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Uh, I'm not an electrical engineer--but I play one on email lists--but
it sounds like your mechanic isn't one either (not uncommon). The
diodes in an alternator perform the same function as the brushes and
commutator in a generator; i.e. they rectify the alternating
current--produced by rotating an electromagnet--into DC current, which
your battery requires for charging and all your accessories require to,
well, accessorize. Hence, they aren't doing squat when the engine isn't
running--when there should be no current coming to the alternator to
energize the rotor--and if you had a short somewhere and your alternator
was getting current with the engine stopped it would likely heat up (see
if your alternator feels warm after sitting overnight). Diodes can fail
by either shorting out or opening up; most alternators have 6 of them
and when one fails by opening up you lose one-sixth of your charging
ability (I'd have to research it, but as solid state devices--sort of
one-third of a transistor--they probably mostly fail by opening up as
the silicon junctions are relatively fragile). I'm not sure if it's an
open or shorted diode that causes it--maybe both--but with a bad diode
you will get all kinds of radio noise, which varies with engine speed
(when I used to fly light aircraft you could tell when someone had a bad
diode with serious noise in radio transmissions).
Others have offered good suggestions so I'll just add that it's
conceivable your ignition switch is worn allowing some current flow when
in the 'off' position (after all, it's Lucas ;)). See if it feels warm
after sitting for a while (in fact, if you can get to them see if any of
the electrical devices in your car feel warmer than ambient after
sitting in the shade). Your battery ground doesn't pull a
load--something has to draw current that the ground cable returns to the
battery--so it isn't the problem. A nearly half-amp current draw in a
12V system will produce 6 watts of heat (nearly half of what a 60W
equivalent CFL light rated at 13W will generate).
Bob
On 6/7/2019 6:13 AM, Michael Salter wrote:
> Real Healeys don't have alternators.����
>
> On Fri, Jun 7, 2019, 8:55 AM Per Schoerner, <per@schoerner.se
>
> It sounds more like your cutout switch is the culprit here.
>
> Per
>
> Skickat från min iPhone
>
> > 7 juni 2019 kl. 14:47 skrev R. Lindsay <050.rpl@gmail.com
> >
> > For the past few weeks I have been fighting a parasitic draw on
> my BJ8. It was running about .48 amps and would drain my battery
> in a day or so. I was told by a mechanic that a diode in my
> alternator must have blown therefore causing the draw. He also
> mentioned that my ground wires in the boot were loose which he
> tightened. I brought the car home, turned off the power in the
> boot and 2 days later the battery was dead.
> >
> > So, hereâ??s what Iâ??ve done. I jumped the battery and started the
> car and ran it for 10 minutes. I disconnected the charger and
> tested the battery - 12.48 volts. I then removed the cutoff switch
> and bolted the grounding wires together. I tested for a draw and
> got a zero reading. That was last night. I went out this morning
> (12 hours later) and the battery reads 12.33 volts.
> >
> > My questions are 1. Is .15 volts a reasonable drop in voltage
> for a sitting car in 90 degree heat and 2. Can a diode in an
> alternator work sometimes and not at others. If itâ??s either â??good
> or badâ?? all the time, I guess I need to presume the alternator is
> not the source of the draw. If a .15 volt loss over 12 hours is
> reasonable then removing the cut out switch in conjunction with
> presuming the alternator is good (as it works fine now) solved my
> problem.
> >
> > What does the wisdom of this great resource think?
> >
> > Price Lindsay
>
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<p>Uh, I'm not an electrical engineer--but I play one on email
lists--but it sounds like your mechanic isn't one either (not
uncommon). The diodes in an alternator perform the same function
as the brushes and commutator in a generator; i.e. they rectify
the alternating current--produced by rotating an
electromagnet--into DC current, which your battery requires for
charging and all your accessories require to, well, accessorize.Â
Hence, they aren't doing squat when the engine isn't running--when
there should be no current coming to the alternator to energize
the rotor--and if you had a short somewhere and your alternator
was getting current with the engine stopped it would likely heat
up (see if your alternator feels warm after sitting overnight).Â
Diodes can fail by either shorting out or opening up; most
alternators have 6 of them and when one fails by opening up you
lose one-sixth of your charging ability (I'd have to research it,
but as solid state devices--sort of one-third of a
transistor--they probably mostly fail by opening up as the silicon
junctions are relatively fragile). I'm not sure if it's an open
or shorted diode that causes it--maybe both--but with a bad diode
you will get all kinds of radio noise, which varies with engine
speed (when I used to fly light aircraft you could tell when
someone had a bad diode with serious noise in radio
transmissions).</p>
<p>Others have offered good suggestions so I'll just add that it's
conceivable your ignition switch is worn allowing some current
flow when in the 'off' position (after all, it's Lucas ;)). See
if it feels warm after sitting for a while (in fact, if you can
get to them see if any of the electrical devices in your car feel
warmer than ambient after sitting in the shade). Your battery
ground doesn't pull a load--something has to draw current that the
ground cable returns to the battery--so it isn't the problem. A
nearly half-amp current draw in a 12V system will produce 6 watts
of heat (nearly half of what a 60W equivalent CFL light rated at
13W will generate).<br>
</p>
<p>Bob</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 6/7/2019 6:13 AM, Michael Salter
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAB3i7LLeik9r7cZyMa3Ph8tObwjUZA1VXpT5mQ0jPFZQGQVoKw@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="auto">Real Healeys don't have alternators.����</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Fri, Jun 7, 2019, 8:55 AM
Per Schoerner, <<a href="mailto:per@schoerner.se";
moz-do-not-send="true">per@schoerner.se</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">It sounds
more like your cutout switch is the culprit here.<br>
<br>
Per<br>
<br>
Skickat från min iPhone<br>
<br>
> 7 juni 2019 kl. 14:47 skrev R. Lindsay <<a
href="mailto:050.rpl@gmail.com"; target="_blank"
rel="noreferrer"
moz-do-not-send="true">050.rpl@gmail.com</a>>:<br>
> <br>
> For the past few weeks I have been fighting a parasitic
draw on my BJ8. It was running about .48 amps and would drain
my battery in a day or so. I was told by a mechanic that a
diode in my alternator must have blown therefore causing the
draw. He also mentioned that my ground wires in the boot were
loose which he tightened. I brought the car home, turned off
the power in the boot and 2 days later the battery was dead. <br>
> <br>
> So, hereâ??s what Iâ??ve done. I jumped the battery and
started the car and ran it for 10 minutes. I disconnected the
charger and tested the battery - 12.48 volts. I then removed
the cutoff switch and bolted the grounding wires together. I
tested for a draw and got a zero reading. That was last night.
I went out this morning (12 hours later) and the battery reads
12.33 volts. <br>
> <br>
> My questions are 1. Is .15 volts a reasonable drop in
voltage for a sitting car in 90 degree heat and 2. Can a diode
in an alternator work sometimes and not at others. If itâ??s
either â??good or badâ?? all the time, I guess I need to presume
the alternator is not the source of the draw. If a .15 volt
loss over 12 hours is reasonable then removing the cut out
switch in conjunction with presuming the alternator is good
(as it works fine now) solved my problem. <br>
> <br>
> What does the wisdom of this great resource think?<br>
> <br>
> Price Lindsay<br>
<br>
</blockquote>
</div>
</blockquote>
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