[Healeys] Electric Draw

[Bob Spidell]

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 at schoerner.se 
> <mailto:per at schoerner.se>> wrote:
>
>     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 at gmail.com
>     <mailto:050.rpl at 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, here’s what I’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 it’s either “good
>     or bad” 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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