In <9607031941.AA02111@genesis.torrington.com>, Mark Jurras wrote:
>> Reply-To: Kevin Sullivan <kevins@borris.khoros.unm.edu>
>>
>> > Does anyone know how to tell the difference between a negative and positive
>> > ground fuel pump?
>>
>> I was told that the diode was reversed; no other difference. This is not
>> first hand knowledge, however.
>
>What Diode???
When current is passed through a coil, it builds up a magnetic field
around the coil. That's what makes the pump pump. When the armature
has moved as far as it can go, if the points are adjusted properly
they open, stopping the current. The field collapses and the armature
returns to rest position as encouraged by the spring. But there is
more to the story. There's energy in that thar magnetic field, and
when it collapses (due to interruption of the current) it induces a
_backward_ voltage in the coil. Ah, you say, I recognize that
notion--that's how spark coils work. But spark coils spit out their
spark to plugs, whereas the poor fuel pump is unrequited, and has no
legitimate outlet for its excess energy. This ends up being burned up
across the points, which is a major cause of failure in earlier SU
fuel pumps.
About 20 years ago SU got smart and discovered power diodes, which had
only been around for about 20 years before that. They are connected
_backwards_ across the coil. When the coil and points are conducting,
it has no effect. When the points open, it offers a low-resistance
pathway for the back voltage generated by the coil, and it helps to
save the points.
Because of the circuitry, it's nearly impossible to tell from the
outside whether or not your pump has a diode or what the polarity is.
If you take off the endcap, you can see the diode near the points (if
you know what a diode looks like--little black plastic thingie with
two wires). If you connect a positive ground pump to a negative
ground car or vice-versa, you will surely blow a fuse, if not the
diode. Because the polarities used for testing diodes on multi-meters
vary, I am reluctant to suggest that you try to figure it out that
way. Note also that there are other differences between
SU pumps, such as low-pressure and high-pressure. I'd be VERY sure
that you are using the correct one.
A. B. Bonds
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