I have a Holley Electric Fuel pump installed. All works with ignition key switch. Turn the key and the fuel pump comes on.

I am now installing a oil pressure switch to shut off fuel pump if engine stops. I got a Holley switch. It has 3 contacts. One to fuel pump and one to the ignition switch. What is third for? It goes to the starter silenoid positive contact according to the diagram that came with the switch. Hmmmm?

Thanks

 

It has 3 contacts. One to fuel pump and one to the ignition switch. What is third for? It goes to the starter silenoid positive contact according to the diagram that came with the switch.

Can you post the wiring diagram?
heres my take on it.
The volts for the fuel pump need to be on when the motor is running or when you are cranking it. The feed from the ignition switch should be the switched power because you do not want power to the pump when the ignition is off.(Just in case. The lack of oil pressure should turn it off anyway) The line from the starter should be the line from the solenoid (control wire, not +volts) so that when you crank it Volts gets to the pump. there should also be a diode in circuit to stop the volts feeding back to the Starter solenoid, which may be internal to the switch.
Hope this helps.

 

Okay I think I get it
Fuel pump needs + power from two sources...at two different times once pressure is high enough!

One to start up:
turns on while cranking from one source (starter silenoid)

Two to keep running:
fuel pump stays on from ignition key switch ON. (because the on position and the start position...)

Maybe I don't get it. Why would this thing not work with just power to it one terminal and enough oil pressure?

Help

 

Clarify:
Only letting the key rest in ON position feeds the power?
When I crank the motor with key in start position...
It shuts off this power until I let go of the key?

 

Sometimes the accessory power is switched off when cranking. It depends which ignition feed you send to the switch.
I have a lot of experience in electronics and that was how I think it works. Can you post the instructions and the circuit diagram that came with the switch? Or email it to me if you can't post.
You know the old adage: If all else fails, read the instructions

 

My Scanner sucks at home (another forum)

I'll scan the daigram tomorrow at work
Thanks for your responses too

 

From the diagram, the connections on the switch are marked NO, NC and C which mean Normally Open, Normally Closed and Common.
Although it is badly illustrated, the NC connection is fed from the solenoid and I believe it should be the control wire, not the permanent +12V otherwise the pump has +12v when the motor is off. The NO connection is fed from the ignition switch and will connect this source of +12 to the fuel pump when the oil pressure is up.
So the series of events when you try to start the car are
1/ +12 applied to the solenoid to start the car is fed through the NC contact of the oil pressure switch to the fuel pump so you have fuel while cranking.
2/Oil pressure rises and switches to the NO contact of the switch to feed the ignition +12 to the pump.
3/ When the car is running the oil pressure maintains the ignition +12 to the pump via the NO contact.
4/ If the motor stalls the oil pressure drops and removes the +12 from the ignition feed to the pump.
5/ if you turn off the ignition the +12 to the pump is removed. the oil pressure drops and it switches back to the solenoid contact for the next start.

Pretty much as I posted earlier.The diode is not needed as the switch will be a "break before make" style.
Hope this makes sense.

 

Just had another thought. If the wire from the solenoid is fed from the output of the solenoid to the actual motor that will give you a safe, strong +12 for the pump.The solenoid switches the volts direct from the battery.

 

Thanks and G'Day