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

Kenn Thyrsted quattro at thyrsted.net
Fri Apr 18 00:07:21 PDT 2008

Looks like a problem i've experienced on my 200tqa

Fully charged i could drive ~40+40km=80km thus needing omly to recharge
at night.

But quickly things got worse, and i could drive ~40km at high speed on

Battery voltage was slowly dropping and at about 11-10,5v i'd have to
run the engine at near top revs to charge it.

At that time i'vd allready changed the VR.

Taking the alternator to a repair shop, they tested it and said the
coils were defective.

New coils and the (nearly)new VR and the problem was gone.


tor, 17 04 2008 kl. 20:59 -0400, skrev Huw Powell:
> > 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.

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