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

Huw Powell audi at humanspeakers.com
Thu Apr 17 17:59:00 PDT 2008

> 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
> following:

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 power.

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



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