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Please review my fuel pump wiring diagram

AFK1911

New member
I am in the process of planning an engine swap from a pre-vortec 4.3 mercruiser to a vortec 4.3. Since the new block will need an electric fuel pump, I have come up with a wiring diagram to hook it up. Would the wiring gurus please take a look and see if I have things correct? Also, one thing I haven;pt figured out yet is what size relay? Thank you

fpdpic.jpg
 
I am in the process of planning an engine swap from a pre-vortec 4.3 mercruiser to a vortec 4.3. Since the new block will need an electric fuel pump, I have come up with a wiring diagram to hook it up. Would the wiring gurus please take a look and see if I have things correct? Also, one thing I haven;pt figured out yet is what size relay? Thank you

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that looks correct. As far as what size relay I would find out the amp draw from the pump and ensure the relay can handle it on the switching side.
 
One issue is starter solenoid connection (S).
This would be to bypass oil pressure switch during cranking.

Just make sure it is S and not I (used back in point ign days) for power to coil during cranking.

Second potential issue is the use of the GM relay.
It needs to be marine approved I would think. Or may not be needed at all as the oild pressure switch act as the On/Off switch.

I understand the use of the relay as you are using that as the +12 switch.
But on non efi, i dont belive merc uses any relays.
 
I would get the 40A one used in the Chevy Blazer ,some fuel pumps draw just enough current to burn out the oil pressure switch so the relay is a better approach
 
Thank you for the replies. I will plan on a marine type of relay. I was just unsure of the size because the fuel pump doesn't operate at a high PSI being that it is feeding a carb and not fuel injection. I definitely prefer to wire in a relay to take the load off of the oil pressure switch. I think it's cheap piece of mind.
 
You don't need to bypass while cranking on a carburetor engine.

Incorrect!
If the fuel in the carburetor has evaporated (as if the boat has been sitting for a while), you will want some type of "start-by-pass" circuit that will operate the fuel pump until oil pressure comes up enough to close the contacts within the N/O low oil pressure switch.


Do you mean I don't need the relay?

A relay would remove the X amp current load from the N/O low oil pressure switch.... so it's not a bad idea.

If you use the starter motor S circuit, there is a risk of current back-feed that would continue starter motor operation once the LOP Switch closed.
In this scenario, a relay would prevent that.

If your starter motor solenoid has the I or R terminal, you can use that with no risk of back-feed.
In this scenario, there is no real need for a relay.


Another option would be to not do a "start-by-pass" circuit, and install a "momentary" helm switch that would operate the fuel pump.
Essentially, the momentary helm switch would be your start-by-pass.
The beauty of this, is that you would not need to crank the engine in order to send fuel to a somewhat dry carburetor.
You would simply activate the momentary switch for several or more seconds.
There is also zero chance of back-feed with this scenario.




.

 
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Oil pressure comes up enough to work the oil switch in a single rotation so no, you don't need bypass. Might need to crank the starter a couple times, but the fuel pump will fill the float chamber in a matter of a second or two if the bowl's empty. You don't see this kind of remote electric pump switch for cars with carbs do you? Of course not.

To help Ricardo understand how engines work; oil is a hydraulic fluid and is incompressible. When the oil pump spins, it doesn't "build up" pressure in the oiling system, the pressure is already there but at zero psi when the engine is off. Rotating the engine spins the pump which adds pressure to the system, duh. At low starting rpm, the pressure is low, but above the 8 psi needed to operate the oil safety switch so the pump will come on almost as soon as you begin cranking, assuming everything is good.

Relay is needed so that it kills the pump when ignition or emergency battery cutoff has been switched off.

The main concern here is fire. What you do NOT want is for the electric fuel pump to continue to pump gasoline into a burning engine compartment.

You DO need an oil pressure bypass on a fuel injected engine, or at least a software switch in the ECU that will do the same thing.
 
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AFK1911, it's doubtful that oil pressure comes up enough to work the oil switch in a single rotation.
Even if it did, a dry carburetor fuel bowl will require several seconds (or more) of starter motor operation before enough fuel has been sent to the carburetor's "dry" fuel bowl.
Now having just said that........, if the engine has been recently operated, this is of no concern.

so no, you don't need bypass.
That would be a "start-by-pass" circuit.
Perhaps ponder why a "start-by-pass" circuit is incorporated in many carbureted
marine engine applications.

Might need to crank the starter a couple times, but the fuel pump will fill the float chamber in a matter of a second or two if the bowl's empty.
I would question that.

You don't see this kind of remote electric pump switch for cars with carbs do you? Of course not.
Comparing a car/truck system to that of a Marine application is rather futile.
Carbureted cars/trucks are typically driven more often, meaning that they typically do not sit for long a duration.

To help Ricardo understand how engines work; oil is a hydraulic fluid and is incompressible.
Technically speaking.... oil will compress approximately 0.5% for every 1,000 psi that is applied to it.
But that is getting a bit too technical for the purpose of this conversation.

When the oil pump spins, it doesn't "build up" pressure in the oiling system, the pressure is already there but at zero psi when the engine is off. Rotating the engine spins the pump which adds pressure to the system, duh. At low starting rpm, the pressure is low, but above the 8 psi needed to operate the oil safety switch so the pump will come on almost as soon as you begin cranking, assuming everything is good.
Thank you for that. I now have a better understanding. :rolleyes: ;)


Relay is needed so that it kills the pump when ignition or emergency battery cutoff has been switched off.
A relay can be used for several reasons:
.... it will take the electric fuel pump's amp load from the N/O low oil pressure switch.
.... it will allow for low current triggering in a fashion that prevents back-feed if the starter motor S circuit is used for the start-by-pass circuit.


The main concern here is fire. What you do NOT want is for the electric fuel pump to continue to pump gasoline into a burning engine compartment.
The USCG requires the N/O low oil pressure switch so that in the event of a stalled engine (due to an array of potential reasons.... a severed fuel line being the important one), will cut power to the fuel pump in the absence of engine oil pressure.

The start-by-pass circuit is NOT a USCG requirement.


You DO need a oil pressure bypass on a fuel injected engine, or at least a software switch in the ECU that will do the same thing.
That must be a typo!



AFK1911, if you want to forgo the start-by-pass circuit, you could run a circuit from the helm back to the fuel pump.
The helm switch must be a "momentary" switch in order to NOT violate USCG regulations.

Here are a few other advantages:

.... you can prime a somewhat dry carburetor without using the starter motor. Hit the momentary switch for a few seconds.... and bingo.
.... believe it or not....... in the long run, this will extend the life of your starter motor.
.... again, believe it or not..... it will also extend the life of your cranking battery. (fewer cycles)

It's your boat, do as you wish.

Good luck to you.
 
Thanks again for all of the info. I can see there are multiple ways to accomplish the fuel pump install. I will check to see if the starter has the S, I or R terminal and go from there. I'm hesitant to put the switch at the helm for fear my little ones will want to play with it and end up dumping fuel everywhere. That could be an issue. I like the idea of the relay to take the load off of the pressure switch. I guess the next step is to check the starter but it's pouring out right now.
 
There’s no issue with the helm switch and children. It would be a momentary switch, therefore it cannot possibly be left in the ON position!
Even if a child was to hold it on, it will do nothing more than activate the fuel pump. There will not be any gasoline leaks.


Please note that the S terminal circuit is very different from the I and R terminal circuit!
 
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If they were to hold the switch down, wouldn't the fuel pump keep running and keep dumping fuel in to the carb? How would I know what starter terminals I have, would they be marked on the solenoid?
 
The carburetor’s needle and seat allows for only enough fuel to reach the proper float bowl level. Once the fuel reaches this level, the float closes the needle/seat.

And yes.... the starter motor solenoid will be marked as to “S” , “I” or “R”!
 
Do NOT follow PCKR, AKA Ricardo.....No mercruiser experience or expertise!

Do not add a bypass switch to you system.
If mercruiser intended for that it would be installed in electric fuel pump carbed engines from the factory.
IT IS NOT!
Use your original plan or acquire a schematic of a electric fuel pump carbed mercruiser and use the OEM parts including the wiring harness...(this is the correct and preffered method).

Merc does not use a relay nor do they have issues with oil pressure switch current issues that i am aware of.
 
This is great info. Thank you. I like the idea of setting it up like the manufacturer. I have the Thunderbolt IV ignition. I think I will ditch the Carter pump and go with the Mercruiser pump and bracket even the there is a quite a price difference. I suppose I can just install a tee at the oil pressure sender to mount the oil pressure switch, add the fuel pump, and modify the existing harness with proper heat shrink connectors correct? The only thing a little fuzzy that I can't really see on the diagram is where does the purple/yellow wire connect to on the starter? Does it go to the "T" terminal? I also plan on getting a service manual when I do this.
 
On most thunderbolt ignitions with a carb,
The coil requires power when cranking.
Typically this is handled in the ignition switch.
So when key is in crank position it is also supplying 12 volts to the coil.

That being said,

In the older point ignitions the 12 volts during crank came from the starter solenoid. (Solenoid had two small connections)
One was to activate solenoid and the other supplied 12 volts to coil when cranking.

So,
a yellow with red from slave solenoid to starter solenoid will go to one terminal on starter solenoid.
The other terminal (small) should supply 12 volts when cranking.
You can verify prior to connecting and test terminal output while cranking starter
 
When the pump current burns the pressure switch contacts, feel free to install the relay
Kg and 02 think there experts ih here.
 
Wow lots to learn, but it's cool. I can't wait to start wrenching. Is burning the pressure switch an issue since the mercruiser factory diagram doesn't show one?
 
When the pump current burns the pressure switch contacts, feel free to install the relay
Kg and 02 think there experts ih here.

I guess it takes an expert to know one...
Or so that expert may think of himself one...

I offered nothing but proper guidance.

If those oil pressure switches are so problematic
Then why are there hundreds of thousands in use.

Whats the "expert" opinion on that?

Or maybe lay off the sauce...either way keep it to yourself!
 
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When the pump current burns the pressure switch contacts, feel free to install the relay
Kg and 02 think their experts in here.
There ya have it....... Bt Doctur nails it!
Kg and 02 love disrupting the flow of these threads.

The 5 pin relays can be purchased in either 30 or 40 amp.
5 pin is 5 pin..... most all share the same contact configuration.
You can get the little pre-wired plug-in harness with them.
They will easily handle the fuel pump's amp load.

They are inexpensive and can be replaced easily.

images


I can't wait to start wrenching. Is burning the pressure switch an issue since the mercruiser factory diagram doesn't show one?
The Mercury Marine loves to sell parts. In this case, it would be the N/O low oil pressure switch.
If this particular switch was to fail, it will cut power to the fuel pump and leave you stranded.



In reference to Bt Doctur's post:
Keep you FΠckin opinion to those who deserve it.
(this is the typical attitude from him)

I did not tell not to use the realy design only suggested to use OEM as hundred of thousand boats use and still in use.
Then what is this? >>>>> "Do NOT follow PCKR, AKA Ricardo.....No mercruiser experience or expertise!
Do not add a bypass switch to you system.
If mercruiser intended for that it would be installed in electric fuel pump carbed engines from the factory.
IT IS NOT!"

Some people are simply not able to think outside of the box!


Just remember opinions of you have been expressed to many of us by those who KNOW you, yet we keep it to oursleves. Maybe you should also.
Perhaps practice that which you speak!




(this was prior to his edit)
Point to the correct ones who you do know think they know it all.
Or maybe lay off the sauce... either way shut the F up
Once again, nice attitude!



AFK1911, think outside of the box and install the relay in your system and you can't go wrong.
If you do not want to incorporate a "start-by-pass" circuit, then do the Momentary helm switch.




.
 
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OK I'm pretty sure I am understanding the various methods of doing this. I am gathering that many people do it many ways. I think the last puzzle piece for me to fully understand is the term "start-by-pass" circuit. Is this referring to the wire from the starter solenoid to the relay effectively bypassing the oil pressure switch until it comes up to proper psi to engage the switch?
 
Guess it`s because I have had service calls to replace the pressure switch because the fuel pump stopped working. And after the second pressure switch also stopped because the fuel pump decided to draw more current than the switch could safely handle.
On special request I have installed momentary switches for electric fuel pumps to prime fuel water seps after replacement. Customers find it more convenient instead of prefilling the filter. Either dash mounted or engine mounted momentary.
 
On the starters solenoid there are 2 small lugs, the inner most one has a yellow/red wire .This comes from the slave to operate the starter. the outer most pin is momentary 12volts only during cranking and is sometimes used to suplly power to the fuel pump until oil pressure closes the switch and power the pump.This or it1s equilivent
Airtex OS75 Oil Pressure Switch or Standard PS-64

 
Afk1911

Responses from Ricardo AKA PCKR...

Should not be followed...

If you are going to do this, do it correctly.
I will say again, If OEM thought a momentary fuel pump activation switch was needed/safe they would have installed them in boats.....They have not
If mercruiser simply installs parts that fail in order to sell more parts, that is freakin foolish as the moron who said it.

Do some parts used by OEM found to be bad over time and superceded by new and better designed, Yes happens all the time.

Its your boat, do what you will but if something goes wrong and it is determined it bscause you made your own design of parts for repair or simply because you wanted a different design,
Insurance will not cover anything, NO authorized service company will work on it.

You will be 100% on your own unless you can get BTDoctor or PCKR to work on it...

Oh and by the way, you dont need a perminant
momentary bypass when dealing with a water seperator......you can activate the pump easily and temporarily and only as needed...
 
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I don't think I will be doing the momentary switch. Too much for the kids to play with and too far away from OEM for my liking. It really boils down to relay or no relay. From what I am understanding, using a relay won't pose any safety issue, but it may offer a greater reliability factor. Again I say "may" because a pressure switch could fail just as easily as a relay I guess. I also don't think people will refuse to work on it because it has an added relay. It'll be a completely different engine than the boat came with in 1992 anyway. Hopefully I'll be the only one working on it if I do my due diligence. I'm also not sure if insurance would deny a claim due to a relay, that many mechanics seem to endorse, was added. I think I have it narrowed down to where I am going to use the OEM fuel pump and bracket, a 30 or 40 amp relay with plug in harness along with dielectric grease (this boat won't see salt water), and the carter or airtex oil pressure switch teed off the sender mount, edelbrock 1409 carb, and edelbrock 2114 manifold. That should wrap up my biggest mystery regarding the switch from non-vortec to vortec.
 
.........
On the starters solenoid there are 2 small lugs, the inner most one has a yellow/red wire .This comes from the slave to operate the starter.
Absolutely correct. This is known as the "S" terminal.
When energized from the key switch (while in the start position), it powers the solenoid's magnetic field that draws the plunger forward, closes the main contacts while simultaneously kicking the pinion gear out into the flywheel.

If the "S" terminal was to receive back-feed current, it would keep the starter motor engaged and running.
Whereas with the "I" or "R" terminal, back-feed is of no concern.


the outer most pin is momentary 12volts only during cranking
Absolutely correct again, and no surprise because Ed knows his stuff!

This terminal is only energized when the main solenoid contacts are closed.


and is sometimes used to supply power to the fuel pump until oil pressure closes the switch and power the pump.
In the old days, this "I" or "R" terminal was used to supply Non-Resisted voltage to the ignition coil during cranking when voltage was low.
In the marine world, the
"I" or "R" terminal can also be used to trigger the electric fuel pump's relay.

I don't think I will be doing the momentary switch. Too much for the kids to play with and too far away from OEM for my liking.
The momentary helm switch simply circumvents the need for a "start-by-pass" circuit. Instead of the system doing this automatically, you would do it manually.

Also, the switch does not need to be at the helm.... it could just as easily be mounted elsewhere and out of reach of the kids.

It really boils down to relay or no relay. From what I am understanding, using a relay won't pose any safety issue, but it may offer a greater reliability factor.
Both of those are correct!

Again I say "may" because a pressure switch could fail just as easily as a relay I guess.
Again, the 5 pin relays can be either 30 amp or 40 amp capable. In other words, they are designed to take the load.
The current required to trigger the 5 pin relay (as in the current passing through the N/O Low Oil Pressure switch) is minimal.


I also don't think people will refuse to work on it because it has an added relay.
Absolutely correct.
If the use of a relay stumps a mechanic, you'd better find another mechanic who is savvy to this.

It'll be a completely different engine than the boat came with in 1992 anyway. Hopefully I'll be the only one working on it if I do my due diligence. I'm also not sure if insurance would deny a claim due to a relay, that many mechanics seem to endorse, was added.
Nope. A sealed 5 pin relay will certainly comply with USCG regs.

I think I have it narrowed down to where I am going to use the OEM fuel pump and bracket, a 30 or 40 amp relay with plug in harness along with dielectric grease
Smart move!
You won't regret it.




.
 
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