Electrical gremlins again (crazy needle syndrome)

[radek at istar.ca]

Here's the latest news:  I took the alternator to a shop to get it  
tested.  They guy hooked it up to a big machine which had an  
oscilloscope on top.  He ran it for a couple of minutes on different  
settings, then declared that the diodes were OK.  Next, he took out  
the regulator and put his finger inside.  His final verdict was that  
the shaft was worn out and brushes were jumping, losing contact.   
Indeed, the shaft was rough to the touch.

So I had a diagnosis, the fix was to get the alternator rebuilt.   
However, he called me today saying everything was falling apart,  
screws breaking and lots of rust inside, and that I'd be better off  
buying a new one.  I think I'm going to get a brand new (Chinese made)  
alternator at Canadian Tire for under $200.  Comes with a three-year  
warranty.  Time to bite the bullet, I guess.

Thank you all for your help and guidance, for an electrically  
challenged individual like me it was a very educational experience!   
What I still don't understand, though, is why would my handheld meters  
show any AC voltage at all, even when measuring across battery  
terminals with the engine off?

Radek.



------------------------------

Message: 5
Date: Mon, 7 Jun 2010 08:03:11 -0500
From: Joshua Van Tol <josh at spiny.com>
Subject: Re: Electrical gremlins again (crazy needle syndrome)
To: urq <urq at pacbell.net>
Cc: quattro at audifans.com
Message-ID:
         <AANLkTikDf-10RNdRyqL60Tb0kZgFYco2lPX7Rhzu3t-6 at mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1

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:


[Show Quoted Text - 59 lines][Hide Quoted Text]
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