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MG MGB Technical - Fuel pump electronic component

I'm replacing the contacts on my SU fuel pump and have spotted what looks like a capacitor, presumably designed to reduce arching as the contacts open and close from the pump solenoid.

Testing the component indicates that the capacitor is open circuit.

But I'm wondering if this is a capacitor or some kind of zenor diode.

This doesn't seem to be available on the MG suppliers.

I'd appreciate if someone could come up with the nature of the component and its value.

Many thanks,
Brian
Brian McIlvenna

Brian - a capacitor will read "open" when tested with an ohmmeter, which is perfectly normal. Let me ask how old your fuel pump is if you can post a picture of the component or send it directly to me so I can see what you have. Earlier pumps used a capacitor, but later pumps have evolved over the years, using a diode/resistor combination (a dome type end cover is required for one of those to fit under it) and recently the use of a Transient Voltage Suppressor has been used. If I can see the component, I may be able to help you with a replacement, if one is needed. Cheers - Dave
David DuBois

Brian
I have an aftermarket fuel pump that worked intermittendly and then just sparked and smoked while testing it. On the inside it is a diode, small barrel that looks like a resistor with color bands on it. I replaced it and it works great. Very cheap at Radio Shack. I found out about this thru another lister, Wray Lemke who I believe is an electronics engineer. here is a note he sent me. All credit goes to Wray, he's the best.



Mike, it is almost certainly not a resistor but is probably a diode. Now
that I think of the numbers you mentioned I bet it is a 1N4001 diode. The
band indicates polarity; you can't reverse the voltage without blowing the
diode. They are cheap from Radio Shack.

Look at the attached picture; if it is like these then you can simply
replace it with another 1N4001. Diodes come in several ratings,
1N4001/1N4002/1N4003/1N4005/1N4007. They are all the same for your
purposes, the higher the number the most current it can pass.

Pay particular attention to the end that has the band on it, it must be
replaced the same way. The banded end on the new diode must be hooked to
the same place as the banded end on the blown diode.



MK Mike

The component is a barrel like one with the inscription BZX1019. My car is a MkI '67 BGT; the pump looks original.

A capacitor should show a lowish resistance initially and then grow over several seconds. I've tried this on other capacitors and this meter pattern is replicated.

I've been practising my electronic engineering, using resistors and capacitors on a circuit bread board. The only capacitor I have is a 4700uf electrolytic one. This succeeds in almost stopping the point contact arcing. I tried using resistor for about 60 to 4,7Kohms; this only cancelled the good work of the capacitor.

Thanks,
Brian
Brian McIlvenna

I forgot to mention that the component inscription also includes the text 'Ampohm'.

Brian
Brian McIlvenna

BZX is a semiconductor number, 2 wires and its a diode. A very small cap will charge up in millieseconds so you will not see the meter move.
Stan Best

Brian - The capacitor used with the earlier pumps is a 0.47 microfarad and like Stan states above, you will not see the charging because it happens too fast. The 4700 microfarad capacitor you tested the pump with is a large electrolytic capacitor and is large enough that you can see it charge up. While it will reduce the arcing it will not hold up long in the fuel pump. An electrolytic is polarized and because it is installed across the points, it will be handling a 12 volt level in one direction and a much larger voltage spike in the other direction as the fuel pump operates. The BZX1019 is a diode/resistor unit that serves as the arc suppression for the pump that you have. If you don't see any burned spots on it, then it is probably good - you can check it by applying the leads of your ohmmeter, first in one direction, then in the other direction. One direction it should read infinite and in the other direction it should read around 100 ohms. If you get those readings, then the unit is good and will work fine as long as it is hooked up correctly. Note that you will still see arcing at the points even with the diode/resistor unit, or any other arc suppression circuit hooked up, but the arcing will be what is called a soft arc. By that I mean that it will be an arc of low enough voltage and current that it will not cause excessive burning of the points. A straight diode, such as the 1N4001, across the points or coil, while it will suppress the arc, it also slows the rate of pumping so it is not an ideal method of arc suppression. Cheers - Dave
David DuBois

Thanks for the replies.

The component does have a bulge spot on the case; I suspect the points began to burn out because of the suppression component failure.

I checked the resistance in one direction only and this read as infinite - an open circuit.

Does anyone know where this component or a modern improvement can be purchased? It is not listed in any MG suppliers.

Thanks,
Brian
Brian McIlvenna

Burlen (the SU suppliers) supply a modern upgrade to replace the old electronic components in the pump. Easily found on their website. Or you can do it yourself from the local electronics shop. I'm on holiday but when i get home on Tuesday I will look up the specs and diagram, if no one beats me to it
James Finlayson

BFS web site is at: http://www.burlen.co.uk/ They can supply either the original diode/resistor assembly or a modern replacement as James suggests (probably a Transient Voltage Suppressor). Cheers - Dave
David DuBois

Thanks again for the replies. I've checked the Burlen site and the part doesn't appear to be there.

James, I'd appeciate if you could send on the component specifications when you get a chance.

I take it that the zenor diode and resistor are connected in series? The diode allowing the current to flow in forward biased mode, when the point part. The resistor limits the current through the diode.

Brian
Brian McIlvenna

Brian - I'll do some research on that part number, I think it may have a revised number. All arc suppression devices are wired in parallel with either the points or the coil. They work by shunting the reverse voltage spike (which can be quite large) away from the points. Cheers - Dave
David DuBois

Hi Dave, I bow to your knowledge on this! My pump has never missed a beat. the TVS you sent me worked perfectly! When I bought some rich needles from Burlen I also got them to through in their suppressor. Cost is 4pounds, part number CZX1004 and here is the link
http://www.burlen.co.uk/partDetail.aspx?pump=&partID=51169

I have not yet re-found the website with the do it yourself diagram!
James Finlayson

OK I found it.... this is a web archive and not all the pictures are here but the ones you need are... Clearly shows the do-it-yourself approach!

Hope it's what you need Brian


http://web.archive.org/web/20041018222320/www.mgcars.org.uk/electrical/pump.html
James Finlayson

James - The link you supplied is a cashed site for Rick Astley's web site that has been removed since he published a book with all the information from the web site. The picture near the bottom of the web page is the diode/resistor unit that Brian is seeing in his pump. The part number that you included in your last post is the revised part number for the unit and is a good number, but I don't know if it is for a positive or negative ground. This doesn't make a whole lot of difference as one can just reverse the leads to accommodate the polarity. A more modern approach would be to go with a TVS, the one that I am using presently is a P6KE18CA, but those may be hard to obtain in single unit quantities.

Brian - I can send you a TVS as I did for James, for my cost on the unit (quite small) and the the cost of mailing it to you. The problem right now is that my wife and I have to make a fast trip to California to get my aging mother packed up and moved up here and will have us tied up for a couple of weeks. Cheers - Dave
David DuBois


Great inputs from you all.

What does a TVS consist of? I may order one of these from you David. How much would the part cost and what would standard postage be to Ireland?

Thanks,
Brian
Brian McIlvenna

I will have a look on the weekend in our garage as David sent me 3 I think, and one is all I needed! If I find them I will post you one for free Brian.
James Finlayson

Hi Folks:

Question, what is inside the SU diode (BZX 1019) component? is it a diode and a resistor in series or parallel?

Rich Boris 67B roadster.
Rich Boris

What about putting in something like this...
http://www.burlen.co.uk/partDetail.aspx?partID=52399
and doing away with the whole problematic end of things altogether.
R Kelly

A TVS will cost about 20pence! That's 40pounds
James Finlayson

The diode/resistor assembly is a diode and resistor (I think that is about 10 ohms) in series. You can make up your own diode/resistor unit and solder it in, just make sure that it is well insulated so it doesn't short against something.

Brian - A TVS looks like a diode, bu tit doesn't have a band painted around one end (at least the ones I use don't because I get the bidirectional ones). I could send you one, but you will have to wait until we get home. We are presently on our way to California to try and get my 86 year old mother's situation straightened out. At this moment I don't know just when we will get home. E-mail me and I'll put you on the list for when I get home. Cheers - Dave
David DuBois

James,

the email address shown in the BBS is outdated; I've changed this on the members' site but it isn't registering.

My current email address is bmcilvenna@yahoo.fr.

I'd really appreciate if you could contact me with a view to the arrangement for sending the component.



David,

I may need to have this repaired asap as the MG is my only car. Thanks.

Brian McIlvenna

Ah so Im not the only Irish guy using a B as my daily driver!
R Kelly

I've done some research on the TVS component. To what voltage does the component need to be specified? The P6KE18CA, refered to by David Dubois, has an 18v specified voltage. I may be able to source locally components rated at 39, 47, 100 & 400v.

Richard, although the MG is my only car I use it little. Most of my journies are made by bike - must faster and stress free than sitting in Dublin's traffic in a car, no matter how nice or expensive a car may be...
Brian McIlvenna

Its Ross, not Richard. I must see if I can change that in my profile. I hate being called R Kelly. It reminds me of that tosser musician. Anyway. No offence taken! I know what you mean about traffic. Ill be cycling into work a lot during the summer. I spend a lot of time in cars. Im a car salesman so it sort of goes with the job.
R Kelly

Brian - the 39 volt unit would work fine. It will break over fully by the time the voltage gets to around 50 volts, which should still provide a soft enough arc that no metal will be removed from the contacts.

I can relate to the MG being your only car to drive, we have a TD and a MGB and an old Toyota pickup used to haul stuff we can't haul in the MGs (gravel ans such). If we go somewhere, it is in a MG. Cheers - Dave
David DuBois

The local supplier doesn't have the component in stock. If I knew the specifications of the diode and resistor I could try to make up the TVS up myself. Is the diode uni or bidirectional, what is the resistor value, what power specification is needed in both components?

Thanks again,
Brian
Brian McIlvenna

Brian I have a TVS for you. Check your email for one from me and send me your address. It will be well travelled having gone from US to NZ then up to Ireland. Probably made in Asia too!
Cheers, James
James Finlayson

This thread was discussed between 01/03/2008 and 09/03/2008

MG MGB Technical index

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