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<p>re: "Hooking it up to a power source in the wrong direction will
probably fry the diode."</p>
<p><br>
</p>
<p>Nope. Diodes allow current to flow in one direction, but not
the other (the 'wrong' direction). In this case, the diode is
connected in parallel with the points, and reverse-biased; i.e.
connected properly, the diode will not pass current, which will
flow through the points and energize the pump coil. Connected
forward-biased, the diode presents essentially a short and current
will not flow through the points and pump body, hence the pump
won't operate (but the diode won't be damaged). The way to fry a
diode is to apply voltage that exceeds the diode's rated value;
e.g. put 200V on a diode rated for 50V*. Older pumps used what
appears to be an electrolytic capacitor to damp the arc, and they
would be fried instantly if connected backwards.<br>
</p>
<p><br>
</p>
<p>When the points open, the collapsing electromagnetic field in the
pump body's coil--essentially a solenoid--causes the diode to be
forward-biased and the diode will conduct ('shunt') the resulting
reverse current back to the 12V power supply instead of across the
points, mitigating wear on the points (I've heard this type of
usage called 'flywheel' diodes, as they act to absorb the reverse
current similar to the way a flywheel absorbs angular momentum).
<br>
</p>
<p><br>
</p>
<p>re: "You should have continuity in one direction across the
diode and none in the other."</p>
<p><br>
</p>
<p>This test should be done with the diode out of the pump or, at
the least, make sure the points are open else you will see
continuity through the points and pump body or diode, regardless
of the connection.</p>
<p><br>
</p>
<p>* Unless it's a Zener--or 'avalanche,' or 'breakdown'--diode
which is specifically designed to break down and pass current
above a certain voltage. These are used in rudimentary
overvoltage protection circuits; when the Zener breaks down it
passes current through--and, hopefully, blows--a fuse. This is
called 'crowbar' overvoltage protection, as the effect is like
dropping a crowbar across across a hot lead and ground. The TVS
diodes favored by some to protect pump points are essentially a
type of avalanche diode: <br>
</p>
<p><br>
</p>
<p><a class="moz-txt-link-freetext" href="https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode">https://en.wikipedia.org/wiki/Transient-voltage-suppression_diode</a></p>
<p><br>
</p>
<p>Bob (yeah, it's a slow day and I'm recovering from a cold so have
way too much time on my hands)</p>
<p><br>
</p>
<br>
<div class="moz-cite-prefix">On 6/23/2018 8:27 AM, WILLIAM B
LAWRENCE wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CY4PR19MB01207B4A888ABE608C337794A5740@CY4PR19MB0120.namprd19.prod.outlook.com">
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<p style="margin-top:0;margin-bottom:0">Better yet test with a
multimeter set for continuity. You should have continuity in
one direction across the diode and none in the other. </p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">Hooking it up to a power
source in the wrong direction will probably fry the diode.
While that will be inconvenient you will at least know what
the problem is...</p>
<p style="margin-top:0;margin-bottom:0"><br>
</p>
<p style="margin-top:0;margin-bottom:0">Bill Lawrence</p>
<p style="margin-top:0;margin-bottom:0">BN1 #554</p>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt"
face="Calibri, sans-serif" color="#000000"><b>From:</b>
Healeys <a class="moz-txt-link-rfc2396E" href="mailto:healeys-bounces@autox.team.net"><healeys-bounces@autox.team.net></a> on behalf of
Bob Spidell <a class="moz-txt-link-rfc2396E" href="mailto:bspidell@comcast.net"><bspidell@comcast.net></a><br>
<b>Sent:</b> Saturday, June 23, 2018 2:39:43 AM<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:healeys@autox.team.net">healeys@autox.team.net</a><br>
<b>Subject:</b> Re: [Healeys] SU fuel pump polarity</font>
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<p>Looks like they're both wired the same, and for neg. ground.
Try hooking them up to a 12V power source, and see which way
makes them fire (pump). If they don't run connected one way,
try the other. If it doesn't run right off the back
disconnect lest you overheat the solenoid body.</p>
<p>Are both diodes labeled the same?<br>
</p>
<p>Bob<br>
</p>
<br>
<div class="x_moz-cite-prefix">On 6/22/2018 3:18 PM, <a
class="x_moz-txt-link-abbreviated"
href="mailto:hspethmann@t-online.de" moz-do-not-send="true">
hspethmann@t-online.de</a> wrote:<br>
</div>
<blockquote type="cite">
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">My
Healey:</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">BJ8
MKIII 1964</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">#
L27347</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">pos.
earth</span></p>
<p style="margin:0px"> </p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">Fuel
pump for the last 20 years:</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">SU
AZX1307</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">now
with a slow, but steady clicking every 15 to 20 sec , when
ignition is on.</span></p>
<p style="margin:0px"> </p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">I
thought, the easiest solution is to change the pump with a
spare pump I have in my trunk for about the same time as
the first one. Both pumps look the same on the outside.
Both pumps are wrapped in black tape. But the label on
the box of the spare pump says: AZX1307 neg. earth.
</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">In
the archives I found a very instructive contribution by
Steve Russel from Aug 2009. And it ended with the words:
"A picture would be worth a lot of words right now."<br>
</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">So I
removed the cap on both pumps to see if there is any
difference in the wiring and took a picture, attached to
this mail.. Both pumps look the same inside. You see the
old pump on the left, the new one on the right. Under the
cap you see a piece that looks like a capacitor, labeled
as BZX1013. But following the explanation with terminal 1,
2 and 3 clockwise beginning with the power input stud and
ending with the internal ground terminal I suppose the
"capacitor" is a diode. But when there is no difference in
wiring between the old and the new pump, why could it be,
that the old one looking like a neg. earth ran 20 years in
a car with pos. earth?<br>
</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">I'm
confused and hoping for your wisdom to shed some light on
the problem.</span></p>
<p style="margin:0px"> </p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">Best</span></p>
<p style="margin:0px"><span
style="font-family:arial,helvetica,sans-serif;
font-size:10pt; color:#000000; text-decoration:none">Achim</span></p>
<p style="margin:0px"> </p>
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