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Thread: Electric Fuel Pump Wiring

  1. #1
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    Electric Fuel Pump Wiring

    When wiring an electric fuel pump I've seen how some wire a bypass from the safety oil pressure switch to the starter wire so it pumps as it cranks. And I've seen many put a push button switch to prime it before starting it if it's been sitting a while. What seems more common and makes more sense to you?
    Tony

    1955 Bel Air Sport Coupe

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    A little more detail would help. Will the electric pump be the only pump in the system or will it be backing up an engine mounted pump? Beyond that, I think it depends on HOW you want to wire it up. Just operating off the normal ignition key or a separate switch for it? If the ignition switch, then yes picking up 12v from the Start connection on the solenoid would allow the pump to run whenever you are cranking, then shutting off when you release the switch. And until the engine lites and provides oil pressure to the safety switch, the pump would not run. If you are running it via separate switch then there's really no need for the momentary contact switch you mentioned. Simply flip it on, prime and turn it off if that's what you want to do with it. When the engine lites, you could then turn the pump back on. And a couple more tips. You didn't say which electric pump you were using but most electric pumps hate to be dead-headed. They will work and for a long time, but it puts more stress on them than having a return type system in place. So don't put an electric pump in dead-headed and expect it to last forever. The other thing is using a relay to power the pump. Whether it is operating off of the ignition switch or a toggle switch, a relay will help reduce the amperage load pulled across the switch contacts, this in turn will lengthen the life of the switch itself plus the pump will get full voltage and amperage allowing it to operate more efficiently.
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    The oil pressure switch will close as soon as you start cranking it anyway, and turn on the pump relay. Assuming your mechanical pump is ok with the right stroke from the pump pushrod, and no other issues, how will the electric pump lower the boiling point of your fuel I'm wondering? The chrome carter style mechanical pump I used from summit said it didn't need a regulator, and it was putting out 9.5 psi, so I added a regulator to dial it back to 6 psi and it holds it steady at all RPM, no matter how hot the engine or ambient temp is. Plus it is pulling it through a filter ahead of the pump using 3/8 line from the tank.
    Last edited by 55 Rescue Dog; 03-09-2018 at 08:39 AM.

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    The electric pump will not change the boiling point of the fuel. It will vastly lower the temperature of the fuel as compared to the hot engine mounted pump. It will be mounted back at the tank where it is cool. I'll put a regulator somewhere BEFORE the hot engine bay and once in the engine bay, a line as short as possible to the carb so the fuel doesn't have time to get so hot. If I need to use a bypass I will use it, I already have it run. Although some pumps run easier when dead headed. Don't even ask about that one.
    Tony

    1955 Bel Air Sport Coupe

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    Maybe at idle fuel could get hot enough to cause a problem, but at full power it is moving too fast to pick up much heat. An electric pump will get pretty hot too, which is why every car made now has them inconveniently mounted in the tank.

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    In my experience, wiring the electric fuel pump thru an oil pressure switch is done to shut off the pump in case of an accident where the ignition switch is still on but the engine is not running. An electric pump isn’t usually wired to the acc ignition switch terminal, but the on terminal, thus the pump is powered during cranking anyway.

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    Quote Originally Posted by TrifiveRichard View Post
    In my experience, wiring the electric fuel pump thru an oil pressure switch is done to shut off the pump in case of an accident where the ignition switch is still on but the engine is not running. An electric pump isn’t usually wired to the acc ignition switch terminal, but the on terminal, thus the pump is powered during cranking anyway.
    In addition to that, some cars have a crash switch to shut down the whole electric system in a crash. I think Ron Francis Wiring sells a setup for that, and that's not a bad idea either.

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    Quote Originally Posted by TrifiveRichard View Post
    In my experience, wiring the electric fuel pump thru an oil pressure switch is done to shut off the pump in case of an accident where the ignition switch is still on but the engine is not running. An electric pump isn’t usually wired to the acc ignition switch terminal, but the on terminal, thus the pump is powered during cranking anyway.
    That is exactly what I was talking about, maybe I worded it poorly? And I think TriR is saying the same thing. The accident kill switch sounds like a good idea, who knows if the engine would continue to run and have fuel pressure and keep pumping. The reason I asked about a primer switch/button is because if my car sits a week or two I can pump the throttle twice and it starts but then stalls right away. Then usually the next pump and crank it will keep running.
    Tony

    1955 Bel Air Sport Coupe

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    Quote Originally Posted by 55 Tony View Post
    That is exactly what I was talking about, maybe I worded it poorly? And I think TriR is saying the same thing. The accident kill switch sounds like a good idea, who knows if the engine would continue to run and have fuel pressure and keep pumping. The reason I asked about a primer switch/button is because if my car sits a week or two I can pump the throttle twice and it starts but then stalls right away. Then usually the next pump and crank it will keep running.
    Anytime I don't drive any car daily for days, or months at a time, I just plan on a couple of false starts, which is the least of my worries. It just mean I need to drive it more. I just fired up a 1950 Chevy truck with a 235ci six in it that hasn't run for several years, and it started in 3 seconds!
    Last edited by 55 Rescue Dog; 03-09-2018 at 04:40 PM.

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    I'm not sure what's meant by "don't run an electric pump deadheaded". If the pump is making the desired pressure when the engine is running, it has the same load on it that it would if "deadheaded". Doesn't matter what the engine's fuel demand is. You need a pressure regulator and it will always regulate the pressure to what it's set at. Pumps also have a relief valve built in to protect against true deadheading. As an example, the trusty old Holley "blue" pump is used with a pressure regulator (actually it's a pressure reducing valve with no bypass). But the blue pump has a pressure relief valve built in it.

    As for the safety switch, there's another setup you can consider. It's called a "rollover switch", if the car rolls it cuts power to the fuel pump. Some, maybe all, Fox body Mustangs had them. I'm sure many others too, but that's the one I know of.

    If it was my car, I'd wire the fuel pump with a relay. The relay would be activated with the ignition switch in cranking and run positions. Using a relay minimizes the electrical load on the switch, and lets you run a 12-14 gauge wire for the pump from the battery to the relay and from the relay to the pump.

    I would not run a "hybrid" system with both an electrical and a mechanical pump. Choose one, either a mechanical pump or an electric pump. Do the electric pump installation correctly, with gravity feed. Don't use the stock pickup for an electric pump, as electric pumps don't make much suction. An in tank pump would be the best way to do this for a street driven car, because most of the other solutions have plumbing hanging below the tank which is not good for the street, as it could get damaged (safety again). Don't use the drain fitting in a stock tank as it's only 1/8" pipe, not big enough for a BBC, or a healthy SBC.

    On the other hand, there is nothing wrong with a properly operating mechanical pump which uses the stock type pickup. But Tony won't admit there's something wrong with his because he can't/won't find the problem and fix it.

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