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Author Topic: What is the purpose of a fuel pump relay for an electric fuel pump?  (Read 5808 times)
sdkar
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« on: May 09, 2007, 02:33:57 PM »

I installed my electric fuel pump and wired it directly.  It works fine.  Why do I need and what is the purpose of adding a fuel pump relay?

Steve
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zdld17
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dd1872
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« Reply #1 on: May 09, 2007, 06:55:26 PM »

In the olden days ( 1960)  I used to do it the  way you did , but now I know better,,, The relay will help carry load better as its connected thru a larger wire from the main power source,  I always put the relay  near my power source and run maybe 10ga wire to pump.   In my case , on my 69, I run electric carter at rear of car, above the rear leaf spring shackle,  at gas tank suction level.  Placed Relay  on cowl with large supply feeding thru relay and using a keyed on signal to close and open relay.   This way I dont have all the pump load running thru my fuse block.   
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Don Dabbs
rich69rs
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« Reply #2 on: May 09, 2007, 06:58:35 PM »

Basic purpose of any electric relay is to use lower amp circuit to activate the relay (liike closing a switch) to activate a high amp circuit.  This isolates the control circuit from the power supply circuit.

Take for example an electric radiator fan.  You can wire it directly through a manual switch in the dash of your fine 1st Gen Camaro, but if you do you will have a high amp circuit coming into the passenger compartment with you.  If you use a relay, when you flip the switch on your dash, a 12 volt, low amp circuit comes to life which puts power to the relay which closes a separate 12 volt high amp circuit to power the fan.  They are safety devices to isolate high amp circuits from areas where you don't want them to be.
« Last Edit: May 09, 2007, 07:00:27 PM by rich69rs » Logged

Richard Thomas
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qwertyme77
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« Reply #3 on: July 01, 2009, 10:38:39 PM »

the other reason to use a relay is so you don't burn out your ignition switch. the run circuit (which the pump would go through) was only designed for so many amps to go through the contacts. if you start adding fuel pumps, electric fans, amps and all that other stuff, it is possible to burn up the contacts. by using a relay which only draws a very small amperage on the control side you should be able to prevent damage to your switch. oh, and be sure to fuse the secondary (device ) side of the circuit.
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hotrod68
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« Reply #4 on: July 04, 2009, 10:47:45 PM »

Are you using a low-pressure cutoff switch, too, Steve? It works off engine oil pressure, and it shuts off the fuel pump if for some reason the engine dies but the ignition switch stays on--say in a wreck. Without a pressure switch, the fuel pump would continue to run and flood the engine and underhood with gas. That would be a nasty fire. Just a thought, and good luck!
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HotRod'68  1968 SS350 coupe undergoing frame-off resto/rod. 386/350/4.11s
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qwertyme77
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« Reply #5 on: July 06, 2009, 10:28:24 PM »

if you are looking for a factory style oil pressure cutoff switch, try the one that was used on the chevy vega. the vega used an electric fuel pump mounted in the tank. if my memory serves me correctly,( this was 30 years ago), when the oil pressure dropped too low, the switch would cut either the fuel pump or ignition circuit out so the engine would die, preventing damage to the aluminum engine block.  the part number was 3986857 from 1971 thru 1975. it was a three terminal switch using a standard pipe thread the same as any gm oil pressure switch.

paul.
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