Fuel Pump Points

[Tim]

I haven't had much experience with electrical fuel pumps before, most of my Minis have had mechanical ones.

Tonight I was repairing an AUF 201 (I think) - cleaning it up and fitting new points. When I tested it it seemed to run very nicely, but there was a lot of arcing from the points. Is it worth fitting some sort of suppression across the points? Maybe a capacitor or diode? If so what is best to fit? I plan to keep it positive earth but would prefer not to fit anything that would force it to have one or other polarity. I see that Burlen sell a supressor (part no. CZX 1004) which looks like a metal oxide varistor. Does anyone know what spec to use?

Tim

[dklawson]

Dave DuBois is an email acquaintance of mine who works on SU pumps. (You will find him as the author of some internet documents discussing a MOSFET conversion to protect the pump points). One of Dave's documents is a list of pumps and technical information about them. He mentions the capacitor on page 2 of the document linked below.

http://homepages.donobi.net/sufuelpumps ... ps_101.pdf

EDIT: I forgot to respond to your pump polarity preference. If you fit an electrolytic cap or diode for your car's present positive earth configuration, it is not a difficult task later to disconnect the component and swap it end-for-end to change the pump for negative earth. If I am remembering this correctly, there was a red band around the body of a positive earth pump to distinguish it from negative earth pumps.

[Tim]

Thanks Doug. I went looking for Dave's website and found a number of useful things he'd written about pumps. I couldn't find the MOSFET article but I can see how that might work.

I googled around a bit more and found a reference to using a transient voltage suppressor, which sounds pretty simple. I've ordered 5 of them at a total cost of $1.50, so I'll give that a try I think.

Tim

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[dklawson]

It looks like Dave no longer has the MOSFET articles easily available on his web site. There are some early copies on the net on other sites (as in the link below).
http://www.jag-lovers.org/xk-lovers/lib ... mp_pos.htm

I have not corresponded with Dave for about 4 years now. I was tinkering with an optical switch solution to replace the points and had OK luck on the bench but did I did not take the time/effort to make the system small enough to fit inside the SU pump cover. (Another of my incomplete projects).

At that time Dave told me he was unhappy with the long term performance of the MOSFET conversions he used to promote. What he found was that with the MOSFET, over time the points appear to glaze or form an oxide coating that limits current flow to the MOSFET gate. That oxide prevented the MOSFET from turning on which in turn meant the pump was just as dead as if the points had burned up. His explanation was that current flow through the points of an unaltered SU pump did allow the points to wear but it also kept the surfaces clean enough to pass power. I believe Dave then started working on his own Hall Effect conversion to address what he saw as problems with Burlen's kits.

EDIT: In the Jag link above Dave talks about using power transistors. He did later use MOSFETs capable of handling 30A to 60A of inrush current instead of transistors.

[Tim]

I wondered whether the points would glaze if the current was reduced. Dave seems to sell a conversion, although I'm not sure if its hall effect or optical (doesn't really matter). Eliminating the points would be the solution though. He charges $60, and I think the Burlen version costs about 40 pounds. But since I'm a tightwad, I'll give my 30c patch-up a try.

Tim

[Spider]

a metal oxide varistor.
Tim

I do really like the idea of these, however when you have a closer look at the specs for them, their break-over voltage always seems too high (for the correct rated voltage) be be of much use.

The humble Capacitor works well for points in the Ignition System, so I can't see why it wouldn't equally as well for this application.

Tong B at Mini Auto Electrics does an Electronic Conversion.