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  1. #1
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    1986 Red Coupe

    Default 86 C4 No power to fuel pump.

    Acquired a 86 C4 that has not been run for a while. Car had a no start issue because of no power to the starter and no power to the fuel pump that the previous owner did not know how to fix. I have solved the starter issue by replacing the starter enable relay. I have replaced the fuel pump relay but still no power to the fuel pump. I have tested the fuel pump relay at terminal A and have verified there is power for about 2 seconds after ignition is switched on. I have also verified there is power getting to the oil pressure sending unit. However when testing the connector at the tank there is no power when key is switched on. I also found that the wires were bare and touching on the fuel pump relay connector. Could this have damaged the ECU and be the reason there is no power to the fuel pump? I have not checked power at the ECU where the red wire connects. I'm leaning towards the ECU since the fuel pump relay and oil sending unit are in parallel. Any info would be greatly appreciated.

  2. #2
    Member tmkassin's Avatar
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    Corvette(s)
    2009 LS3

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    When you turn on the key the relay kicks on long enough to build up proper fuel pressure. Then it cuts off till the motor starts. This prevents flooding. You need to check fuel pressure at the fuel rail. It should hold pressure even after you turn the key off. If it doesn't you have a problem somewhere in the system. If it fails to build pressure at all it is probably the pump. This can be an electrical problem or a faulty pump. If there is 12 volts at the pump momentarily when you turn the key then the pump is suspect. Hope this helps.

  3. #3
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    1986 Red Coupe

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    Thanks for the reply tmkassin. I think that is my problem. I am not seeing the 12 volts at the pump at all. I can see the 12 volts at the fuel pump relay but not at the pump itself. The pump never cycles when the ignition is turned on.

  4. #4
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    Corvette(s)
    89 Brilliant Red Metallic

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    The fuel pump relay will be enabled by the ECM when you turn the ignition On for
    about 2 seconds. It will also enable the relay if it sees distributor reference pulses
    which indicates the distributor is rotating either because the engine is cranking
    by the starter or the engine is running.

    If you have 12 volts on pin A of the relay that 12 volts goes to the FP fuse and
    then the 12 volts goes to the fuel pump.

    If you want to test the fuel pump manually, you can apply 12 volts direct to pin G on the
    diagnostic connector above the drivers right kinee. The 12 volts goes thru the
    normally closed contacts of the fuel pump relay to the fuel pump fuse and then
    to the fuel pump motor.

    The fuel pump should turn on and stay running.



  5. #5
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    1986 Red Coupe

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    Thanks ECSS! I will give that a try to see what happens. I have tried powering the pump directly at the connector going into the tank and it will come on and run. I guess if I try what you are suggesting then i most definitely have a wiring issue somewhere in between.

  6. #6
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    Quote Originally Posted by cutolds350 View Post
    replaced the fuel pump relay but still no power to the fuel pump.
    I have tested the fuel pump relay at terminal A and have verified there is power for about 2 seconds after ignition is switched on.
    when testing the connector at the tank there is no power when key is switched on.
    I have tried powering the pump directly at the connector going into the tank and it will come on and run.
    Checked the fuel pump fuse?

    Quote Originally Posted by cutolds350 View Post
    I'm leaning towards the ECU since the fuel pump relay and oil sending unit are in parallel.
    As you can see from the circuit diagram the OP switch is just that ,
    a mechanical switch completely independent of anything else, wired in parallel as a pump backup should the relay or it's control from the ECM fail.
    Both circuit however run through the fuse before going to pump
    Putting power to the pump through the ALDL connector also puts the power through the pump fuse



    ]
    Last edited by vetteoz; 09-22-13 at 10:02 PM.
    "Being able to "think outside the box" presupposes you were able to think in it." --Bob Lutz

  7. #7
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    I applied 12 volts using a power probe to G but no go on the fuel pump. So there must be an open somewhere on that circuit. So there must be an issue from the fuse box to the fuel pump if i understand correctly.

  8. #8
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    Wow thanks for the diagram Vetteoz. That backs up what Ecss said as well. Now i know for sure there is an issue between the fuse box and the fuel tank. I will let you know what i find.

  9. #9
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    Quote Originally Posted by cutolds350 View Post
    So there must be an issue from the fuse box to the fuel pump if i understand correctly.
    I don't see anywhere where you confirmed the pump fuse is good?

    Quote Originally Posted by cutolds350 View Post
    So there must be an open somewhere on that circuit
    Put your DVM on the pump wire at the tank connector
    ( chassis side ) and one side of the pump fuse holder and see if you get continuity.Try the other terminal.
    If no continuity from one of them you know where your problem lies
    "Being able to "think outside the box" presupposes you were able to think in it." --Bob Lutz

  10. #10
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    Vetteoz you were correct about the fuse. I finally checked the correct one which was labeled FR and it was indeed blown. Fuel pump is now cycling when ignition is turned on. Although the pump sounds like the click clack of a roller coaster right before the drop. I believe the naked wires on the fuel pump relay may have been constantly blowing the fuse. Hence the homemade wire that was connected to a toggle switch under the dash. Thanks for the illustrations i have a better understanding of how this thing is wired now. At least that part of the harness.

  11. #11
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    Quote Originally Posted by cutolds350 View Post
    I believe the naked wires on the fuel pump relay may have been constantly blowing the fuse. .
    Something like this ?
    The early relay connectors are well documented for losing the insulation and causing problems


    l]
    "Being able to "think outside the box" presupposes you were able to think in it." --Bob Lutz

  12. #12
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    Quote Originally Posted by cutolds350 View Post
    Thanks for the illustrations i have a better understanding of how this thing is wired now. At least that part of the harness.
    FWIW.

    Both the relay and the pump switch get their power from the same supply through a fusible link not shown in that circuit diagram
    ( the link is probably on the orange or red wires power wires coming off the starter Pos + terminal on your year )

    If you blow that link you will have no power to any part of the pump circuit,so if you have no power on the E terminal of the relay connector
    or the org wire at OP switch, you know here your problem lies
    "Being able to "think outside the box" presupposes you were able to think in it." --Bob Lutz

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