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MG MGB Technical - Fuel pump condenser(capacitor) dead

Hi everyone,

I have recently pulled my 73 B out of its winter den, and have been enjoying driving it over the past couple of weeks until a few days ago, it left me stranded for the first time since I started driving an MG in 1987. I had been noticing that my fuel pump was not clicking as vigorously as it had typically done in the past, and so I decided to change the points and make sure the valves were intact etc. Having put the new points in, I tested the pump by connecting to 12V, and it ran very quickly, but I noticed a very bright spark occuring at the points. The original points appeared to have been very fouled, but I had suspected that they were as to be expected for a 40 year old pump, which I suspect has never had any work done to it. I tested the condenser by connecting my ohmmeter to it, and then reversing the connections and watching for discharge and recharge signs on the ohmmeter. It only read infinite resistance and did not change. I had a spare .47MF capacitor on hand from when I made a brake light relay, tested it with the ohmmeter and got the expected results, and wired it into the pump. When connected to 12V, the pump ran well, but still had a visible spark, albeit much less noticeable compared to before.
My question is this: Will this capacitor work here? Is a mildly visible spark acceptable (I know that earlier MGs had no condenser)? I know I could just simply order the condenser and be done with it, but I am eager to drive the car again, rather than wait for it to arrive likely after the weekend. Your advice is greatly appreciated!

Erick
Erick Vesterback

Erick - That capacitor should work just fine. It is not going to suppress that arcing completely, but you don't want complete suppression as that would allow the contacts to film over. Cheers - Dave
David DuBois

Thanks, Dave! I think I got the instructions, and therefore that .47MF capacitor, from your article on wiring a relay into the brake light circuit, several years ago. So, I thank you a second time:) I am sure I am not alone in my gratitude towards the many people like yourself who provide great advice to all of us who love to keep these cars going.
Erick Vesterback

While you are at it you could have fitted a diode instead (the correct way round!) and stopped the sparking pretty-much completely.
Paul Hunt

Yes Paul, I had read about that also. I would only have a 50-50 chance of wiring that correctly though, and wiring it incorrectly could cause my wiring harness to melt, so I chose to use the capacitor I had on hand:) If I were to add a diode, which direction would it belong? and does it matter much what kind of diode? I likely have one in my box of electronic parts that I could use...
Erick Vesterback

"If I were to add a diode, which direction would it belong? and does it matter much what kind of diode?"

The diode should be a 1N4001 or a 1N4002 and the end with the band around it should go to the power terminal, with the other end attached to the upper points spring. Be aware that a standard diode as above may slow down the operation of the pump. Cheers - Dave
David DuBois

Hi,

Any 1 Amp diode will do, the 1N40XX series is common and cheap. The last digit doesn't matter, as it is a voltage indicator, ie a 1N4001 is a 1 Amp 100V diode and the 1N4002 is a 200 Volt one, and so on.
The attached pic shows a diode, with the polarity band, compared to the diode symbol. The band is represented by the bar on the top of the symbol. Current flows in the direction of the arrow in the symbol.
To use the diode as a spark suppressor it must be connected with the band towards the positive end of the coil, so that current cannot flow when the coil is energised, otherwise the Lucas smoke will escape.

Also in the pic is a generic diagram of how a diode is connected to a coil to suppress the reverse voltage, when power is turned off to the coil.

Herb


Herb Adler

"the end with the band around it should go to the power terminal,"

This should read 'the positive terminal', or it will fry the harness on positive earth cars. Those cars effectively have the banded end to earth, and the unbanded end to the 12v supply. It's why there were two versions of diode protected pumps.

I suppose I should really have said to use a metal oxide varistor, as those are polarity independent and can be connected either way round with either polarity.
Paul Hunt

I purposely stated that the band end of the diode go to the power terminal because Eric stated that the car was a 73 MGB. I did this after verifying that in fact the car was one that was negative ground. I agree that it would more proper to state that the band end goes to the positive terminal on the pump, but I have found that a lot of people don't know which is negative or positive.

Regarding the varistor - while Burlen Fuel systems is now using them on the points style pumps, but my checks on pumps that I have received with the varistor in them have an extremely high spike voltage spike when the points open. The old method of arc suppression using a capacitor has a better suppression than the varistor does. I consulted with Rick Astley regarding the varistor and he is of the opinion that it is not a very good form of arc suppression. Cheers - Dave
David DuBois

I accept that, but these posts are read by everyone, including those with positive earth cars, who may not realise that the diode must be reversed on those cars.

Clausager said that the diode replaced the capacitor to reduce points burning (it's actually written 'buzzing' but that's a misprint). I'd do find it surprising that SU would replace the diode with something (MOV or TVS) that is worse than the older capacitor quenching.
Paul Hunt

Paul - "I accept that, but these posts are read by everyone, including those with positive earth cars, who may not realise that the diode must be reversed on those cars."
Agreed.

" I'd do find it surprising that SU would replace the diode with something (MOV or TVS) that is worse than the older capacitor quenching."
Both Rick Astley and I find the same thing puzzling. I have used a bidirectional TVS and still do with my optical trigger circuit (as protection of the MOSFET). Rick Astley maintains that the diode/resistor assembly used for many years was actually a TVS/resistor assembly. I suspect that BFS went to the MOV to save money and space under the end cover. Unfortunately, it is a step backwards as far as arc suppression (perhaps they listened to the bean counters rather than their technical people). Cheers - Dave

David DuBois

Very interesting guys. I had a couple of 1N4005 diodes remaining in my electronics drawer, but did not wire it in to the fuel pump as I was concerned it wouldn't be appropriate. I had a car show on Saturday that I wanted to attend, so I installed the fuel pump with the capacitor as I indicated. Good news is, the car ran beautifully for the day to the meet, plus on some errands afterwards. I plan to plumb in a reserve fuel pump as Dave has done, just for that added margin of reliability. I love driving this car around, they are so rare where I live. Its ultra-embarrasing when it stops running! Although to be fair, a week and a half ago was the first time I was left completely stranded, after 25 years of MGB ownership. A few times I have had breakdowns, but I have always been able to at least limp home and repair the car in my garage. Plus, this fuel pump has had zero attention in 40 years!
When I ordered the points for the pump, I also ordered new needle and seat kits, and floats for the carbs since the existing floats have about a quarter of a millilitre of fuel in them. The bags the parts arrived in have the SU logo on them, and the needles are tipped with some form of rubber (viton?) I have read mixed things about these, hope they work OK in my car. Between 9:30 and 11:00 pm tonight I have removed the carbs, replaced the needles, seats, and floats, and reinstalled the carbs. Its actually snowing outside right now, so I'll save the test drive for another day.
Thanks Dave for the instruction on the capacitor, Paul for clarifying for future readers on the polarity of the diode in cases of positive ground cars, and Herb for the lesson on diode numbering and wiring:)
I have added a pic of the car at the meet, parked near the door of my office in the background.


Erick Vesterback

Hi,

Nice car, Eric.

Re: the varistor,MOV device, maybe Burlen decided to use it because they were getting lots of problems with owners fitting pos earth pumps to neg earth cars and vice versa, or owners changing polarity of the car and not thinking about the diode in the pump.
Also with diode quenched pumps they would need to have 2 units in stock, pos and neg. With a MOV device it doesn't matter which way you connect it.

My 2 bob's worth.

Herb
Herb Adler

Herb - "Also with diode quenched pumps they would need to have 2 units in stock, pos and neg. With a MOV device it doesn't matter which way you connect it."

Or they could have done what I did - use a bi directional TVS. As I stated earlier, I use the TVS to protect the MOSFET on my optical trigger board (which is polarity sensitive). I went to the bi directional TVS strictly because I don't have to worry about which way I install it on the circuit board. Cheers - Dave
David DuBois

This thread was discussed between 09/05/2013 and 13/05/2013

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