Connecting a electric fuel pump to 5.7LX

[AKJohne]

1997 5.7 LX Alpha 1 Gen 2
Looking for advice on connecting power to a electric fuel pump.
I am in the process of replacing the old motor with a new long block. Bought the motor and electric fuel pump "kit" from Michigan Motorz which included a NC oil pressure switch. The wiring diagram MM supplied and the wiring diagram I have found here for a carbed Mercs electric fuel pump show the power for the fuel pump coming from
the starter (purple/yellow). However my starter does not have a third lug, only the post for B+ and the yellow/red from the ignition switch to the solenoid.
My plan is to install the oil pressure switch in tandem with the oil pressure sender on the back of the motor (not the alarm switch location) and connect to the purple wire from the ignition that connects to the coil/ dist/alternator.
Does this make sense? Is there something I am missing/overlooking?

Thanks.

 

[flashback]

I did a conversion last year and most all recommended a relay to power the pump.

 

[AKJohne]

I did a conversion last year and most all recommended a relay to power the pump.

I wondered about this, I dont know what the amp draw is on the new pump. I do know the purple wire is not very big, maybe 14 awg and probably shouldn't increase the amp load across the ign switch contacts.

 

[flashback]

I believe the concern lie's with the load on the pressure sensor.

 

[Scott06]

This is what you want to wire. You may be able to get it off the yellow red which fires the starter, if you cant get it off the starter.
you starter has three lugs on it though?
Need to get 12v off starter during cranking then go through the switch off the purple ignition wire when in run . Yes some use a relay but also the factory goes through switch which in the six years ive had my boat hasn’t been an issue.

 

[AKJohne]

This is what you want to wire. You may be able to get it off the yellow red which fires the starter, if you cant get it off the starter.
you starter has three lugs on it though?
Need to get 12v off starter during cranking then go through the switch off the purple ignition wire when in run . Yes some use a relay but also the factory goes through switch which in the six years ive had my boat hasn’t been an issue.

View attachment 356330

Thanks, starter has two lugs, probaly hard to see in the pics. Yellow/red is the momentary hot from the ignition switch to engage the solenoid.
I think I will use the purple wire as that is ignition B+, as suggested I may need to install a relay to take the load off of the ignition switch contacts. I am trying to find out the amp draw on the pump, I thought it was Carter but maybe a Federal Mogul... Will look tonight and see if it has any markings. Its probaly minor but it all adds up. I think it should be fused as well. I could come off the B+ on the starter with a inline fuse holder which I am not a fan of, however the location is dang convenient, otherwise I have a distribution panel on the transom, (port side unfortunately) this is off the house batteries... Prefer to keep all loads associated with house or starter on the house and starter sides.. and maybe that's silly and just me....?
I would use the oil pressure switch in energize the relay... Am I overthinking ....?

 

[AKJohne]

Just finished reading several more posts on this subject and realized I identified the oil pressure switch as n/c and it is actually n/o. You guys undoubtedly knew that.

 

[Eric 3.7LX Alpha 1]

Yes, N/O, as in without oil pressure, with the engine off, the contacts are separated. The starter powers the pump for the few seconds the engine is cranking over until the switch reads pressure and then closes to make contact and take over.

 

[Scott06]

Thanks, starter has two lugs, probaly hard to see in the pics. Yellow/red is the momentary hot from the ignition switch to engage the solenoid.
I think I will use the purple wire as that is ignition B+, as suggested I may need to install a relay to take the load off of the ignition switch contacts. I am trying to find out the amp draw on the pump, I thought it was Carter but maybe a Federal Mogul... Will look tonight and see if it has any markings. Its probaly minor but it all adds up. I think it should be fused as well. I could come off the B+ on the starter with a inline fuse holder which I am not a fan of, however the location is dang convenient, otherwise I have a distribution panel on the transom, (port side unfortunately) this is off the house batteries... Prefer to keep all loads associated with house or starter on the house and starter sides.. and maybe that's silly and just me....?
I would use the oil pressure switch in energize the relay... Am I overthinking ....?

If you only use the purple ignition wire the fuel pump will not be powered during cranking. So when your fuel bowl dries out as typical when the boat sits for a couple weeks it will never refill and engine wont start. two ways around it - wire in parallel a momentary switch to prime before starting, then use the purple to power via oil pressure switch when running. Option two use the yellow/red and purple in parallel to close a relay powering the pump. Not sure if the pump draw would interfere with starter if you take power directly off the yellow red.

The momentary switch might be useful as if mine sits for a week or two I'm cranking for a bit until it gets the carb bowls full again.

Either way the relay is probably better/ more technically elegant but I wouldn't worry about the key switch contacts or the oil pressure switch too much as this works for merc in a stock set up.

 

[AKJohne]

If you only use the purple ignition wire the fuel pump will not be powered during cranking. So when your fuel bowl dries out as typical when the boat sits for a couple weeks it will never refill and engine wont start. two ways around it - wire in parallel a momentary switch to prime before starting, then use the purple to power via oil pressure switch when running. Option two use the yellow/red and purple in parallel to close a relay powering the pump. Not sure if the pump draw would interfere with starter if you take power directly off the yellow red.

The momentary switch might be useful as if mine sits for a week or two I'm cranking for a bit until it gets the carb bowls full again.

Either way the relay is probably better/ more technically elegant but I wouldn't worry about the key switch contacts or the oil pressure switch too much as this works for merc in a stock set up.

Thanks Scott, I presume your statement "the fuel pump will not be powered during cranking" is referring to the fact that there will not be oil pressure initially? While that may be true I dont see a real issue with developing oil pressure before starting, particularly if the motor has sat for a while. The fuel bowl is unvented and we don't have a issue with fuel evaporation here in AK, temps are mild. Even when stuff has sat for long time there is still fuel in the carb, maybe not good fuel, but its still there.
The old motor always started very well, even when it sat, how long does it take for the sensor to close? I am thinking 4-8 PSI, which it should see within a second or two... If we are on the water and using it I dont see this as a issue as the carb will be "topped up".
The motor is on a stand and I will prime the engine and see how long it takes to close the contacts on the switch. I just dont want to add another switch if not necessary and to my understanding Merc did not. I appreciate the help I get here and not trying to be contrary, just want to make the right decision.
I will also check the AMP draw on the new fuel pump this weekend and then make a decision on the relay, as mentioned Merc apparently didn't think it was necessary either.

 

[Scott06]

Thanks Scott, I presume your statement "the fuel pump will not be powered during cranking" is referring to the fact that there will not be oil pressure initially? While that may be true I dont see a real issue with developing oil pressure before starting, particularly if the motor has sat for a while. The fuel bowl is unvented and we don't have a issue with fuel evaporation here in AK, temps are mild. Even when stuff has sat for long time there is still fuel in the carb, maybe not good fuel, but its still there.
The old motor always started very well, even when it sat, how long does it take for the sensor to close? I am thinking 4-8 PSI, which it should see within a second or two... If we are on the water and using it I dont see this as a issue as the carb will be "topped up".
The motor is on a stand and I will prime the engine and see how long it takes to close the contacts on the switch. I just dont want to add another switch if not necessary and to my understanding Merc did not. I appreciate the help I get here and not trying to be contrary, just want to make the right decision.
I will also check the AMP draw on the new fuel pump this weekend and then make a decision on the relay, as mentioned Merc apparently didn't think it was necessary either.

No the purple wire doesnt have power until the key is released from start into run position. Old motor was a mechanical pump right?

merc did essentially have a second switch, the key triggering the starter…which powers the pump during cranking.

 

[AKJohne]

No the purple wire doesnt have power until the key is released from start into run position. Old motor was a mechanical pump right?

merc did essentially have a second switch, the key triggering the starter…which powers the pump during cranking.

The purple wire on my motor powers the coil, ignition module, knock sensor, gauges and alternator.... you might be thinking of a old dodge with a ballast resistor......

 

[Lou C]

Maybe switch the solenoid to a 4 terminal one that way you can wire it the way Merc did it originally. There are 3 terminal and 4 terminal solenoids. My old starter abs the new Arco PMGR starter I put in a few years ago both have 4 terminals. The 4th is not used on my engine because it has the mechanical pump.

 

[Scott06]

The purple wire on my motor powers the coil, ignition module, knock sensor, gauges and alternator.... you might be thinking of a old dodge with a ballast resistor......

No I'm thinking of the mercruiser wiring diagram a posted and how it powers an electric fuel pump …

 

[AKJohne]

No I'm thinking of the mercruiser wiring diagram a posted and how it powers an electric fuel pump …

Look close, purple is B+ from ignition switch

 

[AKJohne]

Maybe switch the solenoid to a 4 terminal one that way you can wire it the way Merc did it originally. There are 3 terminal and 4 terminal solenoids. My old starter abs the new Arco PMGR starter I put in a few years ago both have 4 terminals. The 4th is not used on my engine because it has the mechanical pump.

Good idea, will investigate that. I Will prime the motor this weekend and see how long it takes to close the contracts on the oil pressure switch.

 

[AKJohne]

Pump draws a hair over 1 amp, if I deadhead it goes up slightly. Once the motor is primed (OIL) it takes less than one revolution of the drill (distributor) to close the contacts. I think that's good enough. Will use purple wire, no relay, off to the races.

 

[AKJohne]

Other concerns, maybe start new thread?
I made a connection to the purple wire, merc had a crimped/soldered? & taped connection for the original 5 wires. The copper strands in a couple of the wires were black, specifically the two going to the ignition and knock module's. They are relatively short to begin with and trimmed them a inch or so and never did find shiny copper. I cleaned the strands with some scotch brite and was able to tin them. I added in the new wire to the oil sensor using a bare #10 butt connector with 3 each side, crimped/soldered heat taped. They took solder and felt secure when done and I tugged on them. I am not excited about the black (corrosion?) On the wire. I know this is not the only spot. Mostly concerned about the connections at the ignition module's themselves. These are pretty well sealed. Should I be concerned? Apparently the wiring harness for the engine is available and considering getting one while I can. However I don't believe it comes with the connectors to the modules. Which is not the end of the world.
Thoughts on this please, thanks