Electric fuel pumps are dangerous .

[Sheepdip]

Originally Posted by crakarjax

How do these work at startup? Since there's no oil pressure at startup there won't be fuel pressure, right?

In a nut shell, the oil pressure switch will have three terminal connection
spades, I, P and S.

I terminal connects to your key switch 12 volt in the on/run position.

P terminal goes out to your fuel pump relay as a trigger wire.

S terminal goes to your starter solenoid that is hot only while cranking the
starter.

While cranking the engine to start, the S terminal now hot activates or
opens/connects to the P terminal to energize the fuel pump.

Once the engine starts and and you release the key, the S terminal is no
longer hot, so S and P terminals close/disconnect.

The oil pressure within the switch will by then or has opened/connected the terminals I and
P to make the connection of 12 volts to the fuel pump.

There is a minimum pressure required to keep it open, usually about 4-6 lbs
or so, if it falls below that required pressure I and P terminals will
close/disconnect the 12 volt feed to the fuel pump relay trigger.

No oil pressure equals no power to the fuel pump.