Electrical gremlins again (crazy needle syndrome)

[Cody Forbes]

http://assets.fluke.com/appnotes/automotive/beatbook.pdf

Page three of the Fluke guide shows the proper procedure and indicates  
that more than .5v AC is a sign of a bad alternator. That's how I've  
been testing them for years and there was even a question on my ASE  
test about it. I tend to use 1 volt as I said before, but really if a  
diode is bad it's going to make a lot like yours is.

Also, you shouldn't be disconnecting the battery with the engine  
running. It acts as a filter of sorts to keep voltage spikes at bay.  
On race cars with improperly wired kill switches I've done it a few  
times and never hurt anything, but it's still just not a good idea.

-Cody (mobile)

On Jun 5, 2010, at 2:17 PM, "Radek" <radek at istar.ca> wrote:

> Car drives fine and, after cleaning connections at the starter,  
> alternator and battery terminals, starts without issues.  However,  
> the voltmeter needle still fluctuates at idle.
>
> So I measured DC current at alternator (positive probe to lug  
> terminal, negative to the battery) with a digital meter - fluctuates  
> slightly around 14.4V (14.37 - 14.44 or so).  Next, I measured the  
> same with a needle meter - shows steady around 14V.  In other words,  
> hand held meters show ok, voltmeter in the dash fluctuates.
>
> So next, I removed the volt gauge from the dash and connected it  
> directly to the alternator and the negative bettery terminal, to  
> rule out wiring issues.  Started the engine and it really went  
> crazy!  With engine at idle, abour 900 rpm, the needle would jump up  
> and down between 13 and 16 volt.  With engine running above 2000  
> rpm, it would narrow down, oscillating between 14 and 16 volt.  It  
> would not go below battery voltage, though.
>
> I measured AC current at the alternator. Shows 30 - 31 volt.   
> Alarm!  However, I measured AC across battery terminals and it shows  
> 28 volt, with battery disconnected from the car. Should AC be  
> showing 0?
>
> I swapped the voltage regulator for another used one - no change.   
> Is voltage regulator the only device that converts AC to DC, or are  
> there other electronic devices on the alternator that could have  
> failed?
>
> I cleaned battery teminals and all connections I could find.  Small  
> Audis have no splice, battery is in the engine compartment.
>
> Any ideas?  Do I need a new alternator or a new volt gauge?
>
> Radek.
>
> ------------------------------
>
> Message: 4
> Date: Wed, 2 Jun 2010 05:16:19 -0700 (PDT)
> From: Andrew Dickinson <amdickinson at rogers.com>
> Subject: Re: Electrical gremlins again
> To: quattro at audifans.com
> Message-ID: <407006.58056.qm at web88308.mail.re4.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
>
> Also, the 'crazy needle' syndrome would indicate a short or open  
> wire issue to me if when it drops it is below the battery voltage.?  
> For the short you may be able to follow your nose to burnt wire.?  
> For either instance it appears to be intermittent so banging the car  
> in different places may prove fruitful to narrow down location as  
> well.? HTH.
>
>
> --- On Wed, 6/2/10, Cody Forbes <cody at 5000tq.com> wrote:
>
>
> From: Cody Forbes <cody at 5000tq.com>
> Subject: Re: Electrical gremlins again
> To: "Peter Golledge" <petergolledge at gmail.com>
> Cc: "Radek" <radek at istar.ca>, "<quattro at audifans.com>" <quattro at audifans.com 
> >
> Date: Wednesday, June 2, 2010, 7:44 AM
>
>
> You also might want to check for AC current with a multimeter. Failed?
> diode(s) can put AC current into the system and give all sorts of?
> funny issues. There will usually be a minimal amount, like less than  
> 1?
> volt, anything more than that is no good.
>
> -Cody (mobile)
>
> On Jun 2, 2010, at 12:57 AM, "Peter Golledge"?
> <petergolledge at gmail.com> wrote:
>
>> Since the battery _should_ act like a capacitor I think you still  
>> have
>> either intermittent ground or positive battery terminal to see large?
>> voltage
>> swings.? Since you don't mention below check/clean the battery?
>> ground cable.
>> I had a loose one of the those drive me nuts in a 5000.? Not sure if?
>> the
>> smaller audis had the dreaded "splice" in the positive cable at the
>> footwell, others would have to comment on that as a possible cause.
>>
>> As for the source of the surging voltage I'd suspect a voltage?
>> regulator,
>> they can be replaced with the alt in place and the they are  
>> relatively
>> cheap.? You might want to pull the regulator and check the brushes?
>> as well
>> as the regulator ground to the alt.
>>
>> -----Original Message-----
>> From: quattro-bounces at audifans.com [mailto:quattro-
>> bounces at audifans.com] On
>> Behalf Of Radek
>> Sent: Tuesday, June 01, 2010 8:27 PM
>> To: quattro at audifans.com
>> Subject: Re: Electrical gremlins again
>>
>> Thanks to all you guys for putting me on the right track.? So far, I
>> replaced the ignition switch (old one was broken), and the battery?
>> (old one
>> would drop to 8V when the key is turned to run).? I cleaned?
>> terminals at the
>> alternator, the starter and the jumper strip in the relay box, and?
>> replaced
>> about a foot of wire at the solenoid (old wire had corrossion, white?
>> powder
>> was falling out from between the strands of aluminum).? I also added?
>> an
>> extra ground between engine block and the chassis with a heavy gauge?
>> wire
>> (thickness of your little finger).
>> Car started five times in a row, so far.
>>
>> What worries me, though, is that while there is 13.8 V at the?
>> alternator
>> when charging and nearly the same (like 13.78) at the battery, the?
>> voltmeter
>> fluctuates.???Car idles at about 900 to 1000 rpm (too high, I know,?
>> I'll get
>> to it next).? At 1000 rpm the voltmeter needle is fairly steady,?
>> there is
>> just some barely visible vibration. However, when rpms drop, the?
>> needle
>> (sometimes, not always) goes crazy, jumps up and down.? Could that?
>> indicate
>> a charging problem that could have killed the old battery?
>>
>> Radek
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