S.U. Fuel Pumps:
Adding a Protection Diode

by Roger Garnett
Revised November 2000


The much maligned SP Pump

Some folks claim SU pumps are no darn good, and too prone to failure, but occasionally someone on a British-cars list defends SU fuel pumps by pointing that they just need to be cleaned and adjusted regularly.

Yea, regularly- like about every 15 to 30 years. This is far better than mechanical pumps, where the diaphragm starts leaking, and dump petrol into the crankcase. A time honored, traditional design, the SU electric fuel pump dates back to the 20's, or earlier. This pump does a good job with SU carbs, and has that added audible "fuel low" indicator feature. (The .tic.tic.tic...) An important part of the character of a British car! No matter what you say, I like my SU pumps, and their tic.tic.tic..tic.......tic. I've got the same basic pump on my '34 MG and '74 MG, and both work well after replacing the old stiff diaphragm.

There are times for something else as well, and modern pumps do have their place, such as race cars, engine swaps where you need higher flow rates, or when you need higher pressure, as with fuel injection systems. One of the more popular & reliable replacement brands is Facet. Pumps are available in a variety of pressure and flow ranges, so make sure you still get the correct pressure pump, or add a pressure regulator. Most SU and Zenith carbs call for 1 1/2 to 3 1/2 psi, while many US carbs use around 7 psi. The float valves in SU and Zenith Stromberg carbs can't take higher pressures, and will blow by and flood or overflow. Most of the time, that SU pump will do just fine.

Note also, that if you have Gross Jets (ball valves) in your carb floats, they can be *more* sensitive to overpressure than the original cone shaped needle valves.

All fuel pumps are wear items, and do need attention at times. The points wear & get pitted from the slight spark created when they open, and the surface will sometimes get corroded during storage. After a long life, the rubber diaphragm gets stiff and won't move. Another failure point is the fuel strainer screen found in some models, which can get plugged and prevent flow. Disassembly, and a good cleaning of the strainer & polishing the points will sometimes put a pump back in service, but if yours is farther gone, full rebuild kits are available. With a fresh rebuild, and proper adjustment, your pump will run as good as new. But, we can take it one step further.

Adding A Zener Diode

New SU pumps include a Zener diode, to protect the points from damage due to a spark arcing across them every time they open. The Zener is a cheap, little device that acts as a shunt to bleed off excess voltage spikes generated by the coil when the points are opened. They are used on most relay/solenoid designs today, in order to protect any electronics which is turning on the coil. Adding a Zener will not only keep down the arcing on the points, but will reduce noise spikes sent back into the wiring, which you may hear on your radio.

You can extend the life of your points by adding a Zener to your existing SU pump. The Zener's "breakdown voltage" rating must be higher than the cars normal operating range, as it will try to shunt off any over voltage. With a car's operating voltage range around 12-16 Volts, a 20-28 volt Zener will be a good, safe choice. Connect the Zener as close to the coil as possible- right on the terminals is best. Here's how to connect a Zener diode to a Negative ground car:

The diode can usually be soldered in place where the wires connect to the coil. The end of the diode that goes to positive will usually be marked with a band. On a positive ground car, the shown polarities and Zener must be reversed. If the points are on the ground side of the coil instead of the hot side, the Zener is still hooked up the same way- directly across the coil contacts.

Theory of operation:

Most diodes conduct current in one direction, and block flow in the other. Current flows through a diode in the direction of it's "arrow-head" as drawn on schematic diagrams.

                       | +            | +
                    ___|___         ____
                      /\            \  /
                     /__\            \/ 
                       |            ----
                       | -            | -

Current will not flow through the left diode, this is known as "reversed biased". Current will flow through the right diode, shown "forward biased". If these were "ideal diodes", the reversed biased direction would be seen as infinite resistance (open circuit), and the forward direction would be seen as no resistance (short circuit, or zero Ohms). Using a standard diode would help reduce arcing, by shorting spikes in one direction, but there's an even better solution.

The Zener is a special diode- (and is designated by the extra "feelers" seen in the first drawing.) It works normally when forward biased (no resistance), but only blocks the reverse current until voltage reaches it's breakdown or "Zener" voltage, at which point it begins to conduct. Thus, for the reversed biased Zener hooked to the coil above, nothing will happen until positive voltage rises above 24 volts (as with the spike created by opening the points). But, the Zener will shunt everything above 24 V back to ground, eliminating positive going spikes above 24 volts.. This is an added benefit, as any negative spikes generated by the coil will still be shunted back to the coil in the forward biased direction. Both of these actions will prevent high voltage spikes from reaching the points, arcing across them, and resultant pitting.

You can pick up a 24 volt Zener diode from any electronics supply store. (Radio Shack, etc.) It's pretty easy to remove the back cover of the pump, screw or solder in a diode, and put it back together. Your rebuilt & modernized pump will continue to give you many years of service. Tic... Tic... Tic...


Copyright 1992-2000 Roger Garnett You may publish this in your club newsletter, provided full credits are given, and you send me a copy.

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