PDLJMPR Web Magazine, May 05, 1996
Written
by Norman Nock, British Car
Specialists, Stockton, California
Reprinted with Permission of Author
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The early type SU fuel pump as fitted to British cars of the 1950's 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 1950's and one of the jobs that went with a regular tune-up and yearly service would be to replace the fuel pump points. If the points were not replaced, they would burn up and inevitably the pump would stop after 10-12,000 miles.
The condenser (capacitor) was introduced to help stop the points from burning. When electricity jumps from one contact point to another 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 addition of a condenser slowed down the process, and extended the life of the points to 20-30,000 miles. Still, if they weren't replaced during tune-up, the same old problem came back - the pump 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, and they still have the same problem, the points burn.
The 1970's 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 point. The points don't burn and pit anymore. If a British car with this new type of pump stops, the problem is rarely the fuel pump.
The SU fuel pump is now supplied with this diode installed. You need to request a positive or negative ground type. If you fit the wrong polarity pump you will damage the diode.
Diode fuel pumps designed for positive ground may not have a diode, however. What the major suppliers have been doing is removing the diode from a negative fuel pump and renaming it a positive ground pump. This pump now has taken a step back in time to the 1950's. They say "It will be OK, 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 want to 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, a diode - or nothing at all.
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. 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!
Major repair kits are available with diphragm, points, gaskets, seals, diode, valves, spring, and filter, giving you a great savings over purchasing a new pump. A complete set of instructions are included with each kit.
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