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MG MGB Technical - SU fuel pump diodes

Thanks to the late David Dubois:
If you have a car with an SU fuel pump that is still has points, I have in stock TVS diodes that prevents
the points from burning and or needing cleaning. They fit under the "Flat" cover of MG T type
pump and have ESD protection, are Bidirectional & rated for automotive use. They also
replace the junk condensers on the later MGB pumps. Simple to install.
Only a few "Bucks" for US PS first class mail, .
(Moss Motors sells fuel pump doides of questionable reliability for $ 22.95 + shipping !)
abingdonperforma@aol.com
914 420 8699
Len Fanelli
Len Fanelli

From what I have seen condensers/capacitors were only used on the earlier pumps, being replaced by diodes before the end of MGB production and had the prefix AZX. These diodes are cylindrical with red and black wires coming from one end. That made the pump polarity-sensitive although there was protection against incorrect connection as there was a series diode to limit the current, which also reduced the effectiveness of the diode.

Eventually they were replaced by varistors aka TVS or MOV which could be a flat disc with wires coming from the edge, or a cylinder with a wire at each end. But that was a long time ago now.
paulh4

Bidirectional diodes work differently from the original diodes. They provide excellent protection by shunting anything in excess of the nominated voltage (typically 20-something in this application) across the points.
I am fairly sure that the current suppressor device supplied by Burlen under AUB6179 is one.
We service customers' pumps every 4-5 years (rather than wait till they become intermittent!) and have noticed a considerable improvement in pumps so fitted.
Paul Walbran

Correction to the above - there is a _resistor_ in series with the diode to limit current in the event of reverse connection.

Conventional diodes shunt all (subject to the series resistance) the back EMF (which is what burns the contacts as they open) away as the diode is conducting and offering a very low resistance path. Current devices are not diodes but a type of resistor which 'breaks down' above a certain voltage. There are many different types depending on what voltage it is intended to be used with, typically those intended for 12v shunt away everything above about 20v.

I use these on brake light switch relays, as even the back EMF from a relay as the switch opens is enough to degrade the contacts quite quickly. Modern switches are so cheaply made that the inrush current from filament bulbs also degrades the contacts, hence the need for a relay. LED 'bulbs' have their own issues.
paulh4

Here is more information on my Diodes:
Vishay Brand,1500W 24V.
I have one in my TD fuel pump and it has been working perfectly for more than 5 years.
Len


Len Fanelli

This thread was discussed between 04/05/2025 and 05/05/2025

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