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alternator face thingking

How to detect or diagnose faults in the alternator of your car

Before we have anything light that an alternator is an electric machine, capable of generating electrical energy from mechanical energy, it is usually the drive mechanism of the motor pulleys.

Alternator Parts

Here we present one of my images of an alternator, with its main parts:

partes de un alternador

1-> Stator coil

2-> Coals box, with the coals.

3-> Output voltage regulator.

4-> Rotor winding.

5-> Rotor collector brushes.

6-> Rotor bearings.

7-> Diodes rectifies of AC-> DC



The alternator is basically a generator of three alternate phases. Yes, he did not read wrong, it is an AC generator as it arrives at his house electrical outlets, except that at the end he goes through a panel of diodes that do the job of transforming this AC energy into direct current (DC).

I will not explain here how a generator works, but if you are curious about wikipedia, there is the explanation of the Electric Generator. Basically what I will explain is the ingenious voltage regulation system.

Like any generator, it needs a variable magnetic flux to perform the generation work. The magnetic field is generated in the rotor. By the internal winding that has the same, regulated current of the battery circulates. Once the motor starts, the movement of the rotor produces the variable flow that induces the current in the three-phase stator.

The ingenious thing is that to regulate the voltage generated by the alternator, the current flowing through the rotor is varied, which is much lower and more manageable than the 50A that can generate a medium capacity stator. Then as we accelerate the motor, this current lowers to avoid generating more than 14.5V.

In this way, the alternator charges the battery while the vehicle is running, and supplies the power to all of the car's systems. The following symptoms should give you an idea that your alternator is damaged or is suffering from wear.


Dim lights

The defective symptom of the alternator that most drivers recognize are dim or blinking lights. This is really obvious when the headlights are turned on at night, but it is also noticeable in the dashboard lights and in the central roof light. If the lights are illuminated as the RPM increases, it is an even safer signal.

Battery light on while the engine is running

Signal that the alternator stopped generating energy.

Strange noises

The belt could have stretched with age and could not rotate the alternator pulley effectively, which would cause a lack of load. This usually results in a squealing noise that increases with the lighting of the headlights. On the other hand it is also due to internal bearings that are worn, causing a grunt or grinding noise.

To clear the loose belt, you can wet the alternator belt a little with an accurate water jet. This will wet it and the noise should disappear. If not, then you have bearing problems.

Electric problems

Weak operation of high consumption electrical equipment such as electric windows, front lights, windscreen wipers. This function is due to the existence of a regulation / generation problem.

Engine stop

If the engine suddenly shuts down while driving, it could be an alternator problem. The fuel injection needs a good amount of electrical energy and, without it, the engine stops quickly.

Dead battery

If the alternator does not generate power, the vehicle quickly consumes all the capacity of the battery (approximately in 1 hour, 20 min), leaving it exhausted and the engine will be turned off.

How to test an alternator to identify problems


Alternator problems manifest themselves in several ways, depending on the particular fault. Even so, performing some simple tests at home can help you test if your alternator is damaged.

These upcoming tests will help you verify the functioning of the charging system.

1. Alternator output test

One of the most common problems in an alternator is due to worn coals. But a blown fuse in the charging circuit, a broken fuse link, a defective diode in the alternator rectifier assembly, even a worn out radamiento will affect the operation of the alternator.

Therefore, let's start by checking how much voltage your charging system is delivering.

1.1 Measuring the voltage of the battery base

Set your voltmeter to a setting of approximately 20 on the DC voltage scale, and connect the tips to the battery terminals following the correct polarity (red for + and black for -). If your battery is fully charged, it will read approximately 12.6 volts. Take note of your reading. It will use this voltage as a reference point for the next two tests.

1.2 Measurement of the "no load" generation system

-> Start the engine and let it idle.
-> Ask an assistant to keep the engine speed at approximately 1500 rpm.
-> Turn off all vehicle accessories (this is referred to as "no load").
-> Connect your voltmeter through the terminals of the battery.
-> Take note of the voltage reading.

You should get between 0.5 and 2 volts above your base voltage. Any voltage higher than this will indicate an over-voltage condition. The alternator is generating energy but the voltage regulator is not doing its job. This condition damages the battery quickly and is dangerous, since the overcharge of a battery causes overheating and violent explosion and fire.

However, if your voltage reading remains equal to or slightly lower than the base voltage, the charging system does not work. It is possible that you have a bad connection in the charging system (check that the connectors and cables are in good condition and that they are making good contact). If everything is in order, it is already an internal problem of the alternator, a voltage regulator burned, an open or burned diode bridge or very worn carbons.

1.3 Measurement of charging voltage of charging system

If the charging system passed the previous test, you should see if the charging system produces enough energy for your battery and the different electrical systems.

-> This time, ask your assistant to start the engine and increase and maintain the engine speed at approximately 2000 rpm.
-> Now, turn on all accessories such as radio, air conditioning, headlights and windscreen wipers.
-> Connect your voltmeter through the terminals of the battery.
-> Record the reading of your voltmeter.
-> Now to the reading you took, subtract the base voltage that you wrote down at the beginning.

Your charging voltage should be 0.5 higher than your base voltage. Otherwise, you may have poor circuit connections (inspect cables and connectors) or a defective controller.

1.4 Defective rectification check.

In general, a defective alternator diode will cause the headlights or instrument panel lights to flash or dim, and sometimes deplete battery power at night or in minutes. This is because part of the AC, manages to pass to the DC system for which it is not compatible.

To check if there is a possible faulty alternator diode:

-> Switch your voltmeter to a low setting (about 2 V) on the AC voltage scale (alternating current).
-> With the engine running, touch the probes of the meter to the terminals of the battery.
-> Your voltmeter should read 0 volts AC.

Any amount of AC voltage would indicate a faulty diode, so you will have to schedule to service the alternator.

Video how i do it

Well, it's all for the moment. If I missed any test, below you have the questions.


Questions or suggestions

In the box below questions.

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Written by

Dennis García

Chica Mecánica



Tutorial muy bueno

Gracias Dennis.

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    Do you know the most important components of the Air Conditioning System?

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    • Asked by Richard
      on 04/23/2019
      hola chica mecánica, una observación cuando menciona sobre las luces de faro, de salón y luces del panel de instrumentos parpadean, pero al aumentar las RPM, estos aumentan su intensidad, que falla del alternador es.. ? (que componente del alternador esta fallando). otro sobre pruebas de salida del alternador del 1.4 no tengo muy claro eso, menciona. Con el motor en marcha, toque las sondas del medidor a los terminales de la batería. Answer:
      Richard, ese caso es el regulador de voltaje o un diodo rectificador defectuoso. En ese caso, se mide desde el borne de salida del alternador y la baterías para despejar sulfatación. No deberia de haber mas de 1V.
    • Asked by Jorge
      on 05/01/2019
      Hola chica mecanica mi problema fue que se desprendio la pista de los carbones (la parte de bronce donde hace contacto los carbones) la mande reparar ese era solo el problema lo repare pero ahora la luz testigo de la pila no se apaga segun el electrico que me la checo con multimetro estava cargando correctamente pero cada que prendo los cuartos y posterior luces de tablero noto un pequeño parpadeo esto pasa solo al prender las luces y/o accionar boton para bajar ventanilla Grasias!!! Saludos!!! Answer:
      Que tal Jorge, bueno si la luz de la bateria no apaga es indicativo de que el módulo regulador está defectuoso o hubo un problema en el ensamblaje del alternador (la parte donde está el regulador) que impide que trabaje correctamente. Yo lo desarmaria nuevamente y armarlo lentamente. Al medirlo con un voltímetro debe estabilizar en máximos 14,5V.
    • Asked by Ulices
      on 08/03/2019
      Hola chica mecánica, te felicito muy bueno tu blog me a servido de mucha ayuda, quería hacer un aporte de una prueba al alternador que nunca me ha fallado para saber si esta mandando corriente la cual consiste en tocar la polea o la parte trasera del alternador con un destornillador con el motor encendido si sientes que el destornillador se pega al alternador como si fuera un imán es porque esta mandando corriente si al contrario no se pega algo esta mal, gracias espero le sirva a alguien de ayuda tanto como a mi Answer:
      Excelente aporte Ulises. Lo voy a agregar a mi articulo.
    • Asked by Angel
      on 02/14/2020
      como puedo saber si el estator esta dañado?, tuve un problema con el alternador y me dijeron que el estator estaba dañado porque se sobrecalento y se sulfato le salieron unos puntos de una sustancia blanca. Pero que no tenia ningun incoveniente en seguir su funcionamiento aun cuando le haya sucedido esto, solo le cambiaron un diodo y luego lo pusieron a funcionar, este trabajo bien, pero me queda la duda por la condicion del estator, mi pregunta seria, se debe cambiar el estator o puede seguir trabajando asi?. Answer:
      Que tal Angel, estator es una cosa y diodera es otra. Yo hice unos videos de alternador alli te contesto todas tus dudas. Si hicieron la soldadura bien del diodo y no hay daños en el ailante del estator, puede trabajar sin problemas.

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