[Healeys] SU fuel pump polarity

Bob Spidell bspidell at comcast.net
Sat Jun 23 11:39:29 MDT 2018

re: "Hooking it up to a power source in the wrong direction will 
probably fry the diode."

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.

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).

re: "You should have continuity in one direction across the diode and 
none in the other."

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.

* 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:


Bob (yeah, it's a slow day and I'm recovering from a cold so have way 
too much time on my hands)

On 6/23/2018 8:27 AM, WILLIAM B LAWRENCE wrote:
> Better yet test with a multimeter set for continuity. You should have 
> continuity in one direction across the diode and none in the other.
> 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...
> Bill Lawrence
> BN1 #554
> ------------------------------------------------------------------------
> *From:* Healeys <healeys-bounces at autox.team.net> on behalf of Bob 
> Spidell <bspidell at comcast.net>
> *Sent:* Saturday, June 23, 2018 2:39:43 AM
> *To:* healeys at autox.team.net
> *Subject:* Re: [Healeys] SU fuel pump polarity
> 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.
> Are both diodes labeled the same?
> Bob
> On 6/22/2018 3:18 PM, hspethmann at t-online.de 
> <mailto:hspethmann at t-online.de> wrote:
>> My Healey:
>> BJ8 MKIII 1964
>> # L27347
>> pos. earth
>> Fuel pump for the last 20 years:
>> SU AZX1307
>> now with a slow, but steady clicking every 15 to 20 sec , when 
>> ignition is on.
>> 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.
>> 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."
>> 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?
>> I'm confused and hoping for your wisdom to shed some light on the 
>> problem.
>> Best
>> Achim

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