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

L DC ldc007usa at yahoo.com
Wed Apr 16 13:50:32 PDT 2008

Thanks Huw and Vittorio!!

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

So to confirm, the blue exciter wire serves as

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. 

2- Trigger to light up the "light bulb" in battery
icon on the instrument cluster to indicate problem
with the charging system. 

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?

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).  

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.

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 power.
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.

Thanks again to all.


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

> You might just want to dig in and take out the
> voltage regulator/brush assembly and check the 
> brushes.  It's a pain to unscrew from the back of 
> the alt., but worn down brushes are the main reason
> alternators stop charging (if all the wires are
good, and yours sound OK to me).
> New, the brushes stick out from the VR about 1/2" or
> so, when wiped out perhaps barely 1/8".
> Oh, and reading this:
> http://www.humanspeakers.com/audi/alternator.htm
> can't hurt much, although it might waste some of
> your time.  I should add pix of good and worn
> brushes...
> -- 
> Huw Powell
> http://www.humanspeakers.com/
> http://www.humanthoughts.org/

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