Three-Prong Oil Pressure Switch and Electric Fuel Pump - Is a Fuel Pump Relay Needed?

[sdowney717]

I am converting the boat to a holley red marine electric pump.

The safety switch I am going to use is a gm 3986857 3 prong oil pressure switch. Has a common, NO, NC prongs.

Can the electric pump use the switch without a relay OR is the electric load running threw the switch too much for it and it needs a relay?


The reason I ask is simply I want to replace the current low pressure idiot light switch with this 3 prong switch without a 'T' hookup.
the 3 prong switch will just be turning grounds on and off. I will put a relay running off the start wire to the starter and that relay will just ground the pump negative wire to make it run. If I do this, then all I need is the one pressure switch to run the idiot light and the pump. The relay will turn on and ground the negative pump wire when the starter is cranked.
When the oil pressure builds up, the safety switch will close and ground the negative on the pump and it will run.

If I dont do this, I will have to put in a T and have 2 pressure switches. I currently have an oil gauge pressure switch, so really then I would have to have 3 oil switches on the block. See what I mean?

Currently there are 2 oil pressure switches on the block. One for the idiot light-buzzer, the other for the pressure gauge. I dont want to loose the buzzer idiot light.

[teejayevans]

Originally Posted by sdowney717

I am converting the boat to a holley red marine electric pump.

The safety switch I am going to use is a gm 3986857 3 prong oil pressure switch. Has a common, NO, NC prongs.

Can the electric pump use the switch without a relay OR is the electric load running threw the switch too much for it and it needs a relay?


The reason I ask is simply I want to replace the current low pressure idiot light switch with this 3 prong switch without a 'T' hookup.
the 3 prong switch will just be turning grounds on and off. I will put a relay running off the start wire to the starter and that relay will just ground the pump negative wire to make it run. If I do this, then all I need is the one pressure switch to run the idiot light and the pump. The relay will turn on and ground the negative pump wire when the starter is cranked.
When the oil pressure builds up, the safety switch will close and ground the negative on the pump and it will run.

If I dont do this, I will have to put in a T and have 2 pressure switches. I currently have an oil gauge pressure switch, so really then I would have to have 3 oil switches on the block. See what I mean?

Currently there are 2 oil pressure switches on the block. One for the idiot light-buzzer, the other for the pressure gauge. I dont want to loose the buzzer idiot light.

What's the draw of the pump?
I had a 2A pump directly connected, but now have it drive a relay because the run to the engine panel was long and wires small. Also I can drive the electric pump to prime the lines after changing the fuel filter without needing to turn the engine over. The NC and NO are connected to the ignition switch 12v ACC/START lines.
Tom

[sdowney717]

it draws 2 amps
I came up with 2 ways to do this, one switches positive side, the other switches negative ground side.
I am going with the positive switching.

positive switching with momentary prime switch and relay


negative switching allowed the use of only one sender but someone said it is bad if boat burns and pump ground energized pump will run.

[sdowney717]

I found even 18 gauge runs the pump fine with a distance of 12 feet.
I have 2 pumps, both marine, one is carter the other Holley red.
The wire is cable from a vacuum cleaner, has heavy fiber strands in the cord to resist breakage and is tough. One wire is 18 gauge, the other 17 gauge. I too wondered about the gauge but It pumps fine.

The flow is like this,
tank
anti siphon valve at tank fitting
separating filter
pump mounted behind engine on a wood support
regulator
carb

Putting filter before pump works. the filter top is at same height as tank top, the pump about 5 inches lower than filter top. The filter acts like a small tank right next to the pump so it flows. I do fill the filter housing with fuel first after a change but dont know if need to do that.

USCG has a rule that pump should be mounted within a foot of engine, mine is about a foot behind the engine. I have seen pumps on other boats mounted all over bulkheads even far from motor. So dont understand that rule except perhaps have it close to engine for monitoring function?

[GordMay]

Originally Posted by sdowney717

I found even 18 gauge runs the pump fine with a distance of 12 feet...

FWIW, ABYC specifies:
11.14.1.2 Conductors shall be at least 16 AWG.
And:
11.14.1.3 In engine spaces the conductor insulation shall be oil resistant and have a temperature rating of at least 167oF (75°C) dry.