Alternator Question

[Ben Swann]

Huw,

As it turns out, the new/rebuilt alternator was bad after all - Sean will
be getting a new one from the local dealer for about the same cost.

I am still interested in exactly how the exiter/feild current wire is
supposed to operate.  My understanding is that when it is hooked to ground
(-) it enables the alternator to pass current.  Seems it's there to prevent
the alt from passing current backward through the alternator when the
engine is not running, therefore acting as a switch.  The oil switch is
used in conjunction with some other circuitry/relays to connect the feild
wire to ground when the car is off.

essentially the problem Sean was having was the alternator(s) in question
were allowing current to flow back through the alternator when the engine
was off.  He recreated the alt circuit sans car to verify this.  What was
strange all three of the alts tested exhibited this phenomenon somewhat,
the newest being the worst.

Anyway, well see what happens with a new alt.  There may still be some
wiring issues, but a bad alt was confirmed.

Ben


On Wednesday, September 11, 2002 12:10 AM, Huw Powell
[SMTP:human747 at attbi.com] wrote:
>
> > Huw,
> >
> > I have been helping Sean troubleshoot.  A few questions/comments below
> > preceeded by --
>
> I created this (partially finished?) diagram while we were discussing
> this:
>
> http://www.humanspeakers.com/audi/altpix.htm  (second image)
>
> I left out the way the other warning lamps are tied into the oil
> pressure relay for testing.
>
> > -- it is interesting that we seem to have several bad alternators that
came
> > out of working cars, and in fact one rebuilt, and all tested good by
local
> > shop - I can't say how good the shop's testing methods are as they are
not
> > specialists.  My understaning is they "load tested"  the alts which
showed
> > good.
>
> You're judging them "bad" by their lack of function in this one car?
> Seems a bit precarious.
>
> > -- Is the exciter wire supposed to connect to ground(-), or positive.
 We
> > looked at the circuit diagram, and as best as we can tell, the working
> > operation is a circuit is completed through the activation of the oil
> > pressure switch (engine on), and that completes the exciter wire
circuit to
> > ground (via relays, etc.), but diagram does not show sufficient detail
to
> > be sure exactly how this works.
>
> 12 volts - switched, as you note, by the oil press. unit.  it is
> grounded when there is no pressure, and this is what causes the 3.5
> light test display when the key is on with the engine off.
>
> > -- Sean just by passed the oil switch stuff and went directly to ground
for
> > the exiter wire.
>
> which won't work, of course.  no idea what sort of weirdness or even bad
> things might ensue there...
>
> > - seems all of the alternators exhibited these characteristics to a
degree.
>
> In one car?  I'd look more closely at the car.  it'll run with an alt,
> and you can test the output of the exciter wire (off, and running), with
> it *hooked up right* to see what is going on.  Those '84's have pretty
> nasty electrical wiring still (got a lot better in 85, 87, 88...), and
> all kinds of things might not be working right.
>
> > - The upshot of Sean's testing is that the alternators appear to have a
> > current draw when not operating.  The testing was to rule out other
> > comjponents in the car.  The test is just a simple circuit:  Battery
(+) to
> > Alt (+) terminal  and Alt. chassis to Battery (-)/Ground.  Exciter wire
> > could be hooked to gound or not.  Irregardless, it appears that there
is
> > current draw through all of the alternators we tested, and even the
rebuild
> > had current draw.
> >
> > -- Sean can clarify on my comments, as I was not checking over his
shoulder
> > on every step.  These results are certainly a mystery to me.
>
> It should not be possible to run current against the diodes (output wire
> to ground).  But it is also hard to perform useful diagnostics on a
> device when its operation is not fully understood.  I struggled mightily
> (and failed I am sure) to explain how these things work on my alternator
> file.  But they are actually quite simple.
>
> PS, there are unfused circuits running off the fusebox, btw...
>
> --
> Huw Powell
>
> http://www.humanspeakers.com/
>
> http://www.humanthoughts.org/