Alternator Question

[Huw Powell]

Well, this was sitting around waiting to be sent, maybe some will find
it amusing, and I fixed that file that I pointed to with the new
alternator schematicy thingie.

Jes' goes to show you, though, referring to Mr. Shoes' final solution...
it's hard to make these cars run right until you fix all the intervening
hacks behind the dash.

btw, the black/blue wire that powers the headlight switch illumination
does go on to power up the idiot lights, among other things.

the blue/black wire is the (low) oil pressure switch signal, without
which none of these will work right, either.

Glad you got it fixed after all!

> Are there perhaps two variations on the feild wire one that goes - and
> others that are +.   ??  Or perhaps I /we misinterpreted the diagram and
> they are always go to + when engine running.  I think Huw mentioned that it
> goes to ground per oil switch when the engine is off - in which case we had
> things bass-ackwards.

There really is too much confusion here over a very simple device.

The alternator is a device for turning motion into electrical current.
it does this by spinning an electromagnet inside a set of fixed windings
in which a current is then induced, and recitified by a bunch of diodes.

The electromagnet is a winding on the center part of the alternator, the
rotor, and gets its electric field via the brushes, this field is
controlled by the voltage regulator.  On our Bosch alternators the
brushes and regulator are integrated in one unit.

The voltage regulator itself is basically a switch, a single transistor,
that is "off" above 14.5 volts or so, and fully "on" below 12.5 volts or
so.  In between these points it exhibits some resistance, delivering
somewhat less current to the rotor.

The higher the electrical load in the car (recharging battery, lights,
etc.), the lower the voltage the regulator will be fed, and the more
current it will deliver to the rotor winding, making more current at the
output.  This little feedback loop happens very quickly, of course,
essentially maintaining the cars electrical system at around 14 volts as
long as the engine is running (fast enough, anyway).

The "D" terminal on the alt/vreg is the wire we have been calling the
exciter wire.  When the alternator is expected to make juice (engine
running, not trying to start), this wire is fed system voltage, ie it is
simply hooked up to the rest of the fusebox junk.  The oil pressure
warning control unit is used to do this.

When the car is not running but the key is on, the oil pressure unit
grounds this wire - but not to perform some alternator function - this
is to light up the alt light and a couple of other warning lights to
test their bulbs.  When the car is running and all is well, the alt
light ends up with 12v on both sides and thus goes out.  The oil unit
might determine this by the presence of oil pressure, or the engine rpm,
since it has both bits of information.

To sum up, as I have said about 6 times now, when the alternator is
functioning, the "D" wire, the little blue one, gets 12 volts, or, more
properly stated, the current operating voltage at the fusebox.

PS, I realized I had not added the new drawing of the alternator
schematic to my web site after I made it...

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

the second picture

( http://www.humanspeakers.com/audi/images/charging2.gif )

--
Huw Powell

http://www.humanspeakers.com/

http://www.humanthoughts.org/