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4.3 - SU fuel pump electronic conversion ( Mike Eck,  June 17, 2005 )

One characteristic of switches that most people are not aware of
is that they require a certain amount of current to work properly.
Under normal circumstances a certain amount of oxidation will build
up on the switch contacts which has the effect of insulating the
contacts from each other.

Packing the switch with dielectric grease and designing the
contacts so they wipe each other helps solve the problem, as
does wetting the contacts with mercury. However, none of these
design techniques are present in the SU fuel pump. It needs to
rely on current flow to burn through the oxide layer.

If you put a transistor in the circuit you eliminate the current
flow through the points so they don't burn and wear out but this
creates the aforementioned second problem instead.

The points in the SU fuel pump are perfectly capable of handling
the 3 Amps that the pump draws, but they get burned by the high
voltage arc that is generated when the points open (just like an
ignition circuit).

The solution is simply to install a suitably sized (3 Amp) diode
across the coil. Such diodes are available from Radio Shack as part
numbers 276-1141 through 276-1144. Diodes are polarity sensitive,
so connect the diode lead that is marked with a band to the coil
terminal that connects to the positive side of the battery. Forget
the transistor and just use the points to carry the current.

There's no need for the capacitor if you have the diode.

In spite of the fact that I love to add electronic devices to our cars,
this is the best solution for this application.


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