I need to rig a secondary fuel pump to kick on at a specific fuel pressure as a failsafe. I know most people wire them to a hobbs switch referencing boost pressure. I need to do the opposite of that. I want to reference fuel pressure instead. If it drops below 40psi, I want the second pump to kick on. Except for wiring a seperate pressure sensor to a arduino and programming it, how else could I do that? A pressure switch that triggers from a pressure drop instead of increase? I can't find any.

 

IF the secondary fuel pump is wired to "Kick On" as pressure drops to 40 PSI.... it'll just turn right back OFF when fuel pressure rises above the setpoint.. unless you use a Latching Relay. Not really sure I see what problem you are solving.

 

I already did what I was trying to do but I'll explain it. Its the same premise of the hobbs switch for boosted cars as I've no doubt you already understand. Underload the engine demands more fuel and MAY need more than the single pump could provide. I don't want BOTH pumps running 100% of the time. So the switch is set to 40PSI falling, when that pressure is reached it triggers a relay that runs the second pump ONLY IF the first pump cannot support the desired pressure on its own. I don't care if the pump is cycling, its rated for PWM anyway. Turning on and off a lot wont hurt it and the pressure switch itself is built with a small buffer. It activates at 40PSI falling, deactivates at 2% above that, 42~43psi. (43psi being my set fuel pressure) So it doesn't cycle quickly anyway. As long as the PSI doesn't fall it serves its purpose without needing to be on all the time working the fuel unnecessarily. It has been functioning as I've just explained perfectly. It also leaves the car with a back-up pump incase the primary pump fails at any point. I just take my back seat up, swap the wire, and continue on my way.

 

I'd be nervous about using that approach because by the time it senses a fuel pressure drop, the engine has probably run lean and knocked under load...

With an NA car you could still use something like a Hobbs switch, but triggering when MAP crosses a much lower threshold, like 10psi absolute pressure, rather than 10psi boost. Or trigger the second pump when RPM crosses a threshold, maybe 4000 or so.

The fuel-pressure switch is still a nice idea in case something else goes wrong, I'm not suggesting to get rid of it. Just saying that I'd be more comfortable with a pre-emptive trigger on top of that. You'd still have the 2nd pump dormant most of the time, but this would prevent any lean moments in between the fuel pressure dropping and the 2nd pump kicking in.

 

What kind of fuel pressure regulator are you using? You need one that has a port on the high pressure side. Usually you see a pressure gauge in it. Then a normally closed pressure switch that will open at 42 to 45 psi. Most popular would be a higher volume pump so the regulator will bypass most if the time and the pressure will be consistent.