Alternator Not Charging? (Here’s How To Fix It)

An alternator charges your car battery while you drive, and if it does not, your car stops fairly quickly. Here's to do if your alternator does not charge

Battery Light On

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 and stop charging the battery?

The most common reason an alternator is not charging the battery is due to worn carbon brushes or a damaged alternator itself. It can also be caused by a blown fuse or bad wirings. If your serpentine belt snapped off, the alternator will, of course not charge then either.

Now when you know the most common reasons, you probably want to know them in detail also. Here is a list of the 5 most common reasons why your alternator is not charging the battery.

Causes of Alternator Not Charging

1. Worn out carbon brushes or damaged alternator

Alternator Car

The most common reason why your car is not charging the battery 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.

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 replacing the whole alternator.

If you have already replaced it with a brand new alternator, but it’s still not charging the battery, you should keep reading in this list.

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.

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.

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.
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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.