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Thread: Your friend the circuit opening relay (or how to wire fuel pumps)

  1. #1
    Chookhouse Chooning Automotive Encyclopaedia Hen's Avatar
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    Default Your friend the circuit opening relay (or how to wire fuel pumps)

    A circuit opening relay is a useful and often overlooked part of many cars.

    What is it?
    A circuit opening relay (COR) may as well be called a fuel pump relay, as that is what they control. They simply switch power to the fuel pump and are fitted to most cars with an electric fuel pump (recently cars have begun to use fuel pump ECUs for the job)

    Why do you need one?
    Because you need to know when to run your fuel pump. But most importantly you need to know when to stop it. You could run your pump from ignition power (runs whenever the key is in the ON position) and things would work, however there are two main problems with this:
    1. The pump will continue running if you crash. If you break a fuel line then fuel will spray everywhere and not be fun for anyone involved.
    2. If you leave your car not running, but with ignition on then you'll flatten your battery.


    How to wire one.
    A COR is simply a relay with one switching contact and two coils. The switching contact controls 12V to the fuel pump. The two coils are there to close the contact when either the car is starting or the car is running.

    The starting coil is simply triggered when the key is in the STA position.

    The running coil is triggered differently on AFM and MAP engines. For AFM there is a contact in the air flow meter which closes as soon as the flap opens at all (meaning the engine is sucking air, so must be running). For MAP there is a pin (usually FC) on the ECU which is earthed whenever the engine is running.

    The wiring to a factory COR should look something like the diagram below:


    However if you don't have a COR then you can make one easily from two normal relays. This is shown below:


    If you aren't sure which pins on the AFM to connect to then they are easy to find with a multimeter. Just find two which are an open circuit when the flap is closed and then go to zero ohm as soon as the flap is opened. They also tend to be the two pins at one end of the connector.

    Also if you are running a carby fed car with electric fuel pump then Timbosaurus has come up with a very good solution in this thread, take note here and here

    Hopefully that helps.

    Hen
    Last edited by Hen; 31-01-2007 at 10:52 PM.
    I need a working 4AGE bottom end. Pref smallport GZE, but all others considered. Also complete motors.
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  2. #2
    Junior Member Automotive Encyclopaedia infotechplus's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Excellent explanation for us noobs to auto electrics.

    +rep to you for this very timely writeup Hen.

    Do you have one on EFI Relays?

    Cheers,
    OMG - Winner of the Official "Forum Comment of the Week"!

  3. #3
    Forum Sponsor Carport Converter TurboRA28's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Good write up !
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  4. #4
    Chookhouse Chooning Automotive Encyclopaedia Hen's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Infotech - EFI relays are even simpler. They are just a normal relay that supplies 12V to the EFI system when ignition is on.

    Also note in the above diagrams I haven't included any fuses. But the fuel pump supply needs to be fused before the COR.

    Hen
    I need a working 4AGE bottom end. Pref smallport GZE, but all others considered. Also complete motors.
    Drift Volvo. Was fun. 2JZ next time.

  5. #5
    MR 18RG Chief Engine Builder The Witzl's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Also worth noting is the explanation of why the COR connects to the starter motor signal....

    ... when cranking the engine, it's possible for your AFM flap or FC output from the ECU to "flutter" on and off (more prevelant to AFM engines), and thus not supply constant power to the fuel pump. By having the secondary coil in the COR that is activated by starter signal, it ensures that full power is provided to the fuel pump during cranking.

    In aftermarket systems, they dont connect the fuel pump relay to starter signal in this way - it's all done through the ECU.
    ...... butt scratcher?!


  6. #6
    Yep they look great Carport Converter gianttomato's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Nice write up for the novices.

  7. #7
    Junior Member Domestic Engineer
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    tachometric relays are even better
    they only allow fuel pump to run only when there is an rpm from the coils
    can be used on anything with a spark
    you can even get ones where u wrap the rpm wire around the coil lead and thats enough
    of a signal to turn the pump on

    COR need a signal from the ecu to turn on
    if you turn them on all the time then they are not different to a normal relay

    they can usually be used only on toyotas

    so at end of day they are not very useful

    i buy packs of 20 tachometric relays and fit them to alot of cars cause itsd illegal to wire up fuel pump to run all the time

  8. #8
    ethanol sniffing Backyard Mechanic adamaw11's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    are these tachometric relays available from the local stupercheap?
    and are they subject to the possible 'flutter' when cranking as witzl mentioned?
    http://www.toymods.org.au/forums/showthread.php?t=83259
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  9. #9
    Junior Member Domestic Engineer RobertoX's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    sideshow, how expensive are these relays? I was told by an auto sparky that they were around the $150 mark so I just got a normal COR from a wrecker.
    They would be useful for a carb with electric pump though

  10. #10
    Junior Member Domestic Engineer
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    they range from 35 to 45 bucks new
    i have them in stock always or just goto an lpg gas parts supplier

    dont know what fluutering is but ive used them on many 1jzs and no probs

    when u turn ign on they send a pos signal trigger for a normal fuel pump relay
    and run pump for 2 seconds

    then they wont trigger the fuel pump relay untill there is an rpm signal

    then they stop pump if rpm if engine stops

    there is 2 types

    one is more sensative and sometimes picks up very small signals from coil if ign is jusst on anmd enigne not running

    the dearer one doesnt do this

    but the cheaper one i use most for 1js 2js 1uz etc


    with a std COR is has 2 triggers
    the fuel pump frig from the ecu (fc) is just a hold trigger
    it holds the relay on
    the start signal actually turns the COR on then the "fc" from the ecu holds it on

    some people have used COR as a normal relay but sometimes dont work properly
    i think this is cause they have not wired up the main start signal to the COR thus not givin it the initial on trigger

    not sure if this is right but its the only thing i can think of people are doin wrong when using COR as normal relays

    the tachometric relays can not power fuel pump alone

    they need to trigger a relay which then turn power onto the pump

  11. #11
    Junior Member Carport Converter TA-022's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    so could the above replace this

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  12. #12
    Junior Member Domestic Engineer
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    thats the cor and the 2 speed relay resitor setup
    i always fuk off the 2 speed part of it cause up to 4000 rpm the pump only sees 8 to 10 volts
    and on a turo car if you start making more power then u need 12vs all the time

  13. #13
    Junior Member Backyard Mechanic
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    I have this linked on another thread but is usefull here.



    This is for a MAP engine but for AFM the (FC) connection off the circuit relay goes the the switched ground on the AFM instead of ecu

    anyone with any ideas on putting a timer circuit in here so that the Fuel pump will run when the key is turned to "RUN" just for a few seconds in order to pressure up the lines?

  14. #14
    Rest in Peace Conversion King ViPeR_NiPPleX's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    It should be noted if not already that the coil for the STA signal is of a lower resistance which means it will turn on with a lower voltage. Which i'm imagining is incase of voltage drops while cranking, this ensures the fuel pump keeps going.

    But otherwise great article

  15. #15
    My Missus is a Domestic Engineer BradW's Avatar
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    Default Re: Your friend the circuit opening relay (or how to wire fuel pumps)

    Injected VK commodores and (I think) injected XF falcons used a tachometric relay for the fuel pump. Cheap from Pick n payless.
    Brad

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