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wblazer

Regular Contributor
Hi Guys,

I posted this question a while ago with no response, going to try again. The engine is a 1986 4.3 with electric fuel pump. The pump is being power by a red pig tail that is spliced into the red wire. The pump is not tied into the oil pressure switch but will be in time. The engine cranks strong but only getting 6 volts from this red wire. It has run in the passed with this wireing set up. Could someone tell me what would cause the red wire to only have 6 volts? And where does this wire get its full 12 volts from. Thanks for any help given.
 
wblazer said:
Hi Guys,

I posted this question a while ago with no response, going to try again. The engine is a 1986 4.3 with electric fuel pump. The pump is being power by a red pig tail that is spliced into the red wire. The pump is not tied into the oil pressure switch but will be in time. The engine cranks strong but only getting 6 volts from this red wire. It has run in the passed with this wireing set up. Could someone tell me what would cause the red wire to only have 6 volts? And where does this wire get its full 12 volts from. Thanks for any help given.
Click to expand...


Are you sure nothing has changed? If nothing did change, it sounds like a bad connection somewhere upstream of that wire.

But it sounds like it could possibly be the "load end" of a resistor wire feeding a reduced voltage coil. Some coils are designed to run on a lower voltage, usually around 7 volts, and utilize a resistor wire or ballast resistor in series to reduce the 12 VDC to the required voltage. If you connected to the "load side", then that could explain the "6 volts". Follow that wire you spiced into back towards the supply and see if there is another splice in it. If there is, test the voltage before the splice and see if it's 12 VDC. That would be where you connect in the Fuel Pump..

Normally, the fuel pump is conneced to a relay, which is connected to the ignition circuit, battery B+ and the oil pressure switch, to keep the load on the ignition wire to a minimum.
 
In the case of this style ignition there is NO relay,

See schematic attached and if you did not wire it this way you should do so.

I agree you may have taken the signal from the wrong location but we cant see that from here...

I have two schematics, One from a earlier manual and it does not show the fuel pump in the wiring, The second one for a thunderbolt IV shows the fuel pump.

It looks like the fuel pumps gets in "cranking power" from a puple with yellow stripe wire directly off the starter solenoid, There may or may not be two small posts on the solenoid, one would be crank (yellow with red stripe) S terminal post, the other would feed power only when cranking the starter I terminal post, It then gets its "run" power from the Purple wire which would be connected to the + side of the coil and would go thru the oil pressure switch (which should be present) and the output of the oil pressure switch changes to puple with yellow to the pump The same wire that come from the cranking circut.

A side note: When this first came out with the electric fuel pump on V6 motors I do not believe they had it wired a slightly different way as we had to jump the pressure switch on new boats on the first time starting them to get the fuel system primed and started. Once this was done the temporary jumper was removed.

Check the voltage at the + side of the coil it should be 12 V

Get back with what you find...

I am not to crazy about this uploader as the pic quality does not seem to be the same as the saved image on my pc, The pics seem way to small.

Dont know how to link it to a better picture.................
 
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wblazer said:
Hi Guys,

I posted this question a while ago with no response, going to try again. The engine is a 1986 4.3 with electric fuel pump. The pump is being power by a red pig tail that is spliced into the red wire. The pump is not tied into the oil pressure switch but will be in time. The engine cranks strong but only getting 6 volts from this red wire. It has run in the passed with this wireing set up. Could someone tell me what would cause the red wire to only have 6 volts? And where does this wire get its full 12 volts from. Thanks for any help given.
Click to expand...

Ayuh,.... That's Not the way Merc put it together, So,... It's hard to say just What you've got there....

In 1986, it was equipped with a Mechanical fuel pump...
 
The manual I got the first schematic out of which does NOT show the fuel pump in the schematic does show the installation of it an another page deeper inot the manual. Not sure why it is missing in the wiring diagrams.............That is why I looked in a newer manual to get the second schematic.


In my opinion he should be wiring it if he is useing a elec fuel pump like the drawing on the right!! that is the correct way.

Any thoughts why the other manual is incomplete or did they do a general update later?
 
bondo said:
Ayuh,.... That's Not the way Merc put it together, So,... It's hard to say just What you've got there....

In 1986, it was equipped with a Mechanical fuel pump...
Click to expand...
My thoughts exactly.
In 1986, I believe that the OEM fuel pump would have been mechanical. This likely means that the electric fuel pump has been added.
Follow this power wire to it's source. If it's coming from a portion of ignition system, perhaps via a ballast resistor, that may explain the lower voltage.
You do not want to power the fuel pump with this as the source...,. and if so, you'll want to remove this and rewire it..... and definitely include the low oil pressure switch in the circuit.
The oil pressure shut down switch is mandated by the USCG.

The circuit that supplies power for the fuel pump should be as close to 12 volts as possible.

This is a N/O (normally open) oil pressure switch, not the oil pressure sender unit.
Power the N/O oil pressure switch with a good fused power source.
From the oil pressure sw, power the fuel pump.


You can use one of the five pin relays for a portion of this new circuit.

330-070_s.jpg
relay.jpg



The relay can be brought in for your "start-by-pass" circuit so that the pump will operate during zero or low oil pressure until the engine fires up.
This involves triggering the relay via the starter motor solenoid circuit and removes any liklihood of power feedback.

Or.... some install a Momentary helm switch to provide the same function.... but it must be a Momentary switch.
This method is prefered by some in that you do not need to crank the engine over to build fuel pressure to prime your fuel system.
Less cranking.... less battery draw..... faster starts after having sat for a while.


.
 
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Attached would be the wire connection suggestions for a relay system with a optional momentary switch to apply power to the relay during cranking. This momentary power could be taken directly from the wire attached to the starter solenoid I stud if present instead of the momentary switch. this is shown in one of the schematics.
 
Kghost, in your scenario, the pump would be powered the moment that the key switch was turned to "Ignition ON"......, and that's what the USCG does not want!

If you installed the N/O oil pressure switch to interrupt the "power from ignition on purple wire"......... you got it! :D
The momentary helm switch over-rides this until oil pressure comes up.

The load for the electro-magnetic coil is so low that the ignition circuit won't mind at all.

Fused fuel pump supply would then feed #30.

This takes the main load from the N/O oil pressure switch, and puts it on the five pin relay....... and most of these will handle 30 amps...... some will do 40 amps.
Either is much more than what the fuel pump will require.





.
 
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How do you get the pics in the body of your reply like that?

I agree I left all that out just for a basic wiring, I will add it and repost.
 
The relay would require power independent of the momentary helm swith.

An Ignition source would trigger relay once the oil pressure switch closes the circuit.

Prior to oil pressure, the over-riding momentary helm switch will trigger the relay.

 

Attachments

  • five pin relay schematic for electric fuel pump.jpg
    five pin relay schematic for electric fuel pump.jpg
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Thanks all I will check the wiring this weekend. I need to purchase the oil pressure switch to make the installation legal. Always a pleasure dealing with people on this forum. I will post back in the future when the problem is solved.
 
RicardoMarine said:
The relay would require power independent of the momentary helm swith.

An Ignition source would trigger relay once the oil pressure switch closes the circuit.

Prior to oil pressure, the over-riding momentary helm switch will trigger the relay.

Click to expand...

DUH


Was not looking at that picture the correct way, tHank you!!!

I completely missed 30 as the power input..............duh!!

My bad............gatta wait for the second coffee to kick in before doing some of this stuff!!
 
That's OK, Kghost.... I have more than my share of "duh" moments as well, and if more coffee does the trick, then that's good! :D

I had to scratch my head when I first began thinking of this, and whether or not the relay would pose any issues regarding any current back-feed.

This may not be the same as the OEM, but I believe that this schematic will work just fine!
We just want to make sure that the oil pressure swith has full capability of shutting off power to the fuel pump.


.
 
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The only issue I have with the relay senerio is the relay is or would be the weak link. It coil will wear out first after many on off cycles or the contacts will get carboned up........ I understand full well it is common place in fuel injected auto's..........

The OEM has no relay. (not saying the starter solenoid is a relay but in the big pitcure it could be called one)

the crank power is through the starter solenoid,
the run power is thru the ignition on curcit and the OP switch.

It has been a while for me but if I remember the original V6 electric fuel pump motors did NOT have the crank power to the pump. That came later after too many issues with trying to start with no fuel pressure after long non use periods of time............I believe that is why we used to jump the pressure switch out on a initail start of a new boat so equipped. I could be wrong......been a while.
 
A few thoughts.

kghost said:
The only issue I have with the relay senerio is the relay is or would be the weak link.
Yes, it sure could be. They are very inexpensive, and it would be a good idea to keep a spare on board.

It coil will wear out first after many on off cycles or the contacts will get carboned up........ I understand full well it is common place in fuel injected auto's..........
Understood. However, if you purchase the right five pin relay, they'll be continuous duty for 30 amps or even 40 amps.
His fuel pump circuit will require no where near that.

The OEM has no relay. (not saying the starter solenoid is a relay but in the big pitcure it could be called one)
Yep.... agree.

the crank power is through the starter solenoid,
Only if the momentary helm switch is in lieu of the starter motor by-pass circuit.
If he forgoes the momentary helm switch, then he'll need to make the start-by-pass circuit.
Personally, I like the idea of the helm switch...... no cranking needed to get fuel going.

the run power is thru the ignition on curcit and the OP switch.
Actually, it's through the relay contacts. Power IN #30 and Power OUT #87.
The relay electromagnet pin #35 is triggered by the ignition circuit via the closed oil pressure switch.

Prior to oil pressure, the momentary switch triggers the relay.... or the Start-By-Pass circuit as long as there's no feedback potential.

Click to expand...
 
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