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Re: Fuel Pump Failure (again!)

Subject: Re: Fuel Pump Failure (again!)
From: "A. B. Bonds" <>
Date: 12 Jan 1996 08:50:09 -0600
In <>, Jay Tilton wrote:
>>Actually the QA at SU is now pretty good, and the point and
>>suppression technology is now way better than it was in the olden
>>days.  It is a HUGE mistake to rebuild a pump without a
>>suppression diode.  Point erosion is the primary cause of SU failure,
>>and the diode extends the life by a large factor.
>This is fascinating.  How does this diode work?  Is it intended only for
>fuel pumps, or can it be used for any set of switch contacts?  Could I get
>such a diode at Radio Shark?
>The reason I ask is that my brake light switch has a nasty habit of failing
>at least once every year.

The suppression diode is effective for _inductive_ (coil) loads only.
Unfortunately, brake lights aren't inductive.  The idea is that when
the coil is energized by contact closure, it builds up a big magnetic
field around it.  After all, that's its job.  The killer is when the
points open.  The field collapses when the current is removed,
inducing a _backward_ voltage in the coil.  Because of the speed of
the collapse of the field, this back voltage can get up to several
hundred volts (sort of like how spark coils work....)  The suppression
diode is across the coil, not across the points, and its job is to
absorb the back voltage to prevent it appearing across the points and
eroding the points.  The diode is hooked up with backward polarity
(i.e., with the cathode (band) toward the plus terminal of the
ignition).  This might seem as though it does nothing, unless you know
about the back voltage.

Unfortunately, I have no hints as to how to solve your brake ligh
problem.  You might get a meter and check the amount of current the
lights are pulling.  Shouldn't be more than 1 to 1.5 amps or so.
                A. B. Bonds

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