My First Engine Out In My Soob

Marc,

This is similar to what I remember doing using Stan's version, but do you have a check valve in your fuel line? Mine does so that the auxiliary pump won't backfeed through the dead fuel pump.

If anyone is intending to use this system, I think you have to have a check valve in your fuel delivery system.
 
Great to see Thom that you rescued the situation in quick tim and yes, its a big effect when the noise stops . Its a good idea to run a fuel pressure gauge and on a EFI system with a fuel injected engine, it seems the high pressure pumps seem to be prone to failure around 1500 hours. Thats what I have found and I replace the pump every 1000 hours now.
 
Attached is a page of the schematics I drew up for my SH. I changed quite a bit of the wiring from the kit and so wanted to document the way I built it....

Marc, I don't know if I'm reading this diagram right, but according to it you get a short circuit when the relay is active. There is a direct connection running from the main bus to terminal 6 of the relay and then to ground. Are you sure this isn't a problem in the schematic?

-- Chris.
 
Marc,

This is similar to what I remember doing using Stan's version, but do you have a check valve in your fuel line? Mine does so that the auxiliary pump won't backfeed through the dead fuel pump.

If anyone is intending to use this system, I think you have to have a check valve in your fuel delivery system.

My high pressure pumps have built in check valves so there is no need for an external check valve. YMMV.

Marc
 
Marc, I don't know if I'm reading this diagram right, but according to it you get a short circuit when the relay is active. There is a direct connection running from the main bus to terminal 6 of the relay and then to ground. Are you sure this isn't a problem in the schematic?

-- Chris.
Chris,

There is no problem with the schematic. When the pressure switch closes, the relay is activated and the bus voltage on terminal 6 is connected to terminal 5, which then both illuminates the warning light and energizes the coil thru terminal 1 in order to latch the relay regardless of the subsequent state of the pressure switch. There are no shorts, just current flowing thru the warning light and coil.

Marc
 
Brent: Sorry I just got back to this thread. I see Marc already posted his wiringdiagram. My diagram is just a hand penciled one.

My pumps did not have built in check valves...so I ordered some from Aircraft Spruce. When the main pump fails....the pressure of course drops triggering the fuel pressure switch. I simply let this ground out a solenoid that kicked in the aux pump. The fuel pressure goes up and the check valve prevents it from shutting off the fuel pressure switch. A light goes on in my panel and tells me that the main fuel pump pressure is gone.

You can simply override this if you want on takeoff...but in my fuel injected 2.5, it would run too rich with both pumps and I would lose some rpm. But...if I were taking off over some hostile territory...then I would kick on both pumps even though I lost a little rpm...but at altitude I would kick off my aux pump and save it. Why run both pumps all the time?

I liked the idea of having a brand new fuel pump ready to kick in automatically as soon as the main fuel pump quit for any reason.

It was simple and cheap for this changeover as well.

Stan
 
Whose fuel pressure gauge & how much ?

The gauges are cheap , it's the aircraft sending units that are over $200 !

What type are you guys using ?
 
Stan

Do you happen to have your pencil diagram where you can post it?

I am having a time trying to decipher this one.:tape:
 
What pumps have the check valves in them?

The high pressure pumps on the SH have check valves. I think that is true of most FI pumps, but I'm no expert on the topic. You can tell by running the pump and then turning it off and seeing if the fuel system remains pressurized (when not running of course). The Facet low pressure pumps I have on my FW are an interrupter type pump and also have integral check valves. I think all interrupter pumps will have check valve built in.

Marc
 
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