Alternator Question

Ben Swann bswann at
Fri Aug 30 11:26:21 EDT 2002


I have been helping Sean troubleshoot.  A few questions/comments below
preceeded by --


> Using good old deduction I believe I have narrowed down my problem, but a
solution still eludes me.  I have had earlier postings about electrical
problems and many, many incorrect assumtions on my part.  I have finally
listened and worked on eliminating systems.  I found that I was getting a
1.5 amp draw on the battery with no fuseable equipment connected.  I
disconnected the alternator and the draw went away.  I wanted to prove what
the problem was so I performed the following experiment.
> Here is my test that in included 4 alternators and one known good
battery.  I constructed the circuit with only the battery and the
alternator on a bench.  I ran the ground to the alternator chassis and put
my multimeter, set to measure current, between the postive battery post and
the alternatorpost.  I tested with the exciter wire on and off of ground as
I was not sure about the proper testing conditions.

hooking up the alt chassis to ground and the big wire to battery + should
result in no draw, unless there are bad diodes leaking current the wrong
way through the stator windings

-- 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

the way the exciter wire works is it is hooked up to the battery
(viafusebox, etc.) when the car is running.  Depending on the voltage on
it, up to about 5 amps will be drawn via the brushes through the rotor
windings.  if the alt is spinning, this will result in charging current
flowing out through the big wire.

-- 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.

-- Sean just by passed the oil switch stuff and went directly to ground for
the exiter wire.

> I guess what I am looking for someone to tell me why this test isn't
showing me the right thing

I think it may be showing you something, but it is hard to tell what.

I would reassemble it all on the car, and re-verify that current draw.
 Undo the exciter wire, does the draw go away?  reconnect and undo the big
red wire (watch out for sparks!) and retest for draw.  (I would disconnect
battery ground while undoing and reconnecting these wires, reconnect ground
to test, or put your meter there).

I don't think your test has isolated any components usefully - you may have
bad diodes, or a bad voltage regulator, or voltage to the exciter wire when
the car is off (bad ign. sw?  bad X-relay?), or some sort of random short
in the  alternator.

- seems all of the alternators exhibited these characteristics to a degree.

- 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.

Ben (for Sean)

Huw Powell

More information about the quattro mailing list