Raymond A. Carbone (First published in the February 2012 PEDC newsletter)
I‘m always amazed at the communication and camaraderie that exists among LBC owners throughout the world. In a recent web response to a call for help on an SU fuel pump problem, a Healey owner in Italy referenced an article by Eric Lembrick in an English MGT club newsletter called T-Type2. Eric‘s article, along with additional research, formed the information- base for this presentation and suggested modification.
The Old Ticker
Over the years I have found the SU pump to be quite reliable and the familiar ticking a comforting sound. However, many of us have experienced a fouled-points failure that had stopped our SU fuel pump in mid-tick. Stranded, it is not unusual for a driver to resort to the elegant hammer for relief from frustration and sticking points. Even with the potential for problems, I and many others still prefer to retain their points-triggered pumps, and some have even installed a second modern pump to dampen any fuel- delivery concerns.
However, can we improve point reliability and pump longevity to the point of eliminating the need for insurance?
The Heart Attack
Since many of us infrequently drive our LBCs and often store our vehicles over the winter, naturally forming oxidation on the tungsten points usually leads to a non- functioning pump. Dressing the points prior to a post-hibernation start is not an uncommon act; however, more often than not we rely upon the oxidation being burned off by the high switching voltage developed by the pump. Although a remedy for oxidized points can be a high-voltage burn-off, high point-switching voltage is the common cause of flashed and burned points and the most common cause of pump failure.
Many of us were under the misconception that because power to the pump normally ranged between 12 and 15 volts, the voltage across the points was also within that range. However, since a coil is used as an active component, several hundred volts are actually generated across the opening points, similar to the way the ignition system coil produces a high voltage across the ignition points. Under these high voltages it is not uncommon for the points to burn, flash, and even weld together during operation.
And the Beat Goes ON
Over the SU pump‘s existence, a number of different diodes have been applied by the manufacturer to help control high-voltage point flash. Although moderately successful, one unintended consequence is the creation of polarity restrictions. Finally, the manufacturer introduced a transistorized triggering mechanism for the SU pump that would eliminate the points and their problems all together, but even these transistorized switching circuits are somewhat sensitive to high voltages and power fluctuations.
More disturbing is the fact that if a transistorized pump fails, there is little to no chance it can be revived to get you home.
Within recent years, electronic components called transient-voltage suppression (TVS) diodes have been designed to protect sensitive electronics from power spikes. This device controls overvoltage by shunting power to ground when it exceeds a specific brake-down threshold.
When applied to our SU pump‘s points, a TVS will shunt off excessive flash voltage while allowing the pump to function within a normal voltage range. The result of applying a TVS to the points of an SU pump is the elimination of high flash voltage with a substantial extension of point/pump life.
TVS Selection
When selecting an appropriate TVS, a number of operating conditions must be considered. First, the device must be able to pass power through an LBC‘s normal range of electrical operation. Also, we would like the pump to function even when a (generator/alternator) voltage regulator fails and puts out as much as 19 volts. Additionally, since our objective is to prevent high-voltage point flash and burn, voltage must be kept below a level where this condition is created.
TVS Cost
My choice of a P6KE20CA TVS (manufactured by Vishay), at a cost of around $1.50, was made to meet selection objectives. This device will pass power below 21 volts and safely allow the pump to operate under our stipulated conditions. Additionally, the TVS will react at millisecond speeds to impose a clamping (ground-shunt) voltage of 27 volts to eliminate point flash and burn, before automatically resetting for the next points cycle.
Please keep in mind that my TVS component selection was based on the criteria set above, and others may identify additional or alternative conditions from which to select and apply an alternate device. It should not be difficult to identify an appropriate TVS to meet your selection criteria, as there is a large selection of TVSs from which to choose.
Life-sustaining Operation
Installation is rather simple in that a TVS is wired between the ground screw and points fixing- screw on the head of the SU pump. It should be noted that although not sensitive to polarity (positive/negative), a TVS must be directionally oriented toward the ground. Although some manufacturers print an arrow on the TVS pointing toward the ground end, some have writing on the unit that is read from the ground end. Once the TVS is installed and the points are adjusted per the standard SU procedure, you can pre- test and drive with added confidence.
NOTE: Photos for this article are courtesy of the author.
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