7.4 electric fuel pump

ericcarr2000

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Ok, I have a 1990 7.4 motir in which I will be installing an electric fuel pump on. My question is before I start prodding around, can anyone point me in the right direction to where to splice into the wiring?
 

Bt Doctur

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can tap into the purple wire at the coil .Needs a switched source and an oil pressure switch wire in.
 

achris

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You need 2 sources for the pump. First source is from the ON position on the ignition key (+ on the coil will do the job) via a N.O. oil pressure switch. Second source is from the starter motor, the 'I' terminal, which powers the pump while cranking before the oil pressure is up enough to close the oil pressure switch.

Chris......
 

alldodge

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Ok, I have a 1990 7.4 motir in which I will be installing an electric fuel pump on. My question is before I start prodding around, can anyone point me in the right direction to where to splice into the wiring?

Hope this helps
Merc Elect fuel pump wiring.jpg
 

ericcarr2000

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Ok, I'm kinda dumb when it comes to wiring issues. Can you just tell me where to connect the red & black wires from the fuel pump, lol
 

ericcarr2000

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Okay I think I got it what I did was hooked up the positive from the fuel pump to the purple coil wire and then the ground from the fuel pump to the ground boat itself
 

duped

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You NEED to add an oil pressure switch! You don't want the pump on if the engine isn't running! If you blow a fuel line and/or lose your engine and the pump is still going, you will be pumping gasoline right into your bilge! They are very inexpensive and you can just "T" it in at the oil pressure sender with a few fittings or install it in the port by the oil filter. 10 minutes to do it right could really save your life on this one.
 

ericcarr2000

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So on top of me hooking up to the coil purple wire, I need to "t" into the oil pressure sending unit also? How can I do that if I only have the 2 wires coming from the pump?
 

duped

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The "T" I was referring to is for plumbing the oil pressure switch in line with your oil pressure sender, not wiring.

And it's not on top of your purple wire to the pump, it's instead of...

Your starter should have a small "I" terminal that's hot when cranking. One wire goes from that to the "S" position on the oil pressure switch ( S is for starter ). The next wire is purple from the coil to the "I" terminal on the switch (ignition). Then your pump's hot wire would be connected to the "P" (pump) terminal on the switch. I bought my pump and oil pressure switch on eBay. If you search oil pressure fuel pump switch you will see one with three terminals on one end and pipe threads on the other to plumb it. I did this install a few years ago as stated above and never had an issue. The pump is energized only with the engine is either cranking, or has adequate oil pressure. If the engine stalls, the pump shuts off. Another bonus is if you should lose oil pressure for any reason (oil hose breaks, oil pump fails, etc), it will cut your fuel before your engine is destroyed.
 

ericcarr2000

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Wow, this is confusing. So you're saying I need to plumb the oil pressure safety switch into my fuel line?
 

achris

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Add a Tee and oil pressure SWITCH into the same hole as the oil pressure sender.
Run a wire from the coil+ to one of the connectors of the oil pressure switch.
Run a wire from the 'I' terminal of the starter motor solenoid to the other connector of the oil pressure switch.
Connect the fuel pump red wire to the same terminal on the oil pressure switch as the wire from the starter motor solenoid.
Connect the fuel pump black to a good ground.

Chris......
 

achris

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Does this help?? (and all the better if you can get purple wire for the coil to pressure switch wire, and purple with a yellow stripe for the other 2 new wires. ;))

oil circuit.jpg
 

ericcarr2000

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I was wondering would that really hurt anything to leave it hooked up to the coil and just run it the way I have it now, I mean as long as I make sure to turn ignition switch off I should be okay right?
 

achris

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I was wondering would that really hurt anything to leave it hooked up to the coil and just run it the way I have it now, I mean as long as I make sure to turn ignition switch off I should be okay right?

:nono:

That oil pressure switch is there to save your life. Should the carburettor flood (say from a stuck needle and seat), once the engine stalls the oil pressure drops and the fuel pump stops filling the engine with petrol... Last thing you need after the engine stalls and you're looking in the engine bay is for a hot engine to still be being filled with a highly volatile liquid.. :boom:

Chris......
 
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