Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switch.

[jmz]

Hello all,

I have a 93 King Cobra 454 boat that I have put a new 502 in. I have eliminated the spitfire ignition and wired up a marine HEI ignition from DUI with a CDI shift interrupt module ( Thanks Don S. for your previous posts and wiring schematics) This set up looks like it should work pretty good.

I am now on to wiring up the fuel pump. The wire harness that I currently have is completely shot, dry rotted and in bad shape. The easy thing would be to just buy a new harness and wire it up the same way that OMC did. The harness (complete) is NLA so I can either build up a new one with all the diodes and such or find another way to power the fuel pump.

I am getting some help from a guy who works on mercruisers so he wanted to wire the pump off of an oil pressure safetys switch. I like the idea of the fuel pump only running while we have oil pressure and possibly shut down in the event of an oil pressure drop but I seriously doubt it would save the engine...

Anyway,

Can anyone suggest a simple way to wire up the fuel pump from an oil pressure safety switch? I want to make sure I don't burn anything up or backfeed the alternator or starter or something bad.

Thanks!

Jay

 

[Silvertip]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

The reason for wiring the pump through the oil pressure switch is not to save the engine but to save your behind if there is a sudden engine failure (or fire) where the engine dies. If there is no way to kill the pump it continues to run pumping fuel on the fire. Or the situation exists where the engine dies because of a carb issue. The pump stays running thus pumping the engine full of fuel. YOu hit the key again to fire the engine and the resulting KABOOM turns things to toast. And don't assume you will have the foresight to turn off the key in an emergency. That likely won't happen in an extreme emergency.

 

[Bondo]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Ayuh,.... Welcome Aboard,... The fuel pump is powered through the key-switched ignition circuit,..

It goes through the OP switch...

There's usually another feed from the outer tiny terminal of the starter's solenoid, just to provide power, While turnin' over the motor, before the OP builds, 'n closes the OP switch...

 

[jmz]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Hey guys,

Thanks and I get all that. I understand that it stops the flow of fuel just when the engine dies so that you don't have a flood of fuel and a fire. I also understand that OMC and Volvo do it differently by powering the fuel pump from the alternator and use diodes and relays and such. (alternator not running no power to fuel pump same effect as no oil pressure switch activated)

Can anyone assist in converting the OMC/ Volvo method over to the oil pressure safety switch method and keep me from cooking something? I want to know where to gather the power to the OPS switch (hobbs switch)

The fuel pump gets power from the green wire (light) from the alternator the orange wire from the assist solenoid and the red wire from the assist solenoid. I am trying to find a simple way to wire the fuel pump and don't want to run a bunch of diodes and relays if I can help it. ...OK maybe a relay.

Thanks again.

 

[jmz]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Ayuh,.... Welcome Aboard,... The fuel pump is powered through the key-switched ignition circuit,..

It goes through the OP switch...

There's usually another feed from the outer tiny terminal of the starter's solenoid, just to provide power, While turnin' over the motor, before the OP builds, 'n closes the OP switch...

-- also considered using a two pole safety oil safety switch to power the fuel pump rather than a three pole. ...getting 12 v power from almost anywhere for the OPS switch and then using the tiny terminal on the starter solenoid to power the fuel pump while starting. I am slightly concerned that if the engine doesn't light up right away that the fuel pump will be getting power from both OPS and the starter solenoid terminal. ...and possibly then sending current back to the solenoid. -I guess I could fix this by putting a diode into the wire coming off the tiny terminal but I'm trying to make this as simple and safe as possible. ...eliminating extra things to break is kind of a good thing.

Thanks again.

 

[Bondo]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

-- also considered using a two pole safety oil safety switch to power the fuel pump rather than a three pole. ...getting 12 v power from almost anywhere and then using the tiny terminal on the starter solenoid to power the fuel pump while starting. I am slightly concerned that if the engine doesn't light up right away that the fuel pump will be getting power from both OPS and the starter solenoid terminal. ...and possibly then sending current back to the solenoid. -I guess I could fix this by putting a diode into the wire coming off the tiny terminal but I'm trying to make this as simple and safe as possible. ...eliminating extra things to break is kind of a good thing.

Thanks again.

Ayuh,... Yer completely Over-thinkin' it...

The 2 connector switch is what ya want, it simply passes the power through when closed by the oil pressure...
'n back-feedin' is a non-issue,...
The power is all from the same battery, 'n it's just like runnin' 2 wires, it's still just 12 volts...

The tiny terminal on the solenoid is only powered when the starter is Crackin', 'n dead(open circuit) when not...

 

[jmz]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Ayuh,... Yer completely Over-thinkin' it...

The 2 connector switch is what ya want, it simply passes the power through when closed by the oil pressure...
'n back-feedin' is a non-issue,...
The power is all from the same battery, 'n it's just like runnin' 2 wires, it's still just 12 volts...

The tiny terminal on the solenoid is only powered when the starter is Crackin', 'n dead(open circuit) when not...

...Yup, I tend to overthink things when I'm in uncharted waters so to speak.

So if the fuel pump is getting power from the OPS switch AND it is also wired to the tiny terminal on the starter, won't power be going back to that terminal from the 12 volts now powered to the fuel pump? If this is yes then what happens? -nothing?????

 

[Don S]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

...Yup, I tend to overthink things when I'm in uncharted waters so to speak.

So if the fuel pump is getting power from the OPS switch AND it is also wired to the tiny terminal on the starter, won't power be going back to that terminal from the 12 volts now powered to the fuel pump? If this is yes then what happens? -nothing?????

Since the starter solenoid isn't engaged it does't do anything. The terminal on the solenoid is just a dead end.

 

[Bondo]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Originally Posted by jmz
...Yup, I tend to overthink things when I'm in uncharted waters so to speak.

So if the fuel pump is getting power from the OPS switch AND it is also wired to the tiny terminal on the starter, won't power be going back to that terminal from the 12 volts now powered to the fuel pump? If this is yes then what happens? -nothing?????

Since the starter solenoid isn't engaged it does't do anything. The terminal on the solenoid is just a dead end.

View attachment 187490

Ayuh,... 'n even when Both are energized, Nothin' comes of it, as it's All the same battery....

This ain't yer house, with 2 separate legs of power....

 

[jmz]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Thanks guys. I appreciate the help. I may have a question or two more later.

Cheers,
Jay

 

[Don S]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

And then I missed post 4 completely :facepalm:

 

[jmz]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

Thanks Don. I'm in the middle of a repower (502 crate motor just dropped in).

I'm finishing up with the wiring right now.

I've pulled out the spitfire stuff and figured that part out with your helpful schematics.
The fuel pump cable (harness) is pretty much shot (dry broken insulation, green wire) so it needs to be replaced. I either need to find one laying around somewhere (NLA), build a new one or wire the fuel pump up kind of like a mercruiser through an OPS.

The reason I started this thread was to ask how to trick my OMC motor and power the fuel pump off an OPS. -thinking that it might be fairly simple.

Thanks again,

Jay

 

[Don S]

Re: Please help with '93 454 King Cobra fuel pump wiring of oil pressure safety switc

I actually posted the right info to begin with, then when I seen the post again later on and noticed it was in the OMC forum I thought I gave you the wrong info, and edited out without re-reading the whole thread. :embarassed: :facepalm: