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Re: Alternator light

To: "Florrie & Allen Bachelder" <>,
Subject: Re: Alternator light
From: "Harlan Jillson" <>
Date: Wed, 10 Feb 1999 21:30:59 -0600
   I kind of have the same sort of problem with my '72 B (light on a
little....).  I went and read the alternator primer on skyes web site and it
gives a clue as to what causes the light in the first place, and as to why
the diode appears to work.
   Apparently the 'warning' light come on when either the battery voltage is
lower that the alternator output, or when the alternator output is lower
than the battery voltage.  The diode restricts the current flow through the
light in the first case and not the second ( if installed the correct way
around) so the light appears to work.  In starting, the alternator isn't
putting out any voltage, so current flow through the light and diode and the
alternator bridge and regulator circuit to ground (earth) and the light does
it's thing.
   In my case the light comes on real dim at idle, then gets bright when the
engine revs come up, especially when there is lots of extra load (lights,
radio, etc...).  After running for a while the light goes out.  All of this
indicates that the circuit balance is in favor of the alternator (aka
battery is low...), which your diode would block, so the light would be out
if it were installed.  I really think that what is being indicated is that
there is probably series resistance in the sense circuit from the light back
to the battery ( theory) so it appears lower than it really is until the
charge is peaked up to full.  (The battery is new, and measures to be fully
charged when check directly at the battery.)
   I guess the next step to prove or disprove my theory is to measure the
voltage level at the battery side of the light and see if there is a drop
compared to the level at the battery.  I'm guessing that this is just
another manifestation of corroded connection syndrome that our LBC's are so
prone to.
   IMHO, YMMV, etc....

-----Original Message-----
From: Florrie & Allen Bachelder <>
To: Paul Hunt <>
Cc: <>
Date: Wednesday, February 10, 1999 6:53 PM
Subject: Alternator light

>>...Fitting a diode in the indicator lead to cure this is absolutely
stupid -
>>all you are doing is stopping your alternator telling you when it has
>>completely, it would be far simpler to remove the light altogether.  But
>>preferred option is to fix the real problem.
>Paul - and  List -
>Maybe it is ...absolutely stupid... but I can duck that one 'cause I'm not
>the one who did it.  At any rate, the offending diode is no longer there.
>But even then I ask why, when the diode was still there, did the light
>still come on brightly when switching on the ignition (as it is supposed
>to), only to promptly quit when the engine started (again, as it is
>supposed to)?  Why did the diode not prevent it from lighting at all?
> As I think about this, I may be starting to answer my own question.
>Hopefully you can  either verify or show me the error in my thinking.   The
>light simply indicates a voltage desparity between alternator output and
>the ignition circuit, right?  What if the alternator output voltage is
>slightly  higher than that (from the battery) at the end of the white wire?
>The light would glow then too, right? But since that is not due to an
>alternator fault, we don't need that information.  So, (1) can we defend
>that in-line diode on the basis that, since it allows current flow in one
>direction only,  it is simply keeping the light off when the alternator
>output is greater than the ignition circuit, and (2) is there a type of
>battery defect that can lower its output enough to cause this problem (the
>glow is rpm-independent)?  I would presume that if this is due to line loss
>from poor contact in a bullet connector or the ignition switch, that in 23
>years, total failure would have resulted.
>I suppose another answer that it apparently is charging - enough to be
>functional even - but not as much as it should.  This battery, of
>indeterminate age when I bought the car in '91, still spins the starter
>faster than any of my other Bs.
>But really, despite the fact that we all know the light isn't supposed to
>glow, if the car is operated that way without electrical failure for over
>two years and perhaps 20,000 miles - mostly commuting at hours when the
>headlights are on - is it not unreasonable to presume that there is no real
>Oh-oh - I should never talk that way - lest tomorrow morning I should go
>out to start my B and find the battery dead.

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