How does an ALTERNATOR work ? How to TEST each COMPONENT with a multimeter and how to REPAIR it ?

It all began with the battery light on  my dashboard which one day decided to stay on after starting the engine.  Finally, I decided to make a video where you will see how to test each element  of an alternator with a simple multimeter. Like for example the regulator,  the windings of the rotor, the diodes of the diode bridge and so on ... I will try to be as clear as possible by showing  each time the diagrams of what I test. The idea is to understand how each element works and to  replace only

the one that doesn't work, if obvious first it is the alternator which doesn't work,  and that is what I'm going to look in first time ! Hello everyone, so here is my battery  indicator light stays on while the engine is running. But why ?  First I accelerate a little. On some old cars it's just that the engine runs  a little too slowly. Here the light remains on anyway. So it can't be a battery issue since the  car starts perfectly and everything works fine. Now the first question to ask  is:

does the alternator turn ? And here, yes, it turns. So  it's not the belt that's broken. On some alternators there is a disengageable  pulley. If this is broken, the pulley turns but the rotor inside the alternator no longer turns  and therefore the alternator no longer works. This cannot be my case today because  this alternator has a conventional pulley which is therefore not disengageable. Now I am looking to see if the battery is charging. Effectively it could be just a light issue. To be

sure of that, I measure the battery voltage with a voltmeter. On this BMW 5 series E39, there is a positive terminal here. And the  negative pole, I take it anywhere on the car body. I start the engine and I have the answer. I should have at least 13.8  volts while there I read 12 volts only. This means that the battery is not charging. If you have  13.8 volts or more it means that your battery is charging but you have a problem with the battery  light: the light is on when it shouldn't. In this

case, you can drive, but you will still have to  solve this indicator light problem. I advise you to look at the fuses first. But in my case  the battery does not charge. I can drive, but as long as the battery has enough energy to  power the systems. The engine need electricity to operate, to power the engine control unit,  the fuel pump, the fuel injectors and so on ... I was able to drive about two hours.  And then, it has been the breakdown. Now I want to know if this is  really an alternat

or issue, or if it is another issue on the car. Effectively  I could have a blown fuse, a disconnected or cut cable for example, which would mean  that the alternator has no issue. For that, I show you the connection  diagram of the alternator. The alternator is here. It is connected to  the car by three points. It is connected at the negative pole of the battery through  the car body : there is no connector: it is the metal body of the alternator which  corresponds to the negative pole. Then, i

t is connected to the positive pole of the battery  by the big red cable, but it is also connected to another pin named Ignition. In summary,it's  as a positive signal after the car is turned on. Before incriminating the alternator  itself, I will first test the two positive connections, and for that I  have to remove some parts around it. When you work with electricity, I advise  you to disconnect the battery. And of course I would reconnect it back in to  do the tests. On this car, I have to r

emove the intake airbox to easily  access the alternator connections. Now, I can see more clearly. I didn't  show you, but to do this test, I have reconnected the battery. I will first  look that the big positive connection is well connected to the positive terminal of the battery. On the alternator,  it is under a big rubber. I measure the battery voltage well, so the big  connection B+ is well connected to the battery. Then, there is another cable connected  to the alternator. I unplug it. And

what interests me is the middle pin. This  pin should change to a positive voltage once I turn on the car. I have just added a little  wire in the middle of the connector. And I look at the voltmeter that the voltage  increases well once the car is turned on. So there I read zero volt, and the  car is turned off. I turn the car on. And I read almost 9 volts. So that's good, it means that the alternator has  received the enable command from the car. I don't measure exactly the battery voltage, b

ut  that's because the Ignition signal is controlled by the engine control unit which can decide to  disable the alternator in certain situations. Well now I'm sure the issue comes  from the alternator and not from the connections in the car. Now I remove  the alternator and test it on the table. Here is the alternator removed!  Now I will test it with a drill. I connect it to a 12 volt battery.  Here it's a motorcycle battery, but the voltage is exactly  the same as a car battery. About connect

ions, the negative terminal of  the battery goes to the alternator body. And the positive terminal of the battery goes  to the “B+” pin as well as to the middle pin of the small connector. And I added a  voltmeter to measure the battery voltage. To make it turn, I use a corded drill, simply because cordless drills  don't turn fast enough. Let's go ! Well, unsurprisingly, the battery voltage  does not increase. I should read at least 13.8 volts. That means that the  alternator is not working at a

ll. Before opening the alternator, I will just do  a little parenthesis on its operation. And yes, you noticed that it takes a battery to test  it. This is because the alternator is not a dynamo. It may seem paradoxical, but the  alternator needs energy to then provide more. Let me explain: Here I symbolize the stator  of the alternator, or the not turning part, with a small winding taken from an old relay. In  order to display the maximum voltage measured, I connect it to a voltmeter  that I pu

t in AC and MAX mode,. I start by taking a magnet which symbolizes  the rotor and I move this magnet in front of the stator. There is a small  voltage. And if I go faster, well the voltage increases. So what I just did  here is a dynamo and not an alternator. In fact, in an alternator, there is no magnet at  all: the rotor is also a winding. I take this. And what happens if I move that  rotor there in front of the stator? Well, nothing happens at all: the rotor  is not magnetized and therefore t

he stator does not generate any voltage: So  I measure 0 volt. The way for the rotor to behave like a magnet, I have to supply it.  And I do it by connect it to the battery. And now, it works! I measure  a voltage. And what happens if I supply the rotor with a lower  voltage battery than this battery? Well it still works but I  measure a lower voltage. In fact, since the alternator cannot choose its speed  of rotation, it will vary the voltage in the rotor, in order to have the correct  voltage

at the output of the stator. Now I can show you the diagram with  the internal parts of the alternator. I will introduce the parts and their role and then  I will show you how to test them independently. First here is the regulator. It is supplied  by the battery and it supplies the rotor here. When the ignition pin has a positive voltage, it adjusts the rotor voltage in order  to have about 14 volts at the battery. When the ignition pin is not supplied,  the regulator does not supply the rotor

at all and the alternator does not  work, and that is a normal behaviour: it is just disabled. Moreover,when the rotor  turns, it is powered by brushes shown here. Now here is the stator. The stator is the fixed part of the windings. And  there are several windings. Here, there are three windings connected in star at a  midpoint M. So no, it's not the time convector. Finally, the voltages that come out of  the windings are alternating. So these voltages must be rectified in order to have  a posi

tive voltage to charge the battery. And that is the role of the  diode bridge which is here. The diode bridge is made with 6 rectifier diodes. Now I open the alternator, and  I'm going to show you each part. I have just three nuts to remove. Done ! Now the stator is behind the fixed part here. And you can see the end of the rotor here which is supplied by copper slip rings. The diode bridge is the big black part  here. And finally, the regulator is here. The easiest way is to remove it.  There a

re just 2 screws to remove. I can see that the brushes are not  damaged and have the same length. So it's good. It's important that these  connections are good. The connections are clean. A very simple first thing  to do is to check that the rotor winding is not cut, that it is  not in short circuit and that it is well connected to the slip rings. To do that, I put my multimeter in ohmmeter mode, and I measure the  resistance between the two slip rings. I can't read the value immediately and  it

's normal because the windings have a strong inductive effect. I measure about three ohms. It's  good, it is a correct value. Now i'm sure that the rotor winding is well  connected to the slip rings and that it's not cut. Another very straightforward test to do is to  check that the rotor winding isn't in short circuit with the metal body of the rotor. I  put my multimeter in continuity tester : it is the same behaviour as the ohmmeter except  that it beeps when the measured resistance is very l

ow. When I touch one of the slip rings and  the metal shaft of the rotor, it should not beep. It doesn't beep, good ! Now I  know that the rotor has no issue. For those who are wondering if it possible  to read the value of the inductance, the answer is yes. And I'll show  you that later in the video. Now I will show you how to test  the regulator. I remind you, it is this part which supplies the  rotor through the brushes. For that, I take the battery and  the multimeter in DC voltage mode. The

metal body of the regulator corresponds  to the negative connection! I connect it to the negative terminal of the battery. And I  connect the positive terminal of the battery here : on this pin through the spring contacts. I  follow the connection and I connect the wire here. Now nothing should happen, the  regulator is disabled because its ignition pin is not enabled. I  measure the voltage on the brushes. And I measure almost nothing.  The scale here is in millivolts. Now I enable the regulat

or  by connecting the ignition pin to the positive voltage of the battery. The voltage measured on the brushes is  4.5 volts. This value is not correct, it's not enough: the voltage should  be almost the battery voltage which is around 12 volts. So this regulator is damaged. And I have a good news ! I have a new one. Here. Now I show you the same tests with the new regulator. First I reconnect it to the  battery without connecting the ignition pin. And there I measure a few millivolts  between t

he brushes : it's good : it means that the regulator is disabled.  I enable it by connecting the ignition pin to the 12 volts of the battery, and  there I check the voltage on the brushes. And I measure more than 11 volts, which  is almost the battery voltage. It's a correct value. It means that this  regulator can supply the rotor well. Now I want to know if the alternator  works well with this new regulator. I refit the new regulator. And I connect the alternator as before. And go ! Well : tha

t doesn't work. Remember,  I should measure at least 13.8 volts. That means that there is another  issue elsewhere in the alternator. So now, I will check the  stator and the diode bridge. For that, I remove the new regulator. And I remind you : the diode  bridge is the big black part here. In the diode bridge there are somehow three areas.  The top area is the plate here, it is connected to the positive voltage. Then, the area below or this  plate here corresponds to the negative voltage. And i

n the middle area, here, there  are the connections between the diodes and the windings of the stator  whose ends are soldered here. There are six connections because the midpoint M  is also connected on this plate. The diodes are here : those are the little circles you can see  here. On this diode bridge there are 6 diodes in all. There are three diodes on the positive side,  and you can see the tail of the three diodes on the negative side. So the first test I can do is  very simple: if you lo

ok closely at the diagram, the ends of the windings should not be connected  at the positive side or at the negative side. I take my multimeter in continuity tester mode,  and I will check that there is no short circuit between the ends of the windings and the negative  side or the positive side. I put one probe on the body of the alternator which is the negative pole,  and the other probe on the end of the windings. And. Beep. that's not good! It shouldn't beep! It beeps on all six connections,

so it means  that there is a short circuit somewhere. I didn't show you but it doesn't beep when  I do the same test on the positive side. There is so a short circuit  on the negative side only. And to find the short circuit I have  to remove the diode bridge in order to separate it from the windings. To remove the diode bridge, I begin by removing the three screws. Then, I see the ends of the windings which get into these plastic guides here. These connections are pinched and soldered. So I

tried to remove the solder with an soldering iron  and a heat gun like that, but I didn't succeed. So I cut the plastic guides  and pried off the ends of the windings with a cutting pliers and a flat pliers So be careful, I don't cut the  wires of the windings. I show you. Here you can see : the end  of the windings is released. I do the same operation for the other ends. Now it's ok, all the ends are released.  I can finally remove the diode bridge. Since I've separated the stator windings  an

d the diode bridge, I'll start by testing the stator windings first. You  can notice that there are two wires at each end. This is normal. It's simply because  the windings are assembled in pairs. In fact there are not three windings,  but three pairs. I didn't want to complicate the diagram either, and that  doesn't change the tests I'm going to do. First I take the continuity  tester, and I check that the windings aren't in short circuit  with the body of the alternator. 1. Ok. It doesn't beep

. 2. Ok. 3. 4. 5. both wires Ok. and 6. Perfect! There is  no short circuit. Then I check that the windings aren't cut. So the tester should beep when I  touch both ends of the same winding. It's okay ! Moreover the windings shouldn't be in short  circuit with each other. So the tester shouldn't beep between two different windings. Here it  is good, I check the next one. Ok it's not cut. And this winding isn't in short circuit with  the last one. And the last one. Beep. it's good. In summary I h

ave three pairs of windings  which work well and which are not in short circuit neither between them nor with the  body of the alternator. I can deduce that the stator is good and that the short  circuit should be in the diode bridge. Before checking the diode bridge, I do a  second parenthesis. I just want to show you another measurement you can do on  the windings. I do it with this device: It looks like a multimeter but it's not really a  multimeter. It is an LCR meter. With this device, I ca

n measure resistances since there is an  ohmmeter, but I can measure inductors and capacitors too. I'll start by measuring  the inductance of the rotor winding. I connect it in on the left side, on the right  side it is to measure the resistances, on the left side it is to measure the inductances or the  capacitors. And I select the inductance measure. On the rotor I measure 20 mH. And on the stator I measure 82μH  on the first stator winding, 79μH on the second one and 84μH on the last one.  I

show you these values just to give you an idea of ​​those magnitude. It's just important to  measure roughly balanced inductance values. So it's ok for me and I confirm that  the stator is in good condition. Now I close this parenthesis  and I'll check the diode bridge. Here is the diode bridge ! The midpoint M of the  windings is on the three pins connected here. Then here, it's the positive plate side.  The positive pole of the battery is here. The positive pole is connected to the regulator 

by this contact. The positive diode of the first winding is connected like this. Then the positive  diode of the second winding is connected like this, and then the positive diode of the third  winding is connected like this. And now I flip the diode bridge to show you the negative plate side. The negative pole of the battery is connected to the body of the alternator by the three  screws. The negative diode of the first winding is connected like this, then the negative  diode of the second wind

ing is connected like this. And finally, the negative diode of  the third winding is connected like this. I begin by checking the simplest thing : I check that the three pins of the midpoint  M are well connected to each other. Beep. It is connected. Here, it's  also connected. And of course, it's connected here too. So the  connections of the midpoint M are good. Now I check the diodes. I put my  multimeter in diode tester mode. You can test a diode like that : You have  to connect the probe of

the multimeter's positive terminal to the anode of the  diode and to connect the multimeter's COM terminal to the cathode of the diode. If the diode is in good condition, the multimeter should display about 0.6 volts: this voltage  corresponds to the threshold voltage of the diode. I begin by testing the three diodes on the  positive side. I am testing the positive diode of the third winding. I measure about  0.6 volts: that means the diode is good. I do the same thing with the positive diode 

of the second winding. 0.57 volts. It's good. And the last positive diode of the  first winding : these diode is good too. So all the diodes on the positive side are good. Now I check the negative side of the diode  bridge. I connect the positive terminal of the diode tester to the negative plate.  This is where the anodes of all the diodes are connected . And I am testing the negative  diode of the third winding. Oh : I measure 0. That's not good. If the measure is zero, it means  that the diod

e is in short circuit. Well, I have therefore found the issue on this alternator.  I still check the last two diodes: the negative diode of the second winding is good and the  negative diode of the first winding is good too. In conclusion, this diode is damaged:  this diode is in short circuit. And that is the reason why the alternator does not  work at all. I remind you, in a diode that works correctly, the current can only flow in one  direction: it flows from the anode to the cathode. You can

imagine that a diode is an  arrow that gives the direction of the current. In the other direction, it  is a barrier and the current cannot flow. If all the diodes are in good condition, the current generated by the windings can flow like this: it get out of the windings, flows  through the positive diode, flows into the battery and allows it to charge, and then the current  returns in the alternator through the body, then flows through the negative diode to end  up in the winding. The loop is c

omplete ! Whereas if one of the diodes is  damaged and behaves as a short circuit, the current flows directly into it,  it creates a current loop inside the alternator and it obviously  does not charge the battery. So maybe I could just replace the diode in the  diode bridge but that's not that I'm going to do. I'll refit a new diode bridge. Before that, I just show you the same test  done on the new diode bridge. And here I read 0.63 volts on the diode  tester: so this diode bridge is good. Now

I'm going to refit the new  diode bridge on the alternator. I start by removing the excess  solder from the ends of the windings. In order to not dirt the alternator with  solder, I protect it with a paper towel. Then I clean the solder with a soldering gun and a  wet sponge. I don't show you everything, I've done the other ends. You can also use a desoldering  pump to remove solder. Once it's finished, I remove the towel and I shake the alternator to  remove any dirt: nothing should be left in

it. Now I am going to lengthen the  connections a little bit because otherwise it will be difficult to  solder them to the new diode bridge. To ensure reliability, I hold the  new wire by wrapping another small wire around it. Then I solder the  whole. It must also be not too thick otherwise it will not pass through  the plastic guides of the diode bridge. I show you that on one wire. Here I've  already done two wires. I take a very fine thread caught in multiwire cable. I start  to wrap it aro

und the wire of the windings. Then, I place a big wire:  it's a big electric cable. And I wind the tiny wire around. It  holds but I add a little more wire. Then, to solder that, there are  two methods knowing that the one with the electronic soldering iron  like this one, does not work at all. The first method is with  a heat gun and I will show you the second method when  I'll solder the diode bridge. I put the heat gun at the maximum temperature and I direct the hot air upwards  so as not to

burn anything. I first warmed the connection up and then I  bring solder, I put it on top so it will flow. Here I wait for it to cool down and I  cut. Now I finish all the connections. It's done. And I refit the new  diode bridge on the alternator. I screw it back. And then I'm going to hold the  wires together by pinching them. And then I solder them. And this time I  take a soldering gun. Here I have already made a solder and I show you the second  one. I put some solder flux on the solder con

nection first. The solder flux is a kind  of paste which allows the solder to flow well. I take the soldering gun. I set it  at the maximum temperature and I use the larger tip. I heat the two  parts then I bring the solder. The solder should flow onto the wire  and the diode bridge connection. Done ! And then I cut off what is extra. Now I finish the other four solders. I have finished. Now I refit  the regulator on the alternator. And I test the alternator  in the same way as before. 14.4 volt

s is good, the battery  is charging, it works well! I finish by refitting the cover and  I refit the alternator in the car. Are you ready ? I start the engine! And the battery indicator goes out.  It's good : the issue is fixed. And how much is the charging voltage? Well the charging voltage  exceeds 13.8 volts, that's okay! This video is now complete. I hope you liked  it and was useful to you. The thing one can still wonder is why there were two damaged  parts on my alternator. So my hypothesi

s is that it heated up because of the damaged diode  and that it then damaged the regulator. You can tell me what you think in the comments  below the video. You can also share it, put a thumbs up or consider subscribing to  the channel. And see you soon on EnjoyWheels!