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Alternator faultfinding info Bosch

Discussion in 'Fuel System & Electrics' started by Mordred, Dec 11, 2020.

  1. Mordred

    Mordred Super Moderator Staff Member

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    Below is a post from a BMW forum with information I have not found elsewhere which has helped me find a fault in the voltage control circuit. I have reproduced the post in full with credit to the original poster. There is a lot more info than anywhere I have managed to find and possibly some may not be relevant to our application, but I feel it should not be edited out of respect for the original poster.

    I have found that when I started the engine without revving the alt light would stay on, and then with a few revs on charging normally at 14v but alt light still glowing. Checking the blue cable (connects to D+) from the regulator I found it only getting up to 10v, with the battery voltage 12.8 and increasing. Different regulators and same result. From the info below it appears that the D+ should be the same voltage as B+ but they have their own sets of diodes, the D+ ones much smaller current rated. I am expecting the loss of one of these will reduce the voltage at the D+ and may show a component of AC voltage. Anything more than .5 of a volt AC indicates that a diode has failed. This arrangement may not be the same for other makes of alternator but might help with at least our Type IV ones.

    EDIT: Just gone down this morning to change out the alternator and did one final check and the problem has disappeared! At least I have a better understanding of the circuitry now. Grumble Grumble.....

    edit edit: it happened again so did an alternator swap(in 55mins, pretty impressed) and new alternator working beautifully.

    Adrian

    Subject: alternator system troubleshooting
    Author: shogun (moderator)[​IMG] : member since March, 2004 : 19372 posts
    Posted on: 2011-01-23 02:16:57 [​IMG]
    There are four connections to the alternator itself. D+, DF,D-, and B+. If you look at the Haynes book, what is not readily apparent, but is true nevertheless, is that the set of diodes that connect to the D+ terminal are a duplicate set (except for lower current rating) to the ones for the B+ terminal, which is the actual high current output of the alternator. The D+ terminal is therefore a duplicate output terminal of the alternator, with less current capability. The lower set of diodes on current track 80 is common to both the D+ and B+ functions, and forms the ground return for both the B+ and D+ outputs. The DF or "Dynamo Field" terminal connects to the ungrounded end of the alternator field winding, and is an input to the alternator. The current supplied to the DF terminal determines the strength of the magnetic field that penetrates the output windings, and thus controls the alternator's output. The D- terminal is connected to the alternator frame, and is the ground return for the voltage regulator. The other end of the field winding is also connected to ground at this point.

    The Bosch alternator is incapable of self-excitation, or "boot-strapping" itself to an operating condition. Older DC generators and some U.S. alternators have residual magnetism retained in the core, or some other scheme to get enough field current to get themselves up and running. The Bosch alternator uses a different scheme. The charge warning lamp is connected between the ignition switch and the D+ terminal. When the car is first started, there is no output from the alternator at either the B+ or D+ terminals. The voltage regulator, sensing no output, is attempting to command maximum field current... it effectively shorts the D+ and DF terminals together. This places the D+ terminal close to ground potential, because the resistance of the field winding is not large. This means that there is +12 volts on one side of the charge warning lamp, and the other side of the lamp is grounded through the alternator field winding. Current thus flows through the lamp, lighting it. This same current, however, also flows through the alternator field winding, producing a magnetic field. This magnetic field is what the alternator needs to start up, and if everything is working correctly, that's exactly what happens. The alternator now begins to develop identical voltages at the D+ and B+ terminals. The D+ terminal is connected to one end of the charge warning lamp, while the other end of the lamp is connected to the battery via the ignition switch. Since the B+ terminal is hard-wired to the battery, and since both the D+ and B+ diodes are fed from the same set of windings in the alternator, no voltage difference can exist between these two points. The warning lamp goes out.

    The voltage regulator "watches" the voltage at the D+ point, which should be the same as that applied to the battery. It now changes the short between the D+ and DF terminals into a variable resistance. This effectively controls the field current (whose source is now the output from the D+ terminal, and not the charge warning lamp) and thus regulates the output voltage of the alternator.

    Fault conditions: When something happens to the charging system that causes it's output to be insufficient, the result is almost always a net voltage difference across the charge warning lamp, causing it to light. For example: Suppose an output (B+) diode opens. The efficiency of the main output is now considerably reduced. The voltage regulator does not know this, however, because it is looking at the D+ point. So, the B+ output is now lower than the D+ point and the warning lamp lights. Let's say that one of the D+ diodes failed: The D+ output is now reduced considerably. This means that the voltage regulator will have difficulty in maintaining sufficient field current for normal output. The field regulating resistance is low or short (between D+ and DF terminals) and the resulting load on the crippled D+ system drops it's voltage well below the battery voltage. Therefore, there is a net voltage difference across the charge warning lamp and it lights.

    The bottom line is that in order for your light to light, you must have a net imbalance in the outputs of the D+ and B+ sections of the alternator (or between the D+ output and the battery voltage, which amounts to the same thing).

    To trouble-shoot the problem, you need to check the various sections independently. Thus the first check: Connect +12 volts from the battery to the DF terminal on the relay board. This is the maximum field current situation, and should result in maximum output of the alternator. Note that this checks the B+ diodes, the alternator windings, and the common diodes. It does NOT check the D+ diodes.

    To check the D+ portion of the system, it is necessary to find out if the D+ output can produce enough current to drive the alternator to full output. To do this, short the D+ and DF terminals on the relay board. This will provide the maximum field current to the alternator that the alternator ITSELF can supply (not the battery, as in the earlier check) and so checks the remainder of the circuitry. If this test puts the light out, then the alternator is good, and the trouble is elsewhere. If it doesn't, then the alternator is almost certainly bad, with one other possibility:

    In the Bosch system, the size of the charge warning lamp bulb is critical. Too low a wattage bulb will not supply enough field current for "bootstrap" operation to be reliable. The Bosch book that I have states that the lamps must be at least 2 watts for 12 volt systems. If you have replaced your charge warning lamp recently, then too small a lamp may be your culprit.

    written by Jim T.
     
    Last edited: Jan 4, 2021
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  2. Mordred

    Mordred Super Moderator Staff Member

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    Alternators & Charging Systems Troubleshooting

    Diagnosing and rebuilding alternators.

    How it works: The part that rotates is called the field, or rotor. The stationary part is called the armature, or stator. (This is the reverse of the little DC motors we had in electric train sets when we were young, and is the source of endless confusion.)

    The field makes magnetism - it is just a big electromagnet. Since it rotates (driven by a pulley), we use brushes and slip rings to get power to the field. This power is supplied by the voltage regulator.

    Remember that moving a magnet past a wire induces a current in the wire. (Works the other way, too: You can move the wire past the magnet and get the same result.) The faster you move the magnet, the higher the current in the wire. Also, the stronger the magnet, the more current you get. This is why we need a regulator: At 5,500 RPM the alternator would be putting out 1,000 volts and promptly fry our expensive eurolights and trick stereos. (And climate controls and window motors and trip computers . . . )

    (And yes, I am using the words "current" and "voltage" interchangeably here - this is not correct, but will work for the purposes of this discussion.)

    The regulator functions by sensing the output voltage of the alternator - now listen carefully - and *reducing* the power to the field (less power equals less magnetism equals less alternator output) as required to keep the alternator output DOWN to the required 13.2 to 14 (approximately) volts.

    The armature is the part that makes the electricity. Armatures are nothing more than three coils of wire. They are connected together in two ways: in a "wye" or in a "delta". The wye looks like a letter "Y", and the delta looks like a triangle. This is only important when we start troubleshooting - stay tuned.

    The output of the armature is alternating current, but we require direct current to charge the battery and run all the goodies. Alternating current is converted to direct current by a rectifier, which is simply an array of six diodes.

    There are no more electrical parts (except sometimes a noise supressor which is nothing more than a capacitor). The only mechanical wear-item parts besides the brushes are a pair of ball bearings. These are usually sealed and require no lubrication. They are cheap, and should be replaced whenever you have the alternator open. (There are some mechanical differences - some alternators have an unsealed needle bearing at the brush end - same difference.) A bad ball bearing in the alternator will whine - or grind, if it is very bad.

    Before you take an alternator apart for diagnosis, mark both case halves and stator laminations for alignment. These can be reassembled rotated relative to each other, and while it will go back together, none of the brackets will line up and you won't be able to bolt it back on your car. Also, keep track of all the little bits and pieces of insulating washers and spacers - you NEED to put them back where they came from!

    Electrical diagnosis: Ohmmeter or continuity tester required, alternator disassembled.

    Field: continuity between both slip rings and no continuity to the shaft.

    Armature: continuity between all three "ends" or output wires, no continuity to the frame or laminations. Now here's a gotcha - on the wye wound armature, an open coil will be obvious - one of the tests reads open - infinity - test light doesn't light. On the delta wound armature, an open coil is harder to detect because instead of going through the open coil, it goes around it though the OTHER two coils and will check OK with a test light even though it isn't OK. You need an ohmmeter - if the delta wound armature is OK, you'll get readings of (say) two ohms, two ohms, two ohms between the wire pairs. If one coil IS open, your readings will be two ohms, two ohms, four ohms. Bingo.

    Rectifiers: You'll find five connections: three for the armature output, a ground and a hot. There are six diodes in the rectifier array. Each one must measure continuity one way, and when you reverse the wires to your tester, no continuity. Start anywhere you want - I suggest with the ground wire only because you have to start somewhere - connect your meter to the ground wire and any one of the three armature connections on the rectifier array. You will read either continuity or no continuity. Now reverse the wires to your meter - you MUST read the OPPOSITE of what you just read. If you read continuity before, now you must see no continuity. Any one of the six diodes that reads the same both ways is bad. Continuity - continuity means the diode is shorted, no continuity - no continuity means it is open. Replace as needed.

    Brushes: Check them for free movement in the carrier and make sure they are long enough (they wear down).

    85% of all alternator problems are brushes and/or diodes. Most of the rest are bad regulators. The windings, field and armature, give very little trouble.


    There is a lot to know, I guess about troubleshooting the automobile charging system. The first thing to do is take some basic measurements. Be careful - the battery can deliver a huge current if you short it out, enough to burn you badly or kill you, or weld tools in place until they vaporise! The battery is directly connected to the big alternator stud so go easy down there too.

    Before you start, I will assume your battery is a bit low. You must charge it properly before doing these tests, by connecting it to a trickle charger only at least overnight. This is best done out of doors or at least in a well ventilated space.

    Attach the "-" wire of your voltmeter securely to a ground other than the battery terminal. Prepare to take notes. Put the meter on a scale that will read up to 20 volts or more, and measure the voltage at the positive battery terminal and the big alternator output bolt (it has the fat red wires going to it). Turn the key "on" but leave the engine off and measure these again, and also the voltage at the small alternator connection (this wire is usually blue on Audis). Do the same with a bunch of electrical draws turned on - high beams and high fan should do it. Turn them off and note the voltages again. Now start the engine, and make these three measurements with it running - being careful not to get your clothing or test wires or finger caught in any spinning parts! You can even measure the three voltages with the engine running and electrical loads on the system, but chances are wihtout revving up the engine a bit the voltage will drop because at idle the alternator doesn't have enough output to keep up anyway.

    Now with your meter set to resistance, on as low a scale as possible, very carefully measure the resistance between these various ground points (make sure to get a good clean reading): battery negative terminal, alternator housing, engine block, and the car body. You can expect readings in the range of 0.1 or so ohms, or less if you are lucky, any that are much higher than that indicate poor connections which must be disassembled, cleaned, and protected from corrosion and dirt.
     
  3. Mordred

    Mordred Super Moderator Staff Member

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    These numbers contain most of the information needed to troubleshoot your charging system. If you are going to ask someone to help diagnose your troubles, at least have this data ready for them.

    With the key off the battery and alternator should both read the same and this reading should be over 12 volts - the battery is a 12.6 volt device. Anything lower indicates a poor charge. Unequal readings indicate poor ground connections, which you will have identified with your resistance measurements.

    With the key on, engine off, the voltage may be a tiny bit lower due to vehicle circuits being on. The small alternator wire should read this same voltage, although it will almost certainly be a bit lower due to the resistance of the wire and the load it is feeding (the alternator rotor). If the small wire is not reading a voltage close to the other two (say within .2-.3 volts) there is a problem with the circuit supplying this wire - it basically runs off the "X" or Load Reduction Relay in your car. When the electrical accessories are turned on, the voltages will certainly drop, but it should stay above 12 or at least in the high 11's. If it doesn't, your battery may be in need of replacement. The best way to test it is with a device used by most auto repair shops, which puts a very high fixed load on the battery while monitoring its output voltage. They will gladly sell you a new battery.

    With the engine running, the three voltages should all be higher than 12.6 volts. This indicates a charging condition. If the voltage does not rise, the alternator is not charging for some reason. Clean any poor grounds previously found and start again. PS the volt meters that many cars are equipped with in the passenger compartment are nice to have, but not adequate for these measurements, due to their lack of accuracy and the amount of wiring between them and components we are testing.

    If the trouble appears to be your alternator, yank the regulator with the alternator in place (it's on the back, held in by two bolts) and check the brushes. If they're worn down to less than 5mm or so, replace either them or the whole unit, making sure to clean up the contacts nicely. The ground is at one of the bolts, and the positive is a tab on the regulator which presses against a spring contact in the alternator.

    If you would like to test the functioning of the voltage regulator, you will need a variable power supply that can vary from 0 up to about 15 volts at about 3-5 amps, and a voltmeter. Attach "-" to the connection under one mounting bolt hole and "+" to the tab on the other side of the brushes. Set the voltage at zero and turn on the power supply. Hold a parking light bulb so it is being energised by the two brushes. Start to raise the voltage - the light will come on faintly and slowly get brighter. As it reaches around 14 volts, the light should suddenly go out. Lowering it back to around this voltage should cause the light to come back on again. Now drop the light bulb, as it has become quite hot! Please don't hurt yourself or your tools - you should only be doing this if you have experience working safely with electricity.

    You should also check and clean all the connections again -

    battery ground strap (at block and frame as well)
    starter stud (hot)
    alternator output stud (hot)
    alternator grounding - usually this is the mounting bracket and hardware
    alternator signal wire stud
    alternator signal wire at fuse box (if you can find it)

    These should be clean, free of corrosion, dirt and fraying, and tightly attached. The first four can be checked with an ohm meter for quality without disconnecting anything. The fifth and sixth should have battery voltage present when the ignition is on and nothing when it is off. This wire carries about 3-4 amps when running, to make sure it is capable of it disconnect it from the alternator, turn the key on, and run a parking light bulb with it - the bulb should be bright!

    Cleaning connections starts with the obvious - disconnect them, remove grease and dirt, and carefully file, sand, or emery paper them til they are shiny. Then, you should smear a bit of silicone dielectric gel (a tube is about $10) on both surfaces before reassembling them. I like to shoot the exposed studs and such with a quick dose of battery corrosion preventing spray paint after this.

    Typical charging system schematic diagram:

    http://www.humanspeakers.com/audi/altpix.htm

    parts suppliers
    http://www.lima-shop.de/en/


    http://www.alternatorsparts.com/index.htm

    http://www.woodauto.com/
    http://www.nationsautoelectric.com/bosch.html
    http://www.alternatorparts.com/understanding_alternators.htm

    in case you know other sources, please add those.

    E32 750iL 11/88, E32 750iL Highline 03/90
    SHOGUN'S TRICKS AND TIPS FOR THE SEVEN SERIES
     
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  4. oldman

    oldman Super Moderator Staff Member

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    Great find Adrian....;)
    Cheers,
    Mark
     
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  5. Mordred

    Mordred Super Moderator Staff Member

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    Here is a simplified diagram of the alternator and wring to help get your head around the way it all works. I believe the reason there is actually 2 bridge rectifiers giving 2 separate positive outputs is to allow the regulator to have a reference voltage separate from the battery and main power voltages.

    Edit: There is a better and clearer version of the diagram a few post below after some big flaws were brought to my attention.

    Adrian
    Bosch Alternator schematic diagram (VW Bus).jpg
     
    Last edited: Jan 12, 2021
  6. Mordred

    Mordred Super Moderator Staff Member

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    Thanks Syncro, You found my bit of laziness. I just used the generic bridge rectifier symbol in the diagram which really oversimplified it too much without explaining that there is actually 3 windings and the 2 rectifier stages are different, one with 6 diodes and one with 3 smaller. Had a few mins spare and wanted to get the explanation of how the regulator and field winding wiring was separate across and missed out the detail of how they were wired. I should have just shown them as boxes with rectifier written on it. I'll go back to the drawing board and make it right. All car alternators are 3 phase to my knowledge too.

    BTW I wish I could have found that article you posted as that is a much better explanation than most I have found while looking for info. I might have to lift some bits for this.

    Cheers
    Adrian
     
    Last edited: Jan 12, 2021
  7. Mordred

    Mordred Super Moderator Staff Member

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    Syncro, try this.

    Bosch Alternator diagram 3.0.jpg
     
  8. syncro

    syncro Well-Known Member

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    Here is one for the earlier original mechanical regulator. Although I am not sure whether it should have a star or delta wound stator.

    upload_2021-1-12_11-39-38.png
     
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  9. Mordred

    Mordred Super Moderator Staff Member

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    I thought they were star wired stators but it is possible they are delta wired which means they need a higher speed to obtain the voltage but can give greater current. As the Alternator rotational speed according to the Bentley is 2200 rpm at 975 rpm on the engine it may be delta wired. Seems most later alternators are delta wired.

    Interesting on that diagram that the D- is connected to earth. Why not just connect it to earth at the regulator and leave the connection out altogether. I have seen the 2 leg electronic regulators used in place of the 3 leg and they work fine, using the case as the earth connection and not using the extra wire.

    At any rate, most people just need to know what the wires connect to and the voltages they should get, so the simpler diagram with the box containing the basic info about the alternator windings and rectifiers should suffice.

    Adrian
     
    Last edited: Jan 12, 2021
  10. syncro

    syncro Well-Known Member

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    It may have been the 80A conversions were delta. I remember converting some to higher output, but that was about 40 years ago.

    I think the regulators were mounted on the firewall so it would be important to get an accurate reference voltage or you could cook the battery. If I was using a remote 2 terminal vr, I would run an earth to the alternator.
     
    Last edited: Jan 12, 2021
  11. rstucke

    rstucke Well-Known Member

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    That's better
    but you need to find the schematic with the electronic reg (I have one some where I can post)
    The alternators on TIV engines are star wound (55a and 70a westies). If you want extra out of the then connect a diode to the star connection to power
     
  12. Mordred

    Mordred Super Moderator Staff Member

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    Thanks for the confirmation on the star winding question.

    I am intrigued by how that would work. I thought the only way was to convert to a delta connection, which means more current but lower voltage from the windings and a higher rotational speed required.

    If the windings are all balanced as I would expect there will be no voltage difference to ground at the star point, (with mains voltage reticulation systems the star point is connected to neutral and ground to give a return path for unbalanced loads) so there is no voltage from the windings to ground. But if you put a DC voltage in to that point I can't see it would make a lot of difference short of raising the star point voltage which i suppose may increase the voltage produced from the windings by that amount possibly. so theoretically giving you more voltage which would be reduced by the regulator dropping back the field voltage ending in no net gain. Happy to have that disproved if I am missing something.

    AJ
     
  13. rstucke

    rstucke Well-Known Member

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    a lot of modern high output alternators have a diode from the neutral point to the rectifier circuit
    The average voltage of the neutral point is 1/2 of the output DC voltage. While the output current is flowing through the alternator, the voltage at the neutral point is mostly DC, but it also includes an AC portion.
    A diode from the neutral point will add to the alternator output after about 2000 alternator rpm.


    here's a more complicated version of the same thing
    neutral point diode.jpg

    Here is a picture of a Y stator with a lug on the neutral point for connecting to the rectifier


    DSC02778.JPG
     
  14. rstucke

    rstucke Well-Known Member

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    Here's the modification for a Bosch alternator

    unnamed.jpg
     
  15. rstucke

    rstucke Well-Known Member

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    ford fiesta

    Ford-alternator-wiring-diagram-internal-regulator.jpg
     
  16. Mordred

    Mordred Super Moderator Staff Member

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    That is very interesting. Thanks for the pics.

    Adrian
     
  17. syncro

    syncro Well-Known Member

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    This is all new to me, but I've been out of this for 40 years.

    What does the "W" terminal do in the first diagram?
     
    Last edited: Jan 13, 2021
  18. rstucke

    rstucke Well-Known Member

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    unrectified Ac signal used to monitor engine speed (diesel)
     
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  19. syncro

    syncro Well-Known Member

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    Now who remembers the real calculation for the voltage? I knew it in 1972.:eek:
     
  20. rstucke

    rstucke Well-Known Member

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    I=e/r, but in the case of the stator it becomes more complicated. Flux density, stator impedance, phase angles (120deg) , phase factors (phasors) and polar diagrams will give you a headache
    suffice to say the regulator regulates voltage through regulating the strength of the magnetic field in the rotor via the brushes and the design and construction of the stator regulates the maximum amperage, resistance and impedance and a fan in the rotor stop the whole thing from melting
    Regulated voltage, 13.8 to 14.2 for early alternators, 14.2 to 15.8 in later ones (has to do with the type of battery used)
     
    Last edited: Jan 14, 2021
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