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Old 01-18-2012, 04:34 AM
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Oil pressure switch for elec. fuel pump?

I have bought an oil pressure switch to use on the electric fuel pump circuit as a safety switch.

But how do I make the fuel pump work when I turn on the ignition to start? At that moment there are no oil pressure, so I guess the safety switch will cut the power to the fuel pump?
I know this is used by others. How are you solving the problem?

Of course I could use a manual override but I would like to avoid that.

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Old 01-18-2012, 07:32 AM
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pump sw

i'd say ur gonna need a seperate bypass sw.. can't use the ign sw in the start position for after it starts and the oil sw allows power to pump it'll backfeed to the starter again.. not good... a push button momentary type...
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Old 01-18-2012, 09:12 AM
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Diagram

Here is a wiring diagram. This uses a Chevrolet Vega type oil pressure switch.
http://www.jegs.com/InstallationInst...510-12-810.pdf

Nolan
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Old 01-18-2012, 05:40 PM
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Quote:
Originally Posted by staleg
I have bought an oil pressure switch to use on the electric fuel pump circuit as a safety switch.

But how do I make the fuel pump work when I turn on the ignition to start? At that moment there are no oil pressure, so I guess the safety switch will cut the power to the fuel pump?
I know this is used by others. How are you solving the problem?

Of course I could use a manual override but I would like to avoid that.
Older GM starters use a second small terminal marked "I" for Ignition. This wire is hot only when the starter is being cranked. you can hook this to the pump feed side of the oil pressure switch so it will prime when cranking.
That terminal is electrically isolated from the starter energize post "S" terminal, and mechanically Isolated from the Big power lug on the starter, so no danger of backfeeding the pump or starter circuit with this arrangement.

Another option would be If you dont have the solenoid with the "I" terminal is to use the start circuit to the starter solenoid and splice into it with a bosch type relay ,to control the load , so as to isolate the circuit from each other.
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Old 01-18-2012, 05:58 PM
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The switch that u currently have is an NO switch,this means normally open,when u get oil pressure then it closes the circuit, and of course the pump comes on.
If you need for the pump to come on while you r cranking another option would be to also plumb a NC (normally closed) switch to the oil pressure,feed the input terminal of both switches with ign.on when u r cranking the NC switch will make ur pump come on n then open as soon as u get oil pressure ,as soon as this happens the NO switch will close n the pump will continue running.
Nason makes these switches in many different values,I get mine from Lauderdale Speedometer in Ft.Lauderdale.
I use them in almost the same manner for an oil pressure lite to come on at less than 35 psi at over 3000 rpm but not to come on when I let off the throttle n the engine drops in oil pressure.
I hope this helps,just another option.
Whatever u do make sure to fuse everything!
P.S. Some nason switches have both an NO and an NC terminal then the center terminal is marked C for common,I posted all this because it may be helpfull to others as well...

Last edited by topwrench; 01-18-2012 at 06:03 PM.
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Old 01-18-2012, 06:17 PM
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Quote:
Originally Posted by topwrench
If you need for the pump to come on while you r cranking another option would be to also plumb a NC (normally closed) switch to the oil pressure,feed the input terminal of both switches with ign.on when u r cranking the NC switch will make ur pump come on n then open as soon as u get oil pressure ,as soon as this happens the NO switch will close n the pump will continue running.
The NC contacts w/o pressure type switch would defeat the saftey feature he wants to put in the circuit: No fuel if the oil pressure drops, hence the NC with pressure type arrangement.
If the oil pressure drops and the fuel pump keeps running ,engine damage could result.
Also effectively having NC and NO switches would keep the circuit powered all the time, might as well just hook the pump to a key on B+ source and save the money for the switches. Not trying to be flip, just an observation.
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Old 01-18-2012, 06:20 PM
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Easy day….You could take a relay and wire it so that it is energized while the ing key is in the start position, allowing power to pass thru to the fuel pump, after the engine starts and the key returns to the run position the relay opens, disengaging the relay bypass, and allowing the fuel pump to receive it’s power from the oil sending unit circuit…….

Here is a dia of a 2 relay circuit with a one wire sending unit
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Last edited by EOD Guy; 01-18-2012 at 06:49 PM.
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Old 01-18-2012, 06:33 PM
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Quote:
Originally Posted by latech
Older GM starters use a second small terminal marked "I" for Ignition. This wire is hot only when the starter is being cranked. you can hook this to the pump feed side of the oil pressure switch so it will prime when cranking.
Another option would be If you dont have the solenoid with the "I" terminal is to use the start circuit to the starter solenoid and splice into it with a bosch type relay ,to control the load , so as to isolate the circuit from each other.
Post # 4 .........
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Old 01-18-2012, 06:52 PM
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What type pressure switch did you buy? A two wire sending unit would be easier to wire, three way along the same lines etc......
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Old 01-18-2012, 08:17 PM
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Quote:
Originally Posted by latech
The NC contacts w/o pressure type switch would defeat the saftey feature he wants to put in the circuit: No fuel if the oil pressure drops, hence the NC with pressure type arrangement.
If the oil pressure drops and the fuel pump keeps running ,engine damage could result.
Also effectively having NC and NO switches would keep the circuit powered all the time, might as well just hook the pump to a key on B+ source and save the money for the switches. Not trying to be flip, just an observation.
It wont do that if you wire it rite,im sure u no what a diode does right?
Kinda elementary,sometimes I take it for granted people know basic electronics and wiring, so sorry if you didnt understand it.
If he gets a dual switch(one with both NO n NC) terminals this cant happen
Do u still disagree,if so please enlighten me!
C'mon now youve nevr hokked a pump to B+ have you?
You were just joshing about that werent u?
I know what he wants to acomplish,was stated in very plain english!.
I know your not trying to be flip.
Are you familiar with the Nason product?
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Old 01-18-2012, 08:40 PM
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Revised dia to include a three wire sending unit, as stated in the link above
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Old 01-18-2012, 09:23 PM
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Quote:
Originally Posted by EOD Guy
Revised dia to include a three wire sending unit, as stated in the link above
Thanks for posting the schematic for a 3 way (C,NC,NO) I didnt have a pic of it.
This should clear it up for him.
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Old 01-19-2012, 04:27 PM
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Clarify

I don't know what happened to Staleg but I feel compelled to clarify the wiring of an electric fuel pump with an oil pressure safety cutoff.

1) The Holley diagram DOES NOT show the vehicles start circuit to the solenoid, the wiring diagram only shows what is needed to add an oil pressure safety cutoff switch to an electric fuel pump.

2) The normally closed contact(N.C.) is closed in the shelf position(no pressure). The normally open contact(N.O.) is open in the shelf position(no pressure) Depending on the switch used ~ 5-6 psi will cause the contacts to change state(NC opens, NO closes). Vehicle is started through the NC contact allowing the fuel pump to run while starting, then when oil pressure is built up the fuel pump continues to run through the NO contact. At any time the oil pressure drops below ~ 6 psi the NO contact goes back to shelf position which stops the fuel pump.

3) A fuel pump relay is not absolutely necessary if using a heavy duty oil pressure switch rated to run the fuel pump. You may have noticed that the Holley diagram does not call for one when using their switch and pump. However, it would not hurt anything to use a relay with the oil pressure switch as a pilot device. If using a fuel pump that requires high current, the relay would be mandatory.

4) While EOD Guy is real good at drawing diagrams normally, there is an error on the switch labeling on the three wire sending unit.

I hope this clears things up for anyone that was puzzled. Nolan
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Old 01-19-2012, 04:54 PM
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I see it, did it in a hurry sorry, revised dia attached

NC contact Should be connected to the "Hot in Start"
NO should be Switched/fused
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Old 01-22-2012, 08:30 AM
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Quote:
Originally Posted by OLNOLAN
I don't know what happened to Staleg but I feel compelled to clarify the wiring of an electric fuel pump with an oil pressure safety cutoff.

1) The Holley diagram DOES NOT show the vehicles start circuit to the solenoid, the wiring diagram only shows what is needed to add an oil pressure safety cutoff switch to an electric fuel pump.

2) The normally closed contact(N.C.) is closed in the shelf position(no pressure). The normally open contact(N.O.) is open in the shelf position(no pressure) Depending on the switch used ~ 5-6 psi will cause the contacts to change state(NC opens, NO closes). Vehicle is started through the NC contact allowing the fuel pump to run while starting, then when oil pressure is built up the fuel pump continues to run through the NO contact. At any time the oil pressure drops below ~ 6 psi the NO contact goes back to shelf position which stops the fuel pump.

3) A fuel pump relay is not absolutely necessary if using a heavy duty oil pressure switch rated to run the fuel pump. You may have noticed that the Holley diagram does not call for one when using their switch and pump. However, it would not hurt anything to use a relay with the oil pressure switch as a pilot device. If using a fuel pump that requires high current, the relay would be mandatory.

4) While EOD Guy is real good at drawing diagrams normally, there is an error on the switch labeling on the three wire sending unit.

I hope this clears things up for anyone that was puzzled. Nolan
You stated what I was trying to say a lot better,the NO (normally open) means its open with no pressure when it gets pressure it closes,I wire alarms on Detroit Diesels this way.
The short version is when the engine is runnig above 1200 RPM or so it has 50 to 60 psi of fuel pressure,so I nstall an NO switch at the secondary filter that closes at about 40 psi the outlet of this switch feeds aanother NO switch that closes at 35 psi making the alarm come on when the oil pressure drops at high engine speed,so that the engine doesnt self destruct before the original LOP alarm comes on at 5 psi or so,I use a truck back up horn for the alarm. I know a similar set up can be applied to run a fuel pump like he wants to with an NO,NC 3 way switch or 2 separate switches, I would still use a manual by-pass switch in case one of these switches fail,I install a secret cut off switch on all my electric fuel pumps to prevent theft of the vehicle.
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