Re: Battery Light -techie fodder - oscopes and ripples and lights, oh my

["Allyn" <amalventano1@home.com>]

Brian,
    if you have 14v at idle and you are getting the light, only thing i
could imagine is that if a phase was not working, the regulator would have
to max the field out to keep up. this may fool the alternator warning
circuitry into thinking that it cant keep a charge, and make the light come
on somehow. raising engine rpm means less armature current is needed, so the
regulator can back off of the max output. Since yours seems to be non-rpm
related, i can think it may be remotely a brush/slip ring problem, but more
likely an intermittent failure of a rectifier diode or an intermittent open
on a stator winding. The oscope should tell you whats up.
    oscope will show the ripple, you may have to expand the scale out to
something like 0.1V/div and AC coupling so it offsets to 0 reference (so you
can see the trace at all).
    a missing phase will be missing every third ripple on the scope.
Al
(nuclear trained scirocco list consultant :)

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 170k, rocco #6) running - for now
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
driveshafts, wheels :)

----- Original Message -----
From: "Brian Haygood" <scirious@hotmail.com>
To: <amalventano1@home.com>
Sent: Monday, November 05, 2001 9:43 PM
Subject: Re: Battery Light -techie fodder


> Hey Allyn;
>
> You know you are going to get all sorts of Alt questions now, right?
>
> First is whether or not a bad phase (in diodes or windings) could be the
> cause of my intermittant batt light problem:
>
> DVM shows ~14volts regardless of anything else.
> Batt light comes on and off, unrelated to rpm, slight correlation to
> temperature, but not absolutely so.
> Car turns over just fine but doesn't start when heat soaked on hot days
(no
> it's not an 8v, so no thing on the block to move off of the block)
>
> More pertinent question may be:
> If I hook up an Oscilloscope, will I be able to see a ripple in the
> alternator's output corresponding to the overlap of the three phases?  Can
I
> tell from that if I have lost a phase, or do I have to somehow put the
meter
> on a particular phase, and if so, how do you do that?? ?
>
> I've tried putting jumper cables between the alt and batt, cleaning all
the
> contacts I can reach, and staring at it for hours, but nothing seems to
> help.
>
> Thanks Reigersubmarineman.  Hey, imagine a sub painted that Reiger green
of
> yours.
>
> -Brian
>
>
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