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Discussion Starter #1
Guys,
A follow-up to my earlier question of mine about electric fuel pump.

Is a fuel pump relay wiring harness needed if I have my Battey in the trunk (failed to mention that in my previous post).
After reading about a relay, it seems the concern is about voltage drop??

Thanks
jack
 

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The relay is only needed and used to provide a large circuit to run a fuel pump. I have a 250 GPH pump that requires a 20 amp dedicated fuse and the easiest way to provide that kind of circuit is with a 30 amp relay and fuse.

If you are using a fuel pump that does not need that kind of dedicated fuse and circuit then a relay would not be needed. The problem is most of these old cars do not have circuits available to support a large draw circuit such as fuel pumps or electric fans.

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Redrum (or Mike)

68 Corvette - 383 CI, 427 HP, 700R4, 12.56 @ 108 MPH
69 SS Chevelle - 502 CI, 610 HP, 2004R, 1320 unknown
97 Z-28 - totally stock still under 14,000 miles

How Many Roads Must A Man Travel Down Before He Admits He is Lost?
 

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Discussion Starter #3
Redrum,

I'm getting the Mallory 110 Series.

It has a free flow rating of 110 GPH.
Current draw: 5.0A at 7 PSI.
Seems pretty mild to what your running.

But your statement about most of these old cars do not have circuits available to support a large draw circuit such as fuel pumps or electric fans.Now that concerns me.

So now, Im trying to figure out for 30 bucks or so, would this relay prevent/assist the old wiring with this new pump or just be a waste in my case?

Thanks for your quick response.
jack
 

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I think a relay is a good idea. I have mine set up with a relay that is turned on by an oil pressure switch, with a momentary switch wired to the circuit. That allows me to prime my carb when needed. If oil pressure drops, fuel cuts off. The engine will still run for a minute from the gas in the bowls, but if your in an accident, and the fuel line rips, it won't keep running spraying gas all over. Maybe this was already discussed, I didn't see the other post. If so, disregard.


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Steve B.

Life is a dumpster, dive in and enjoy.

69 Malibu ZZ4-350
88 Camaro Sport Coupe
93 Camaro Z/28
01 Silverado LT Z/71
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Do you have a low use circuit? A good one would be the cigarette lighter circuit if you never use a cigarette lighter. Or add a piggyback circuit with it's own 10 amp fuse and you will be fine with that pump. One good thought about a 30 amp relay is the one from Painless wiring will also give you two additional modern fuse circuits. Two 20 amp ignition activated and one 10 amp constant hot. Good to have in these old cars!

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Redrum (or Mike)

68 Corvette - 383 CI, 427 HP, 700R4, 12.56 @ 108 MPH
69 SS Chevelle - 502 CI, 610 HP, 2004R, 1320 unknown
97 Z-28 - totally stock still under 14,000 miles

How Many Roads Must A Man Travel Down Before He Admits He is Lost?
 

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Jack, The basic purpose of any relay of the most common form is to isolate the heavy current flow to the device needing to be powered from the control circuirt which manipulates, or determines when the heavy current contacts open and close. Imagine a direct power source from the battery to the fuel pump. With the appropriate gauge wire for the current draw and the length of the circuit, very little if any voltage drop will take place between the device (fuel pump in your case) and the source of current or battery.
Typically you would want to use a #10 gauge wire if you need to conduct a maximum of 6 to 8 amps the length of the car. For higher currents of 10 to 12 amps a #8 gauge wire is recommended. As current draw increases, so should the conductor diameter. Lower gauge numbers mean larger conductors. Mount the relay where the control circuit wires are convenient to the switches or devices that will control the operation of the relay. It isn't necessary to mount the relay near the device. The control side of the relay only requires 3/10 to 4/10 of an amp to engage the relay heavy current contacts. This allows a very wide range of switches and control devices without running high current through sensitive, small current rated switches or oil pressure switches.

Sinceley,

Louie Hammel
 

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10 gauge wire is not required for a 5A current load. That load doesn't require anything more than 18 guage so go with 16 to be safe. The factory usually uses 16 guage wire to run the EFI pump. I wouldn't go to 10 guage wire until I was powering something requiring over 20A, and only if the wire ran the length of the car.

The recommendation for a oil pressure switch is a good one. I would wire it to run either with the key in the Start position or with oil pressure. That way, you put the car in gear to override the starter (or clutch out) and turn the key to start to turn on the pump. Once the carb is full of fuel, start it. Of course, this is only required when the car's been sitting for a long time and the carb has dried out.

Put the relay by the battery. That would also help eliminate any extra wire runs if you're still worried about it.

Peter
 

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Discussion Starter #8
Thanks guys for all the info.

Redrum, I don't use the lighter.
I'll check out the Painless wiring kit.
Steve B,
An oil pressure switch, with a momentary switch wired to the circuit. Anything for safety is me.

Louie Hammel,
Boy, that was alot of info, like an electric class. Thanks for your time and effort of making it look clearer for me.

Peter F.,
16 guage wire - OK
oil pressure switch and wire it to run either with the key in the Start position or with oil pressure.

With my battery in the trunk and the relay in the rear it will be a short run to the relay.

If anyone has a reference site to a specific setup picture or list of what I should buy that would be great.

I've never wired before and I don't want to start with the gas controlling unit so
I'll print out this page to show to my mechanic.
He's good but mostly works on the newer cars.

Thanks again,
Jack
 

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Here's how I did my setup. I could leave my car sit for weeks and not have to prime it. I now have a mech. pump because the electric pump was too noisey. I'm planning on just running the electric pump at WOT using a microswitch on the carb.


If someone can clean this up a little, I'd appreciate it.

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Steve B.

Life is a dumpster, dive in and enjoy.

69 Malibu ZZ4-350
88 Camaro Sport Coupe
93 Camaro Z/28
01 Silverado LT Z/71
Chevelle pics

[This message has been edited by ZZ69chevelle (edited 05-05-2002).]
 

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Discussion Starter #10
WOW!!
Excellent work. Thanks so much for all your effort.
I'm printing this now.
Thanks again,
jack
 

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How I'd run it (assuming you have a factory stock idiot light for "oil" in the dash AND a carburetor):



This circuit allows for the factory original "oil idiot light" sender to be utilized. By simply splicing into that wire (typically dark blue off the oil sender) you have a protection circuit (kills fuel pump if the power dies in the car OR the car flips over or wrecks resulting in loss of oil pressure).

IF the car sits for long periods of time you may find it beneficial to install the "priming" switch where shown. This should be a NORMALL CLOSED momentary switch that allows overide of the "sending relay" for easier starts.

The anti-theft switch should be regular on/off automotive toggle switch. It needs to be ON for the car to run. Place it in a hidden location and flip it off to prevent the bad guys from stealing your ride.

It works like this:

The bottom relay is the "sender relay", the 87A contact is normally closed to the IGN circuit (this and all references indicate the ignition terminal (ign) in the fuse box--only hot when the car is in the start, run , on position). The minute you turn the key, this relay is activated, the armature pulls away (breaking the connection between "30" and "87a" because the you have power across the relay coil (from "86" having +12 volts and "85" being grounded as there is NO oil pressure instantenously resulting in the idiot light sender being grounded).

This means "86" on the middle relay does NOT see 12 volts. The minute the engine is cranking, oil pressure builds and the idiot light oil sender "un-grounds". Thus "85" on the bottom relay doesn't have a ground connection and the bottom relay is "off". "30" and "87A" now have continuity, current flows from the IGN circuit thru "30" on the bottom relay thru "87a" on the bottom relay to feed the middle relay's "86". Now that the 86 sees 12 volts on the middle relay, there is a complete current path thru it's coil. The relay is NOW activated. Current will pass thru "30" to "87" in the middle relay from the IGN circuit--IF the anti theft switch is in the closed position.

The top and final relay is the main control relay. If the above conditions are met, the number '86" terminal on the top relay will see 12 volts. The top relay will be activated and battery +12 volts power will be supplied to the fuel pump from the "30" to "87" terminal. Use a 10 amp fuse for the "fuse" shown. I'd use at least 16 gauge wire on the "thick" red wires shown. The little wires, use at least 18 guage (i'd recommend 16 guage as it's a long run from the rear of the car to the interior compartment). The top relay should be close to the trunk battery and the middle/bottom relay in the interior of the car for simplicity.

The "priming" switch should be "normally closed". If the car sits a long time, the fuel bowl in the carburetor will be empty. By pressing the priming switch, you'll interrupt the connection between the the "85" terminal and the oil sender, thus current will makes its way to the main relay even tho oil pressure has not built up, allowing less cranking time so the carburetor bowl fills up.



[This message has been edited by Coppertop (edited 05-04-2002).]
 

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Discussion Starter #12
Coppertop,
This is just amazing...

Between you guys explaining and showing pictures, it takes all the quess work out.

And it addresses the problem I'm having with the hard start (fuel bowl)and the kill switch idea.

These drawings should be put into the reference section / archive section on this site.
These will be very helpful to all who want to solve similar problems, or would like to avoid them.
Easy to follow instructions on the way to wire things up.
Thanks guys again for all your help.
Jack
 

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Dean,

"Quality" relays will be five terminals WITH a specific terminal marked "87A". DO NOT assume just because you have a 5 terminal relay that it contains an "87A" contact. Cheapo brands out there have (5) terminals, but there are actually (2) 87 terminals.

Yes,

87 = N.0. (normally open)
87A = N.C. (normally closed)

The above are in relationship to 30.

85 and 86 have the electromagnetic coil between them to operate the contact arm.
 

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I drew up the diagram for my pump from memory yesterday. I need to look at the relay again. It's been a while since I worked with anything this small


EDIT I redrew the circuit I posted earlier. I think I got it right now
Waddya think Coppertop?

------------------
Steve B.

Life is a dumpster, dive in and enjoy.

69 Malibu ZZ4-350
88 Camaro Sport Coupe
93 Camaro Z/28
01 Silverado LT Z/71
Chevelle pics

[This message has been edited by ZZ69chevelle (edited 05-05-2002).]
 

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Lookin' better Steve


Oh, wasn't try to say your way wouldn't of worked, just went about a bit different so the factory "dummy" light oil sender could be quickly used.
 

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Thanks. I had the relay bass akwards
. I actually have it tied to another relay in my car for the MSD as well. I still need to add a microswitch on the throttle to only have the electric pump come on at full throttle. Right now I just have the fuse pulled for the pump and am running the mech. pump.
 

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Coppertop, Interesting circuit but the middle relay isn't really needed. You can put the "burgler" switch into the ground on the relay powering the pump. Or even in the thin red wire going to terminal 86 on the pump relay. Personally, I stick with the simpler is better philosophy.

I like Steve's circuit. Instead of the switch though I would just connect the purple start wire from the key to that point (terminal 85). That eliminates the need to mount a switch in the vehicle. Supposedly, there are switches available that have 2 complete sets of contacts, or a set of NO contacts and a terminal it grounds so the oil light can be left alone while wiring up the relay.

Peter
 

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I have been wondering about the oil cutof for quite sometime...What if you get hit hard enough to rupture a line, but not stall the motor, will you still spew fuel?
With that train of thought, I've been toying with the idea of adding an impact switch (a'la Ferd Mustang, Crown Vic, etc) in addition to the oil switch...Thoughts, good or bad?
 
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