Dave -
Thank you for your description of the fuel pump modifications
necessary to fit a diode. It was very clearly written. Thanks also for
responding to the list, and not just to Simon. I have a few comments to
add.
First, the cathode end of most diodes will have a black band
near that end for easy identification.
Second, the choice of diode type is not very critical. I would
try for at least a 1.0 Amp rating with a peak reverse voltage rating of
at least 500 volts. Types 1N4005, 1N4006, or 1N4007 should work just
fine. I used one rated 1.0 Amp, 800 volts.These diodes are amazingly
cheap - should be much less than one dollar.
Third, concerning your description of how to identify a diode's
polarity.
(snip)
To identify a diode's polarity, using an analog ohmmeter set on a low
scale, it should read low ohms with the meter's positive lead, usually
red, connected to the anode & the meters negative lead, usually black,
connected to the cathode. With the meter leads reversed it should read
high ohms. Since not all ohmmeters are color coded this way, it would be
best to apply the ohmmeter to a separate DC voltmeter to verify
polarity.
(snip)
Most cheap analog VOMs (Volt-Ohm-Meter) are wired such that when
switched to a resistance scale, the polarity of the leads is reversed;
i.e. what had been the positive (Red) lead will have a negative voltage
on it, with respect to the Black (now positive) lead. I verified this
last night on my Sperry and Lafayette VOMs. Your caution to verify
polarity with another voltmeter was an excellent suggestion.
Fourth, while you are working on the fuel pump, you may want to
"transistorize" it. In a stock pump, the points are required to switch
the full current of the pump's solenoid, probably on the order of one or
two Amps. By adding a transistor circuit, the points will only have to
switch a few milliamps to turn on a transistor, which in turn will power
the pump's solenoid. I designed and built a circuit like this for my BJ8
back in the 1970's. It was built in a Bud aluminum box and is mounted on
the wooden block that separates the battery from the rear fender, with
wires running to the pump. There is a switch on the box with three
positions (Normal - transistorized fuel pump, Off - no power to the fuel
pump, and Override - bypass the transistors and run the pump as
originally, in case of electronic failure.) I sometimes use the "Off"
position for theft prevention purposes. I have not published my circuit,
which uses transistors that I got from Radio Shack. However, there is a
circuit by David DuBois that you can find at
http://www.jag-lovers.org/xk-lovers/library/pump_neg.htm or
http://www.jag-lovers.org/xk-lovers/library/pump_pos.htm By lowering the
current the points must switch, the points' life should be increased
significantly.
Bernie Johnsen 1967 BJ8
|