is not charging the alternator has been bench tested and is working fine the battery light goes out when the car is started but using a test meter there is no charge to the battery any help where to look would be appreciated all the fuses seem ok and the v r is inside the alternator
    Please help thanks

  13. I have 05 ford focus zx3 I replaced battery and alternator and after driving for short periods the battery light comes on I rev it up and it goes off but eventually after time gauges die out and eventually whole car died what is the issue I was told battery is good and alternator also

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