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Fuel Pump Diode

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chuck1006

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I have a 1959 100-6 that came with a square body fuel pump (Moss says are early). I switched to negative ground not knowing to switch the diode. I measured ohms across the diode ( with it removed) and it is zero. I assume burned out. I ordered a late model solid state pump to replace it but would like to use this as spare. The diode reads:
EROID
0,047uF
400 V -
250 V ~
,,k"" D3
0,047/400
The Moss diode looks different. Any ideas where I can get the one I have or is it wrong type? The points I can get from Moss I guess. Did the early square bodies have diodes?
Thanks,
Chuck
 
Hi, Chuck,

With those markings it's not a diode it's a capacitor!! 0.047 microfarads, max 400 volts working. The replacement marks just say the same in a different way. A capacitor should NOT be zero ohms, it should show pretty much open circuit with the test meter set to ohms. You might get a bit of a kick on the meter needle as the capacitor charges up, but 0.047 microfarads is quite a small capacitance and maybe not enough to do this.

Rgds

Mike
 
[ QUOTE ]
I have a 1959 100-6 that came with a square body fuel pump (Moss says are early). I switched to negative ground not knowing to switch the diode. I measured ohms across the diode ( with it removed) and it is zero. I assume burned out. I ordered a late model solid state pump to replace it but would like to use this as spare. The diode reads:
0,047uF
The Moss diode looks different.
Did the early square bodies have diodes?
Thanks,
Chuck

[/ QUOTE ]
Chuck,
You are looking at a capacitor, as Mike says. The older pumps had a capacitor instead of a diode. The capacitors are not polarity sensitive so you didn't need to switch it. The newer pumps replaced the capacitor with a diode which IS polarity sensitive. If you got a new diode from Moss, you can use it in place of the capacitor, if you connect it the right way. The diodes for pos & neg ground are actually the same inside. The leads are just cut different lengths & the wire ends are different. Go here & compare what you have with the pictures. It pretty well tells the story.
https://www.mgcars.org.uk/electrical/body_pump.html

I have no idea what kind of meter you are using, but, a good capacitor should be as Mike says, very high ohms or an open circuit. If it is truly zero ohms, the capacitor is shorted. It is unusual for a capacitor of this type to fail in the shorted condition but possible, I guess. If it was shorted the pump would not work at all. Which is maybe why you are changing pumps?
D
 
Thanks Mike and Dave,
I guess I could have just bought new points for my existing pump instead of buying a new one. Oh well, I will still buy new points and switch to a diode to use my old one for a spare.
Chuck
 
If the capacitor had been shorted (0 ohms) and it were in use, the pump would not have operated and the capacitor would, most likley, have quickly turned in to an open circuit shortly after letting the "smoke" out. Many capacitors fail in this manner: first they short, then they get hot and burn, explode or open.
 
Can anyone tell me definitively how to connect a diode to an SU fuel pump? The diode has 2 leads. The C-shaped lead probably goes under the points blade, but the loop-shaped lead could either go under a grounding screw or to the power lead. I would appreciate your help if you know the answer.
 
JamesD said:
Can anyone tell me definitively how to connect a diode to an SU fuel pump? The diode has 2 leads. The C-shaped lead probably goes under the points blade, but the loop-shaped lead could either go under a grounding screw or to the power lead. I would appreciate your help if you know the answer.
Click to expand...

The diode (or capacitor or varistor) go across the points. The second lead goes under the ground screw. The diode is polarity sensitive, so if you've switching a positive ground pump to negative, you'd have to swap the leads of the original diode or use a diode assembly for negative ground. The capacitor and varistor are not polarity sensitive.
 
Thanks, Bob. That is exactly what I expected. What confuses me is the picture of BZX 1019 on www.su-rebuilders.com, which is supposed to be for positive ground cars. In that picture, the black lead would hook onto the ground screw and the red lead would hook on the points blade. That is the opposite of what I would expect. That same site shows part number AUB 6179 as dual polarity. The site did not state this, but 6179 appears to be a transil. The transil is advocated on ttypes.org because it can be used on either positive or negative ground cars and can be installed without concern about which lead is which. One comment was added to the discussion that the transil is dual polarity only if "CA" is in the part number. This is all very confusing.
 
I've owned early pumps with electrolytic capacitors in them; electrolytic capacitors are polarity sensitive (the condenser in a points ignition is an electrolytic capacitor).

Fun fact: A diode used in this type of application--an inductive load--is sometimes called a 'flywheel,' 'flyback' or 'clamp' diode as it dissipates the current induced by the collapsing field of the pump electromagnet, theoretically reducing arcing across the points and subsequent points erosion. I tried the transil setup--I ordered the parts from England--and didn't think it worked any better than a diode. A transil is essentially two Zener (or 'avalanche') diodes paired, so it flows current, in either direction, to ground when voltage exceeds its spec (when the points open, the pump coil operates similarly to an ignition coil). These pumps are funny; some claim decades and hundreds of thousands of miles without ever touching them, Norman Nock said they used to clean and adjust the points as part of normal servicing every 10K miles. If you clean/file the points be sure to slide some paper between them to remove filings.
 
Thanks, Bob. It is good to know that the transil will work as reliably as a diode.
Last night I opened the SU pump AUF305 that has been on my AH since the late 80's or early 90's without ever giving a moment's trouble. I know this was the first time it has been opened, since I had to peel off the SU label in order to get the cap off. Much to my surprise, the wiring is not what I expected. In this POSITIVE GROUND case, the diode's black lead is hooked to the power post (negative), and its red lead is hooked to the points blade. That is the way SU wired it, not me, and it works flawlessly.
14Mar24-1.jpg

I also noticed in the photo provided by ttypes.org showing how to hook up one their transils that it is wired differently. One transil lead is attached to the points blade, and the other is attached to the ground screw. That would agree with Bob's description of how it works. (Photo 2 from ttypes.org attached below)
15Mar24-1.jpg
 
I haven't messed with a pump in a while--thank goodness, knock on wood, etc.--but that may answer a question I've always had: does the coloring of the leads (red, black) indicate the biasing of the diode, or how to install it? IIRC S.U. specifies both pos. and neg. gnd. diodes--not, uh, positive about this--so they could sell just one and specify different wiring depending on the grounding of the chassis. A diode is essentially a one-way valve for current; wired backwards and current would always flow to ground and the pump wouldn't operate. Anyone ever done a post mortem on one of these? An off-the-shelf diode of suitable capacity would be a lot smaller (not much bigger than the transil).
 
Before su-rebuilders website went offline I managed to capture a picture of both diodes. The only visible difference from the outside is the terminals. The diode for positive ground BZX1019 has the C-shaped terminal on the red wire. The C-shaped terminal will fit onto the points blade, and the round terminal (black wire) will fit onto the power post. The diode for negative ground has the terminals reversed. If a diode is all that is in the black tube, you should be able to use a positive ground diode on a negative ground car by swapping the terminals (see BobHaskell's message above). I must add for full disclosure that I have not tried that.
I think all this may be moot. I don't think either of these diodes are for sale in 2024. it seems that our choices today are a transil or the fully electronic pumps and conversion kits that SU offers without any pesky points at all. It would be nice if those were quieter.
BZX1019PosGnd.jpgCZX1004NegGnd.jpg
 
Thanks for posting this. It confirms what I've always suspected: these diodes are the same, just the terminals are reversed for either pos. gnd. or neg. gnd. use. It appears the red lead indicates the anode of the diode, and the black lead the cathode. Why they need to be so large still eludes me. Much smaller diodes are available from electronics supply sources; they just need to be voltage rated.
 
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