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The SU Fuel Pump
by Norman Nock
British Car Specialists
2060 N.Wilson Way
Stockton, CA 95205
Phone: (209) 948-8767
Fax: (209) 948-1030
Updated on 4 May 1995.
The early type SU fuel pump as fitted to British cars of the 1950s era
used to give their owners many problems. It was not unusual to hit the
fuel pump to get it started again. The points in the SU pump are designed
to give a wiping action on each stroke, helping to clean the points.
I was a mechanic working for an Austin-Healey dealer in Toronto Canada
in the 1950s and one of the jobs that went with a regular tune-up and
yearly service would be to replace the fuel pump points. If it was not
done, inevitably the pump would stop.
The condenser (capacitor) was introduced to help stop the points from
burning. When electricity jumps from one contact to an other it takes a
small amount of metal with it, leaving a hole in one contact and a
mountain of metal on the other contact (like distributor points).
The condenser helped to slow down the build up of metal on the contact.
The regular replacement on fuel pump points with a tune-up stopped,
but 20-30,000 miles down the road the same old problem (it stopped).
The SU fuel pump has been replaced with many other types and makes of
electric pumps, but the problem then is - they don't fit - they make a
lot of noise - they are always clicking - the pressure is too high - they
still have the same problem, the points burn.
The 1970s was a big improvement within the regular looking SU fuel pump
with the introduction of a diode. When the points open, electricity tries
to travel across the points in the opposite direction, but when a diode is
fitted it shorts out this reverse flow of electricity. There is now no arc
of electricity across the points; the points don't burn and pit anymore.
If a British car with this type of pump stopped the problem is rarely the
The SU fuel pump is now supplied with this diode installed. You need to
request positive (+), or negative (-). If you fit the wrong polarity pump
you will damage the diode.
Positive-ground fuel pumps with a diode may not have a diode. What the
major suppliers have been doing is removing the diode from a negative fuel
pump and renaming it a positive ground pump.
/ \ ____________ | |
\___/ | | \___/
|========| yellow |===========|
This pump now has taken a step back in time to the 1950s saying "It will be
okay, it will last a long time." Yes, but if you only drive say 2-3,000 miles
a year, it should last about 2-3 years, well past any pump warranty. If you
inspect your fuel pump to see if it has a diode, remove the black round
plastic cover. Reference the diagrams to see if you have a condenser -
diode - or nothing.
If the pump was a negative pump and the diode was removed, we have found
that whoever removed the diode, cut the wires at the terminals, removed
the diode, but the two terminals are still there.
If you don't have a diode, I would suggest fitting a correct SU diode to
avoid any stopping and embarrassment.
| | black / \
| | /| |===============\___/
Diode | |/ | |
| |\ | | | |
| | \| |----------------\__/
If your pump has been on the car for many years, you should rebuild it
completely or replace it with the new type SU with a diode!
| | | |
Condenser | silver |-----------\__/
Major repair kits are available - with diaphragm, points, gaskets, seals,
diode, valves, spring, filter - giving you a great savings over
purchasing a new pump. Complete set of instructions are included with each
This article was published in the CHATTER, the official magazine of the
Austin-Healey Club of America (AHCA) in November 1994. Printed here with
permission of the president of the AHCA George Feild and the author
This article is also part of a comprehensive collection of technical essays
about the Austin-Healey, published in the "Tech Book",128p., US$34.00
Agustin (Gus) de la Calle, MA
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