Correct wiring for fuel pump?

[jbutler67]

Hi all-

I have gone through many posts looking for info about how to correctly wire the electric fuel pump (Mercury 861155A 3). Currently, the electric fuel pump is wired directly to the coil and turns on as soon as the key is turned to the ON position. I believe it should be wired to the starter and oil pressure switch?

How should the fuel pump be wired?

1998 5.7L (M049938) - 2 BBL Carbed engine

 

[Rick Stephens]

You are correct, needs the wire off the starter as a priming circuit and the oil pressure switch as a safety. You surely don't want fuel in the bilge when the motor is not running.

 

[jbutler67]

Thanks for the diagram. A few notes to clarify how to wire, along with some questions.

Wiring plan notes
1. Black wire from fuel pump to ground
2. purple/yellow from fuel pump to a solenoid
3. wire from solenoid to the + terminal on battery, with a fuse

Questions
a) where should the second wire from the solenoid go to? To the oil pressure switch? if yes, then what is assumed about other wiring to/from the oil pressure switch? (How wire between the Starter, oil pressure switch and solenoid?)
b) what amp fuse is recommended on wire between solenoid and the + battery terminal?
c) is there a particular solenoid recommended?

Thanks!

 

[Rick Stephens]

Solenoid should read "RELAY". Idea being that we don't power the fuel pump off the ignition switch. Merc diagram shows the pump powered off the ignition (purple) wire.

The relay gets its power direct from the battery through a fused wire.

The switch part of the relay has two sourced to turn the relay on: 1: The outside lug of the solenoid on the starter, and 2: from the oil pressure switch.

I believe you only need a 10 amp fuse for an electric fuel pump.

The oil pressure switch gets its power from the purple ignition power wire. You can get that from several harness location on the engine, one being the power to the electric choke.

I like a sealed relay - I usually get them from the local NAPA. Looks like this:

 

[jbutler67]

Thanks, Rick. This part just cleared it up! Now, I get it.

The relay gets its power direct from the battery through a fused wire

The switch part of the relay has two sources to “turn on” the relay:
1) The outside lug of the solenoid on the starter
2) from the oil pressure switch

 

[Rick Stephens]

Thanks, Rick. This part just cleared it up! Now, I get it.

The relay gets its power direct from the battery through a fused wire

The switch part of the relay has two sources to “turn on” the relay:
1) The outside lug of the solenoid on the starter
2) from the oil pressure switch

Zachary!

 

[achris]

I'd prefer to see the relay power coming from the red/purple on the starter slave solenoid. That way, it remains as part of the engine wiring, and get the power removed when the battery switch is off. And as it's after the 90A starter mounted fuse, no need for it's own fuse. If you do put a fuse in the line, don't use the glass fuses (3AG). They corrode on the ends and go intermittent. Use a mini blade fuse and water proof holder.

Chris...........

 

[achris]

Rick Stephens I hope you don't mind, I have reworked your drawing... See how this grabs you....

Chris.........

 

[Rick Stephens]

Rick Stephens I hope you don't mind, I have reworked your drawing... See how this grabs you....

Chris.........

I mind so much that I hope you put it in a sticky. Thx!

 

[Bt Doctur]

And the reason for the adding the relay is when the fuel pump starts drawing a bit more than it rated current the pressure switch contacts burn out and the fuel pump stops. Adding the relay allows the low current coil to close the contacts and the relay contacts usually 30-40 Amp rated carry the fuel pump current demands Same relay is used as a fuel pump relay in the S10 Blazer

 

[Bt Doctur]

https://www.summitracing.com/parts/sum-890023/overview/

 

[achris]

I mind so much that I hope you put it in a sticky. Thx!

This do?

https://forums.iboats.com/forum/eng...-add-a-fuel-pump-relay-new-or-existing-setups

 

[jbutler67]

Great info so far! Thanks all!

Recommendations for how to tee the wire at the oil pressure switch? This is the current oil pressure switch.

 

[achris]

Great info so far! Thanks all!

Recommendations for how to tee the wire at the oil pressure switch? This is the current oil pressure switch.

Your serial number comes back as an EFI V6....

So, based on the year (1998), your engine should already have an electric fuel pump. Just cut the Pur/yell wire a few inches from the pump. Join pump side of that cut wire to one of the power contact sides of the relay, and the coil side of the relay to the other side of the cut you made (the pur/yel that goes back into the harness).

Chris.....

 

[jbutler67]

I see what I did. Typo in original post. My bad. Serial number on the engine is M048938.

Its a V8 with a 2 barrel carb. It does already have an electric fuel pump. But it’s wired incorrectly (wired directly to the ignition coil).

I plan to rewire it following the diagram above. Just looking for clarification on how to tie it into the oil pressure switch.

 

[Bt Doctur]

the pressure switch has 3 terminals C common to the elec pump, NO to the ign feed for the pump, NC to outboard small terminal on starters solenoid

 

[achris]

the pressure switch has 3 terminals C common to the elec pump, NO to the ign feed for the pump, NC to outboard small terminal on starters solenoid

That's not the switch on OP's engines. The one on that engines has only 2 connectors, have a look at his photograph.....

 

[jbutler67]

the pressure switch has 3 terminals C common to the elec pump, NO to the ign feed for the pump, NC to outboard small terminal on starters solenoid

Hi BtD - attached is a picture of oil switch I have.