'86 5KTQ: Bad alternator/voltage regulator? UPDATE

L DC ldc007usa at yahoo.com
Fri Apr 18 19:12:25 PDT 2008

Thank you Vittorio, Kenn and Huw!!!

Today I took all your input and did a couple of more

First, I tested, yet again, the battery icon light. It
did not come on with ignition key to the on position.

Next, I took the instrument cluster out to check the
battery icon's light bulb, which turned out to be
fine. In fact, I hooked up another spare cluster and
same thing happened; battery light did not come on
with ignition key to the on position nor while running
the engine.

Just as in previous tests, I put a test light to where
the blue exciter wire connects to the alternator and
the test ligth lit up. I followed by hooking up a
multi-meter and that yielded about 11.9V on a battery
that showed a charge of 12.15V.

Then things got interesting.

I disconnected the blue exciter wire from the
alternator and grounded it to the side of the
alternator's casing by simply holding it against it
while my brother turn the ignition key to the on
position and the battery icon lit up, albeit kind of
dim, which I attributed to the poor ground. Then I
gounded firmly, by hand again, the exciter wire to one
of bolts of the passenger's side engine mount and the
icon lit up brightly.

So, that means that the stud itself on the alternator
became defective somehow??

I then giggled the stud and it wobbled a little, which
leads me to believe that is not making contact inside.
It obviously is not grounded either since the battery
icon did not up.

I bought a rebuilt alternator from ADAP but need to
swap the pulleys because it actually came with a VW
Jetta pulley. 

Thank you all again.



--- Huw Powell <audi at humanspeakers.com> wrote:

I took the VR out and cleaned its brushes, which
do stick out about 1/2" long, so I don't see a
problem there but could swap it for a known good 
one just in case. 
> Cool.  Swapping in another known good VR couldn't
> hurt.
So to confirm, the blue exciter wire serves as

> My comments interspersed...
1- The exciter or field current to engage the
alternator when voltage is dropping, say, below
12V and, thus, begin charging system to power whatever
running accessory. 
> It provides power to the field windings, yes.  It
> also "turns on" the transistor in the voltage 
> regulator.
> The transistor controls how much current the
> alternator makes based on  voltage at the blue wire.
2- Trigger to light up the "light bulb" in battery
icon on the instrument cluster to indicate problem
with the charging system. 
> Yes, in a really arcane, strange way.  But, yes.
If the "light bulb" is burnt or the blue wire is
broken, then the alternator WILL NOT kick in and,
therefore, will not charge the battery, correct?
> Correct (I think for the bulb, definitely for the
> wire)
I believe current flows both ways from "light
bulb" to alternator and vice versa when alternator
(and battery) is working properly, correct?
If so, I guess is even possible for the light to
remain off even when the alternator is bad as long
as it puts some power out through this little field/
exciter stud (to where the blue wire connects).  
> The alternator never "puts out power" through the
> blue wire.  The output is only the big stud with 
> the red wire(s), which go to the battery (via 
> starter) and the fuse box..
Following along this lines (power flow both ways),
provided the "light bulb" and blue wire is fine, which
I believe they are based on the previous tests, I
think the problem lays on the alternator output
stud to where the big battery wires connect.
> You had ~12 volts at the small stud from the blue
> wire, so the problem  should not be there.  You
> might have an internal problem, with the wiring or >
the diodes, in which case, the alternator is toast. >
(Unless  you have an old-school rebuilder repair it)
As I recall, when I had the alternator tested on a
bench at Advance Discount Auto Parts, the
field/exciter stud was the only one emitting

> That's just wrong.
> The big output stud never got any power out of it.

But coincidently, when I requested to have one of
their units tested, it yielded the same results. So
they either did not know how to test both units or
both units were bad.
> I suspect, sadly, that they didn't know anything
> about what they were doing. Try another place to 
> test, or another tech. at ADAP?

> If they hooked it up "wrong enough", they may even 
> have harmed it, but don't go yelling at them just 
> because I said that.
> -- 
> Huw Powell
> http://www.humanspeakers.com/audi
> http://www.humanthoughts.org/

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