Al, that switch will work either way.  I have one and am using it so that is N.C .  It closes when my coolant pressure drops and lights the indicator on engine monitor.   That occurs at about 5 psi.  The only thing I am unsure about in your application is the rated amperage since you want to use it to power a fuel pump.  I can't remember if it was 3 amps or 10 amps.

 

Dave Leonard

 

Oops.  I didn’t look at the diagram carefully enough to note that it could be wired either way.  I’m thinking that a higher pressure switching point is needed, like at least 20-25 psi.  The idea is to catch it before the engine stops, and it’s likely that would happen before the backup would get it up to pressure.  It may be best to simply run both during critical flight phases, which I probably would anyway.  Just seems like it would be good to have the backup come on automatically and keep the engine running so when your droning along at cruise you don’t have a huge pucker factor (and cause cardiac arrest with your passengers) if the engine suddenly quits.

 

I doubt that the pressure switch would handle switching pump amps directly for very long.  A relay is required.

 

Two questions: Does the engine ‘windmill’ if turned off during cruise?

                        How much does your pressure regulator drop the pressure at low MAP?

 

Al