Electric fuel pump design/failure modes

arnoha

arnoha

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arnoha
Friend of mine just picked up a 340A that he's getting some RAM engines on. Along the way they noted that the Low setting on one of the auxiliary electric fuel pumps didn't work, but High did. It's probably wiring or a switch, but I opined that it could be a failed pump motor winding. If there were two windings, with low energizing one and high energizing both, it could be that High now was actually low.

Of course, the picture of the pump says I'm probably wrong, as there are only two wires in the cable. Two settings via windings would need three leads. How does it actually work? Strictly voltage regulation? What are the possible failure modes?

https://www.rulonaviation.com/product-p/4140-00-153.htm
 
Never worked in a 340, but it's likely the same as the Cessna singles with injected Continentals. There are two resistors in the circuit, so there should be three speeds (older airplanes had one resistor and two speeds). There's a microswitch actuated by a cam on the throttle lever on the FCU, and at low throttle it opens the switch. Opening the throttle far enough closes the switch, shorting one resistor and increasing the speed. The highest speed come when you hit "High" or "Prime" on the panel switch.

The original resistors were small and made some trouble. If you look at this PDF and go to the last few pages, you'll see schematics for various models. Nothing complicated. The new resistors are adjustable and are set to give specified fuel flows. The microswitch also has to be rigged to actuate at the correct throttle setting.

And if someone has installed a Weldon pump in place of the Dukes, all bets are off. The Weldon draws less current, meaning that the voltage drops across the resistors are smaller, and the pump produces too much pressure and messes things up.

The 340 might have relays instead of being directly switched.
 
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arnoha said:
Friend of mine just picked up a 340A that he's getting some RAM engines on. Along the way they noted that the Low setting on one of the auxiliary electric fuel pumps didn't work, but High did. It's probably wiring or a switch, but I opined that it could be a failed pump motor winding. If there were two windings, with low energizing one and high energizing both, it could be that High now was actually low.

Of course, the picture of the pump says I'm probably wrong, as there are only two wires in the cable. Two settings via windings would need three leads. How does it actually work? Strictly voltage regulation? What are the possible failure modes?

https://www.rulonaviation.com/product-p/4140-00-153.htm
Click to expand...

The pump you posted is a transfer pump for a nacelle tank. There is between 6-8 electric fuel pumps on a Cessna 340 depending on how many fuel tanks there are. Below is the wiring diagram to the fuel pump for the 3 position sw marked Low, Off, High.

upload_2017-3-30_22-26-14.png
 
Plano Pilot said:
The pump you posted is a transfer pump for a nacelle tank. There is between 6-8 electric fuel pumps on a Cessna 340 depending on how many fuel tanks there are. Below is the wiring diagram to the fuel pump for the 3 position sw marked Low, Off, High.

View attachment 52378
Click to expand...

6 to 8? Damn. 340 is definitely outside my price range!

Looks like there's plenty of ways it could go wrong, but blowing one of two windings isn't it, since there is only one. Confused about the relay in the diagram: with the modification, it looks like it is never energized and doesn't do anything anymore? Both high and low paths seem to skip it entirely.
 
The relay must have been associated with the pressure switch that's been removed. It appears to serve no purpose now with the jumper installed.
 
GlennAB1 said:
Possible failure mode: Heed the warning about running it dry, they mean it.
Click to expand...

Was thinking about this warning (flagged on my unit that is out for overhaul). Since we drained the fuel sump and lines to remove, how do you avoid running dry on initial start up after reinstalling?
 
The 414 I fly has 10 fuel pumps. :eek:

When you say low doesn't work, what exactly are you referring to? Assuming MEB 88-3 has been complied with, there is supposed to be an oil pressure switch in place that won't let you flip the pumps on low when the engines are not running. This exists because on low the pumps are pretty quiet and it's possible to just dump tons of fuel into the engine bay. In more than one situation where this has happened (they got rid of this protection on the 414A/421C) people have literally melted the wings off of their airplane (on the ground). In other words, if you are stationary and put the pumps on low, they won't turn on and this is intended behavior.

If your friend hasn't already, he should join The Twin Cessna Flyer (www.twincessna.org). Cheapest thing he'll ever do relating to owning a 340 and it will also be one of the best money savers. He can also read the 14-page thread started by some long-haired hippie who flies dogs (and hopefully soon people on stretchers) in a 414 that he's restoring after it sat for 6 years.
 
Jeff Cutler said:
Was thinking about this warning (flagged on my unit that is out for overhaul). Since we drained the fuel sump and lines to remove, how do you avoid running dry on initial start up after reinstalling?
Click to expand...
Gravity feed, I'd hope, other than that is there a maintenance manual procedure for bleeding or priming?
 
Don't know about the Cessna but my Beech actually has two pumps for high and low speed.
 
Jeff Cutler said:
Was thinking about this warning (flagged on my unit that is out for overhaul). Since we drained the fuel sump and lines to remove, how do you avoid running dry on initial start up after reinstalling?
Click to expand...
It probably uses the fuel to cool and or lube the pump. If the tank is dry and you run it long enough you will damage it. Running it for a few seconds to a couple of minutes to prime it should not hurt the pump, would be my guess.
 
Plano Pilot said:
It probably uses the fuel to cool and or lube the pump. If the tank is dry and you run it long enough you will damage it. Running it for a few seconds to a couple of minutes to prime it should not hurt the pump, would be my guess.
Click to expand...

If it's a Dukes or Weldon boost pump outside the tank, it's not fuel-cooled. The damage happens if they're run dry because the vanes are lubricated by the fuel. The pump will fill when the fuel valve is turned on again.
 
Plano Pilot said:
It probably uses the fuel to cool and or lube the pump. If the tank is dry and you run it long enough you will damage it. Running it for a few seconds to a couple of minutes to prime it should not hurt the pump, would be my guess.
Click to expand...
My old Chevy Camaro had a note/caution in the owners manual to not operate the car with less than 1/8th tank of fuel, for fuel pump cooling, of course is was submerged in the tank.
 
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