Re: Battery Light (+ good tech info) - techies MUST READ

[16V Jason <jason@scirocco.org>]

Okay, then answer me this. :)

My old alternator (the original on the car), never did this unless the car 
started strangely and didn't bob over 1000rpm before settling to a 950rpm idle.

Then, I replaced it with an (outrageously expensive) brand-new Bosch 
alternator.  Now it needs 1200rpm to turn the lights off.  Needless to say, 
it happens much more often now that the lights stay on.

Mind you, I'm not concerned about this at all, as I know it's basically a 
"normal" quality on older VWs.  But your explanations about why this would 
happen all infer a worn, older alternator.  In my case, the worn, 13-year 
old alternator didn't do it, but the brand new one does.

Jason




At 08:30 PM 11/5/2001, Allyn wrote:
>I'll go into 'the details' for marc (p.s. marc, you know all this stuff
>already).
>
>An alternator has rotating winding (armature), as well as stationary
>windings (stator).
>     Current goes through the armature winding (via the rings/brushes many of
>you have seen), making it one big rotating magnet. This magnetic field is
>felt by the stator windings, causing them to produce an AC output. This AC
>is then rectified (converted to DC) and sent out to your battery. The
>regulator 'senses' the DC output voltage and varies the current to the
>armature, varying the intinsity of the field, thus varying the output
>voltage. You may be wondering 'what the heck does the regulator part have to
>do with the light staying on till i rev the engine*&#$^@', here's your
>answer:
>     Picture the car off, alternator is not spinning, nothing 'generated' by
>the stator, since the armature is not rotating. The reverse is true here: no
>generated voltage, regulator has no voltage to make the armature current
>from (dont go thinking it has battery voltage, the battery cannot supply
>current TO the alternator, since it would be going backwards through the
>rectifier (just go with me on this, it doesnt work that way)). Compared to a
>gas grille, the gas is off and there is no flame.
>     Now, you start your car. Belts start spinning the armature. Wait a sec,
>there is no armature current, and no stator voltage to make it. So now, "The
>gas is on, but there is no flame". What the heck%*&@)%. How does this thing
>ever get going you might wonder???
>     Answer: There is a small magnet built into the center of the armature
>winding. It supplies the initial field, just enough for the stator to
>generate some voltage (our "spark"), just enough for the regulator to send
>current to the armature and 'jack up' the output voltage to the 14VDC we're
>all happy to see from a working alternator. The whole process is called
>'flashing the field', which is why i related it all to a gas grille.
>     Question #2: Why does my alternator do this flashing light thing now,
>when it didnt do it when the car was new?
>     Answer #2: Several possibilities:
>         1. Magnet is getting weak: rust, heat cycles (remember, from high
>school, heh), vibration.
>         2. Rectifier diode(s) are open: the stator on our alternators are '3
>phase AC generators' to rectify this type of AC down to single phase DC, you
>need a group of 6 diodes. 3 phase systems are high reliability (the entire
>power distribution system on my submarine was 3 phase), portions of them can
>totally not work, but there will still be at least some output. 1 or 2 of
>these diodes can be broken causing the DC output to be weakened. While this
>may be ok once the field has flashed, it will require a higher rotating
>speed to get it flashed to begin with.
>         3. Stator field winding open: one of the 2 windings could have an
>open wire. The effect is the same as open diode(s) associated with that
>field.
>     You can 'knock out' one of the 3 phases and still get nearly 50% power
>output. Thank VW for overengineering everything, since your scirocco can
>probably still run just fine on the 3 phases.
>Hope this info helps everyone.
>Al
>
>Allyn Malventano, ETC(SS), USN
>87 Rieger GTO Scirocco 16v (daily driver, 170k, rocco #6) running - for now
>87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
>driveshafts, wheels :)
>
>
>----- Original Message -----
>From: "Marc scirocco Québec" <marc_scirocco@yahoo.ca>
>To: <scirocco-l@scirocco.org>
>Sent: Monday, November 05, 2001 7:36 PM
>Subject: Re: Battery Light
>
>
> > This is normal on some alternators, depending on your
> > model.
> >
> > The alternator needs to get "excited" by some voltage
> > to start producing electricity. Without going into too
> > many details bla bla bla, it is normal for some of
> > them.
> >
> > Do not forget that even though it puts out DC Voltage,
> > it actually produce Alternative Current, which then
> > goes through the regulator who rectifies it (with
> > diodes), and regulates it so that it does not goes any
> > higher than 14V.
> >
> > Cheers.
> >
> > Marc
> > '83 Scirocco
> > '88 Scirocco Slegato
> >
> > --- Bill Collins <photo@intrex.net> a écrit : > This
> > comes up on the Vanagon list from time to time.
> > >  The consensus is that its just one of the things
> > > that makes VWs unique.
> > >
> > > I wouldn't worry about it as long as the light
> > > reliably goes out the first time you rev the engine.
> > >
> > > Bill
> > >
> > > > only going out after I've revved the engine past
> > > 3k.
> > > > Then it goes off and stays off.
> > >
> > >
> > > --
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> > >
> >
> >
> >
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>
>
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