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Electric fuel pump questions - 4.3 L mercruiser-1990 V6- 2 barrel carb

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kirkll

Regular Contributor
Last year I ran the gauntlet on an electric fuel pump issue. I had replaced the fuel pump the year before, and it quit coming on. I tested the pump, and it ran fine bypassing the oil pressure switch, so I replaced that 3 prong switch. But…. I think I may have it wired wrong. Prior to switching out the oil pressure switch, the fuel pump would turn on after the starter engaged and then only run when the motor was running. But after I changed the switch the fuel pump comes on when I turn the ignition switch in the on position. This isn’t right for this carberator system. It shouldn’t be runn8ng with the engine not running unless it’s cranking with the starter. I know some fuel injected models allow the fuel pump to run until it comes up to pressure, but this isn’t the case here.

Is it possible I have the wires on the wrong post of the oil pressure switch that would cause this? I was recently trying to get this baby started, and having a tough time. sometimes the fuel pump would come on when I turned the ignition on, and sometimes not…. Something is not right here. Any suggestions to get this right?

Btw…. When I changed out that 3 prong oil pressure switch, I did replace the connectors with new ones that have a solid connection and are clean.

Thanks, Kirk
 
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Attached is a snap shot of the wiring diagram showing wire colors and connections

Screenshot 2026-01-04 131344.jpg
 
It looks like Cranking wire and wire to fuel pump should be on same connection/terminal. Ignition On wire (Purple) should be on the other terminal. Not sure it makes any difference on which terminals are used as this is a ON/OFF type switch as long as the connections are as I described.

Your theory of operation is correct. Power applied to pump when cranking to supply fuel to carb (Purple with yellow wire). Once engine starts (oil pressure has risen above ~ 5-7 psi) and key is returned to the run position, Power is now supplied by Ignition on (Purple wire) wire.
 
First off why a product from Amazon? Was the original a three wire switch also?

Even if a three wire,

Two of the terminals should have continuity with NO oil pressure to switch (this would be the Normally closed part of switch) and those two would be, wire to fuel pump and wire to solenoid at starter.

The other terminal should NOT have any continuity to either of the other two terminals with NO oil pressure ( this would be the normally open part of the switch) and that would be where the purple wire connects

does this help?
 
Based on the parts structures on this web site the model year you posted does not Jive. Has the engine been replaced with a newer one?
Every one I have looked up 1989-1996 4.3 liter (Gen I and Gen II engines, Not drives) shows a TWO terminal pressure switch

Please post Engine serial number!!!!!!!!!!!!
 
kghost said:
Based on the parts structures on this web site the model year you posted does not Jive. Has the engine been replaced with a newer one?
Every one I have looked up 1989-1996 4.3 liter (Gen I and Gen II engines, Not drives) shows a TWO terminal pressure switch

Please post Engine serial number!!!!!!!!!!!!
Click to expand...
Where can I find that engine serial number? That would help a lot in the future chasing the right parts.

This is not the stock engine. It was a truck engine, supposedly a 1990, that was installed prior to me Purchasing the boat. When I first got the boat and took it into a reputable shop, the mechanic spotted it right off and there were a number of things that needed to be changed out to meet CG specifications. The fuel pump was one of them. the soft plugs were changed out to deal with raw water cooling too. The truck engine blocks used antifreeze with a closed system.

The oil pressure switch was a 3 prong switch.
 
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Maybe next time lead with these details....that oil pressure switch may be GM auto not Mercruiser marine.
Either way I explained how it should be wired.
 
Being a 1990 V6 it should be manual 7 and it does an electric fuel pump but no wiring diagrams show one connected.

4.3L (2 BBL.) GM 262 V-6 1988-1992 0B773243 THRU 0D714106​


Looking up parts with the above serial number range I'm still not able to find an electric fuel pump
Having a 3 prong oil pressure switch it can still be Merc installed and if Merc it may have used it

In any case, it sounds like the common is not connected to the Fuel pump, it's probably connected to the normally closed (NC) side and 12V is connected to the common (C)

So the wire coming from the starter should be connected to the NC
The normally open side should be connected to 12V ignition power. Key is turned ON and 12V on NO post
Connect FP to C
 
I appreciate your help alldoge, but you’re confusing me with your terminology. What is the common? And which one is NC?

Then you say key is turned ON and NO post connect Fuel pump to C… I think Kghost explained it better.

It makes sense to me if the ignition wire (purple) which provides 12v with the key on should hook to the open side of the switch and on the other side of the switch has two wires hooked up to it. one to the starter solenoid And the other to the FP. That way when the starter solenoid is engaged, the fuel pump runs off of that hot wire, and when the oil pressure switch closes the fuel pump runs on the ignition wire, and the starter solenoid disconnects.

It actually makes more sense now to just have a two prong pressure switch. But I can most likely make this one work if I can figure out which two posts are the open switch that closes when pressure is there. I could probably just use a continuity tester to figure it out. I obviously have it hooked up wrong now.

Btw…. I just used Amazon because these oil pressure switched are pretty generic. I couldn’t see a reason to purchase a marine grade OEM part.
 
AllDodge said:
Being a 1990 V6 it should be manual 7 and it does an electric fuel pump but no wiring diagrams show one connected.

4.3L (2 BBL.) GM 262 V-6 1988-1992 0B773243 THRU 0D714106​


Looking up parts with the above serial number range I'm still not able to find an electric fuel pump
Having a 3 prong oil pressure switch it can still be Merc installed and if Merc it may have used it

In any case, it sounds like the common is not connected to the Fuel pump, it's probably connected to the normally closed (NC) side and 12V is connected to the common (C)

So the wire coming from the starter should be connected to the NC
The normally open side should be connected to 12V ignition power. Key is turned ON and 12V on NO post
Connect FP to C
Click to expand...
I also looked at this link and several other model years. The root of the misunderstanding is in one of his posts,

The engine is a GM AUTO TRUCK engine which was used for replacement and not a Marine engine. it has been "Converted" to some degree to be marine (Core plugs etc.)

As I said above, The original oil pressure switch for the GM AUTO engine may have been a Three wire connection thus he replaced it with a Three wire switch. Mercruiser uses a TWO wire switch.

As far as the wiring and NO (Normally Open) vs NC (Normally Closed) and C (Common).

The common is the Output to pump. (Purple with Yellow stripe) Confirm this wire goes to pump.
The NC is from the starter (Purple with Yellow stripe)
The NO is from ignition on (purple). This terminal becomes +12V from Ignition on, when 5-7 psi of oil pressure has occurred.
 
I thought I’d follow up on this electric fuel pump issue. I tried all the different combinations hooking up my 3 prong oil pressure sensor switch, and couldn’t make it work with out the fuel pump coming on when I turned the switch. When I had it wired right, it would fire up, then run out of gas. I was pulling my hair out. So I went to the auto parts store and bought a new switch, and did a continuity test to find the right posts for the starter solenoid wire an FP, and put the ignition wire on the other post. The power source was a purple wire that came into my automatic choke and had a connector there for a second wire.

BINGO! Everything is working as it’s supposed to now. The only thing I could figure is that cheap Amazon pressure switch went boot up on me.

Thanks Ghost!

The only confusion was when using my multi meter I checked the purple power wire coming to the choke where I was T’d into, and I wasn’t getting 12 volts. That kinda threw me and I thought that might be the problem before I tried firing it up. But the fuel pumps running and it’s running ok. Shouldn’t that purple wire be showing 12 volts when the key is turned on?
 
Yes. 12vdc when key is in the run/on position.
All the gauges also have the same power connection as does the ignition.
 
kghost said:
Yes. 12vdc when key is in the run/on position.
All the gauges also have the same power connection as does the ignition.
Click to expand...
It was strange.... I checked the battery level and had 12.65 volts at the battery, but I was getting a lower voltage reading at the choke connection of the purple wire with the key on.... But.... The fuel pump seemed to be running fine. kinda strange...
 
in additon to the 10 pin plug and batt cable check for voltage drop at the helm as well.

what i have seen on two of my boats is the power comes from the key and daisy chains through all the gauges in series, lots of connections that can get corroded or loose. To me this is a bad design vs sending 12 v to the ignition directly. Also will go through the kill /man overboard switch in the shifter, whioch would be a good place to check if you have voltage drop from the key
 
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