Electrical gremlins again (crazy needle syndrome)

[Joshua Van Tol]

Testing alternator diodes directly is a huge pain in the butt. Measuring AC
voltage either with a proper AC meter, or a scope is way easier, and very
conclusive. When I wrenched professionally, that's the way I always did it.
Never a problem. There's not any likely sources for AC on the battery bus
other than a bad alternator. Particularly if the AC goes away with the
engine off.

Josh.


On Sun, Jun 6, 2010 at 9:16 PM, urq <urq at pacbell.net> wrote:

> Neither of the handheld meters have a diode when measuring DC either ...
>
> I don't understand what might make the dash meter jump ... maybe there's a
> bad connection inside that is causing the jumping in particular voltage
> ranges.  When you've got two handheld meters telling you there isn't likely
> anything wrong they are likely the ones telling the tale ...
>
> If you suspect alternator diodes then test them directly, don't infer
> there's a problem based on a voltage fluctuation ...
>
> Steve Buchholz
>
> -----Original Message-----
>
> The voltmeter in the dash possibly doesn't have a diode or such to
> keep the AC voltage out. The handheld meter when set to read DC
> voltage will read ONLY DC, likewise with AC. He stated he's getting a
> whole bunch of AC voltage which could make the dash meter fluctuate.
> I'm not saying the dash unit isn't bad, but all that AC in there is
> certainly no good.
>
> Didn't OP state that the dash meter is stable with the engine off?
>
> -Cody (mobile)
>
> On Jun 6, 2010, at 7:45 PM, "urq" <urq at pacbell.net> wrote:
>
> > ... I thought I read that with external analog and digital meters the
> > voltage was stable ... why are people continuing to recommend
> > anything other
> > than replacing the voltmeter in the dash?
> >
> > Steve Buchholz
> >
> > -----Original Message-----
> >
> > Another vote to check out the alternator, even if it was recently
> > rebuilt.
> > My son's car ('91Golf) had a recently rebuilt alternator in it and had
> > similar problems.
> > As it turned out the first problem was that the filed wiring was not
> > soldered in properly to one of the slip rings. In another year the
> > soldering
> > for one of the rectifying diodes let go.
> > Clearly shoddy workmanship.
> > The car was bought secondhand and came with the rebuilt alternator and
> > starter.
> > I also had to fix the rebuilt starter, that was also nearly brand
> > new, there
> > was hardly any wear on the commutators.
> >
> > Good luck, Peter
>
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