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Thread: How much voltage am I supposed to get at the fuel pump?

  1. #1
    2.0Si User Hazwan's Avatar
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    How much voltage am I supposed to get at the fuel pump?

    While installing my fuel pump kill switch today I've noticed the voltage I'm getting is way under 12v, not even 10v with the engine running. Can't remember how much as my DMM is dead and I'm stuck with this POS analog multimeter lol. Anyway, I'd like to know if this normal?

    Anyone could check and see how much are you getting?

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)



  2. #2
    2ndGenGuy
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    Re: How much voltage am I supposed to get at the fuel pump?

    I'll check mine for ya tonight. Bump this thread a little later on and remind me, or I'll forget.

  3. #3
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    Bump!

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

  4. #4
    2ndGenGuy
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    Re: How much voltage am I supposed to get at the fuel pump?

    Damnit I knew I'd forget. Sorry I was helping my friend put a new tranny in his new 84 Accord. He bought it with a bad tranny and fifth gear was toast. New tranny from a Prelude went right in there. Now its a good running 84 Accord for $500. Sorry thats off topic. I'm tired.

    I'll check my fuel pump before I get to work on my car in the morning. I assume that you checked it while it was running, just kinda stuck the probes up inside the rubber boots that are covering the wire connectors?

  5. #5
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    Quote Originally Posted by 2ndGenGuy View Post
    Damnit I knew I'd forget. Sorry I was helping my friend put a new tranny in his new 84 Accord. He bought it with a bad tranny and fifth gear was toast. New tranny from a Prelude went right in there. Now its a good running 84 Accord for $500. Sorry thats off topic. I'm tired.

    I'll check my fuel pump before I get to work on my car in the morning. I assume that you checked it while it was running, just kinda stuck the probes up inside the rubber boots that are covering the wire connectors?
    Thats alright, no rush

    Yeah I stuck the probes inside the rubber boots, touching the connectors, of course.

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

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    LX User tomatofiasco's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    buzzing in with a digital multi-meter reading of 14.1v at my fuel pump.

    may i suggest the new alternator sir? its electrifying...

    ...ly expensive

  7. #7
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    Thanks! My alernator is good, at least I thought so. I need to get the relay or whatever related checked. Its almost half the voltage that its supposed to get!

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

  8. #8
    2ndGenGuy
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    Re: How much voltage am I supposed to get at the fuel pump?

    Bah! I'm slack!

  9. #9
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    Quote Originally Posted by 2ndGenGuy View Post
    Bah! I'm slack!
    I've waited forever


    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

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    Re: How much voltage am I supposed to get at the fuel pump?

    Same voltage reading, (use DVM) at the pump as at the relay?
    You will read some voltage drop if you are measuring it when it is running. I would check and clean grounds concerning that circuit, and if you are getting drop at the relay it could be the relay not switching good inside (burnt contacts from being old)
    Carefull making those checks at the tank, things go boom ouch.

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    LX User tomatofiasco's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    you cant check it if its not running cause the fuel pump doesnt get power if the coil isnt firing... innit? otherwise it would just fill the float chamber and piss fuel everywhere [mainly the intake but still] when the engine wasnt using fuel, innit? i checked it right at the pump with the engine running anyway. 14v beats your 3 of clubs mister bond time to die

  12. #12
    2ndGenGuy
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    Re: How much voltage am I supposed to get at the fuel pump?

    What's odd is, I put this Holley fuel pump into my 84 Accord. Supposedly that fuel pump doesn't power up until the car is running either. But as soon as I turn the key on, the fuel pump starts running. I know it's on because it's the loudest fuel pump I've ever heard in my life...

  13. #13
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    I'm pretty sure the pump receive power everytime to switch is on the 'on' position?

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

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    2oodoor's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    liar's bluff, streight flush
    Let me claify what I said since it is up for disection. You probably won't get your full battery voltage reading on an appliance that is under load. Just like you won't read your full battery voltage at a starter motor when it is being used, there will be a natural voltage drop to the ground side as the appliance is using voltage and current.
    Just how low it should be on this particular appliance is the question.

    Ok I am just trying to help, I am not the 1 geez expert, but in general troubleshooting of this type problem my post was spot on. Some vehicles get battery voltage to the fuel pump anytime the key is first turned on as a prime. Then again, some use the relay to control the ground going to the pump instead of batt voltage. The circuit is isolated from ground and controlled for obvious reason. Then some are controlled by the oil pressure switch. My first thoughts in troubleshooting are with in tank pumps since that is what I deal with the most.

  15. #15
    2.0Si User Hazwan's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    It reads <10v without fuel pump connected. Even with voltage drop across the wire, I doubt it will drop more than 1v, even 0.5v is a lot. I can't test again since the car is at the shop getting the huge hole in the brakes thread welded up..

    1986 Honda Prelude build thread
    1981 Honda Accord (sold)

  16. #16
    2ndGenGuy
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    Re: How much voltage am I supposed to get at the fuel pump?

    Quote Originally Posted by roodoo2 View Post
    liar's bluff, streight flush
    Let me claify what I said since it is up for disection. You probably won't get your full battery voltage reading on an appliance that is under load. Just like you won't read your full battery voltage at a starter motor when it is being used, there will be a natural voltage drop to the ground side as the appliance is using voltage and current.
    Just how low it should be on this particular appliance is the question.

    Ok I am just trying to help, I am not the 1 geez expert, but in general troubleshooting of this type problem my post was spot on. Some vehicles get battery voltage to the fuel pump anytime the key is first turned on as a prime. Then again, some use the relay to control the ground going to the pump instead of batt voltage. The circuit is isolated from ground and controlled for obvious reason. Then some are controlled by the oil pressure switch. My first thoughts in troubleshooting are with in tank pumps since that is what I deal with the most.
    Oh no, I wasn't trying to debate you nor did I doubt you in any way. I just was under the impression for a long time that the fuel pump was somehow driven off of a relay which took signal from the distributor so that the pump would shut off when the car was running. Which doesn't seem to hold true in my experience.

  17. #17

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    Re: How much voltage am I supposed to get at the fuel pump?

    carb fuel pump wires have also been a mystery to me aswell. all i did when i converted from efi to carb temporarely was hook 12v to the pump and it ran as soon as the ign was on. worked fine, no flooding or problems. i note the stock accord pumps in the 3g at least pump for like half a second when u turn the ign on and then they turn off. does the 2g and 1g do that also? Must be some sort of controller somewhere. but if you suspect error just see what hooking 12v to it does

  18. #18
    3Geez Veteran lostforawhile's Avatar
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    Re: How much voltage am I supposed to get at the fuel pump?

    Quote Originally Posted by AccordB20A View Post
    carb fuel pump wires have also been a mystery to me aswell. all i did when i converted from efi to carb temporarely was hook 12v to the pump and it ran as soon as the ign was on. worked fine, no flooding or problems. i note the stock accord pumps in the 3g at least pump for like half a second when u turn the ign on and then they turn off. does the 2g and 1g do that also? Must be some sort of controller somewhere. but if you suspect error just see what hooking 12v to it does
    it's a safety relay, the idea is if the engine quits ignition pulses stop and the fuel pump turns off, they are all like this, from the early eighties up, when you first turn on the key the pump should run for a couple of seconds to prime the fuel line,often they get old and stop doing this, the general idea is if you get in a wreck the engine will stop and turn off the pump in case the fuel line has become compromised. if it wasn't there and you got in a wreck the pump would continue to pour gas everywhere on the engine if the line or carb was damaged. it's really simple how it works, it "listens" for ignition pulses from the negative side of the coil, if they stop, the pump shuts off. also if you are running a high current aftermarket pump, you need to use the factory circuit to operate a relay, and pull pump power from it's own circuit, or you will damage the relay.

  19. #19
    DX User
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    Re: How much voltage am I supposed to get at the fuel pump?

    Bumping an old thread, but did you ever find a fix or issue related to this? Did your pump work on 10v? What's the voltage with the key on?

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