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Electric Fuel Pump

Occasionally (maybe once a month), the CB pops when switching on the electric fuel pump. When this happens, the CB can be reengaged, and the pump starts running at the second switch-on attempt.

So I decided to take a look. I’ve measured the switch-on current using a 0.1Ohm shunt in series with the pump.

So the pump has a 20A peak current when switching on, dropping to 10A after 100ms (the CB is rated 10A), and further dropping to just below 5A. This looks fairly normal to me, given that the Weldon 8120-G pump, which is a PMA replacement part for this pump, is also specified with 5A continuous (this is a 14V pump).

So I decided to open the pump motor, and was greeted by a large amount of carbon dust:

It looks like there’s still some carbon left, but not much; and furthermore, the contact ring got eroded quite a bit:

After cleaning and making sure the bearings (and the actual pump) turn easily enough, I reassembled and reinstalled the pump. To my disappointment, the switch-on current didn’t go down:

It looks to me that neither the CB nor the switch are the problem. What happens, occasionally, is that the pump doesn’t start to turn, but continues to consume 20A. After some time, the CB trips, as it should.

It looks to me like a good idea to replace the pump in the near future. Or am I overlooking something?

LSZK, Switzerland

I think the 10A CB is marginal, given the turnon current of the pump. It should probably be 15A or 20A (obviously with appropriately sized wires used to the pump). There are many types of these thermal CBs and they come with different response curves, and somebody needs to check the correct profile CB is being used. I would PROB99 bet that nobody ever did that bit of due diligence on your plane.

The commutator looks in a bad state and may even have a short (possibly carbon dust) between a couple of segments, which would push the CB over the edge.

It could be the pump has a shorted winding, and if the motor happens to stop at that point, you get extra current drawn. But then I would not expect the motor to work OK after the CB reset because it will not have moved… You can check this with an ohm-meter across the commutator, in place of the brushes. The resistance should be the same across every pair of commutator contacts. I think yours is OK because the current waveform on the scope doesn’t have any glitches on it.

Also CBs do seem to become more sensitive over years. Being thermally triggered, they are running continually hot inside. A CB whose trip current is say 10A will be running pretty warm at 5A. I have replaced some… Aircraft CBs are cheap jobs and not too well made inside.

The motor is of a suprisingly poor quality – a very low grade Chinese $1 kind of job.

I would replace the pump and the CB.

There are firms that can rebuild the armature. Most of these things are off the shelf. Rebuilding motors is a widespread business. You won’t get any paperwork of course. There may even be markings on the motor so you could buy a new one and transplant bits from it.

Last Edited by Peter at 14 Nov 14:02
Administrator
Shoreham EGKA, United Kingdom

The motor is of a suprisingly poor quality

This is an expensive certified aviation part, so what do you expect 8-)

I don’t quite buy that the CB is at fault, currently, it can withstand 20A for over a second, that’s well over a decade more than what a normal startup will require, so there should be enough margin.

And since there’s no advantage on replacing both the pump and the CB at the same time, only disadvantages (less controlled experiment), and the pump looks somewhat desolate, the pump looks like the prime target for replacement.

There are firms that can rebuild the armature.

Do you think that would be worthwhile? A new PMA pump is around 800$, and given the existing pump was overhauled ~8 years ago and given the outcome, I’m not overly keen on overhauling it. Plus there will be the downtime.

LSZK, Switzerland

At “just” $800 and given the commutator is definitely badly worn, I would change the pump.

Some electric fuel pumps I’ve come across are more like $8000…

Then you can get the old one rebuilt and put it on the shelf.

Administrator
Shoreham EGKA, United Kingdom

Well that commutator is well worn and you will have to replace it one way or the other…. It looks to me to beyond limits for any skimming. Do you have new brushes? (Any 50’s vacuum cleaner by the look of it). It may be they have worn down to a point where the spring has insufficient pressure and the starting behaviour is due to a high resistance at the point of contact. Regarding the CB, they do go flakey and it would not need much additional resistance to cause to motor to behave like this. I had an alternator that kept going off line and no amount of new alternators, regulators, overvoltage sensing trip, etc.,. would fix it until eventually I found in the air that lightly flicking the CB with a fingernail (not pushing it in) would restore power. The problem could easily be a combination of all the above…

EGBW / KPRC, United Kingdom
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