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Thread: 1993 gmc no voltage to fuel pump

  1. #1
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    1993 gmc no voltage to fuel pump

    This problem started yesterday. I got home, shut off the truck, and it will not start now. No voltage to the fuel pmp relay, or fuses. I have an onoard fault code of 54. All wiring seems to be in good order. I don't think I have the fuel pessure shut off oil switch on this truck, if so it is not located above the oil filter (as shown in Hayne's). Any ideas on this one?
    Thanks
    Tom

  2. #2
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    Welcome to the forum Tom.
    Hope you'll stick around after you see hpow fun this one is.

    Your symptoms are pretty much the definition of a code 54.
    Easy things first, and I don't think you have a cutoff sw.
    Check the connection at the oil pressure switch,
    they have a habit of getting bumped loose.
    This is my bet. Some are simple sw, some are combo sw&press-sender,
    does not matter if not tight connection.

    Second, check the relay for function with your finger on it when the key is turned to on.
    Should be able to feel or hear click.

    Third, pull they relay and check for batt voltage to it from
    fusible link by checking it's hot at relay plug (think pin A, org?)
    This would cause no hot anywhere for fuel pump stuff.
    If good, may want to try swapping the brake lamp relay for it if you have one.
    Make sure relay installed again before moving on.

    Fourth and you want to be gentle here, check for voltage at the DLC (ALDL)
    Look for batt voltage at pin G, believe lower left of connector,
    while cranking the engine enough to get about 5psi or more oil presure.

    Relay is only supposed to work for the first few seconds of cranking
    and as oil pressure comes up, the fuel pump gets power through
    the oil switch. That's when the DLC will be hot also.

    Code is set when PCM sees low voltage on pump anytime, so does not tell
    if relay or oil press circuit, Big help since you know the gas isn't moving.
    BTW to clear the code you can wait through about 50 start-ups,
    or after fixing the problem, clear yourself by disconnecting battery for a while.

    Post back what you find. Good luck.

  3. #3
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    hey crab, in 93 did they tie the ecm temp switch to the fuel pump or was that 95
    4 wheels move the body
    2 wheels move the soul
    B.W.A.I
    Snow is a lot like religion and a lot like sex....you never know when you are going to get it!....or how much you will get

  4. #4
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    Far as I know the ECT was tied to the TPS.
    In 96 I believe they switched the design of fuel relay
    to just being under the control of ECM and sharing the ECM-B 20A fuse.
    Still would leave much of the checking the same, just no press sender.

    BWAI ?

    BTW (Big Thought Wanted) We don't know exactly what truck either...

  5. #5
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    Good morning Crabtruck,

    Thanks for the reply, I think I can use all the help I can get on this one! I diconnected / reconnected the oil pump pressure switch, connection seemed fine. There is voltage at the orange power input wire to the relay. I installed a new relay, same symptoms. Testing the DLC at pin G, no voltage while cranking.

    Thanks again for the assist, sure got me scratching my head!!!
    Last edited by Tom Riner; 02-15-2009 at 06:47 AM.

  6. #6
    z07
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    I think DLC in this sense is another name for the Assembly Line Diagnostic Link aka the ALDL located above your right knee.


  7. #7
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    to confirm it's a fuel problem, have you tried dumping fuel into the throttle body? it should at least sputter
    4 wheels move the body
    2 wheels move the soul
    B.W.A.I
    Snow is a lot like religion and a lot like sex....you never know when you are going to get it!....or how much you will get

  8. #8
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    Yes, and it did sputter.

  9. #9
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    Good, making progress. You have hot to the relay from battery.
    What is your truck exactly ? mod, eng, trans? Not an R or V I'm assuming.
    Circuit may be a little diff for those.

    Thanks for the pic z07. If I didn't say both DLC / ALDL I should have,
    never know which name folks are using.

    Tom you didn't say if the relay "clicked" when you swapped for another.
    That would tell you ECM is talking to it. Should have tried to fire up too.
    Not that it's ever happened to me (ahem) but are you sure
    you are looking at the pump relay ?

    When I say "Apply 12v to " below, I mean a battery hot with it's own 20A fuse.
    When I say "Jump" I mean use a fused wire also, otherwise I'd have said "short".
    Don't just jump an unfused hot wire to anything. Safe doesn't take that much longer.
    #3,4,and 5 are basically different attacks for same info.

    1) Check for hot at the oil switch plug, think org wire. Tan is to gauge if used.
    Gray is to relay and pump fuse. Re-connect. This could be done after stuff below.

    2) Remove relay, key off. apply 12v to the ALDL/DLC pin "G" and look
    for a second hot to show up on relay plug "C" (red). This is to confirm
    your truck actually has that "prime" or "test" pin connected on ALDL.
    Some trucks don't and yours could be broken too.

    3) Apply 12v to the ALDL/DLC pin "G" with relay installed, key off,
    and look for 12v at both sides of fuse. Pump run ? If fuse hot both sides
    and pump not running, it's the wire fuse-to-pump or pump. If fuse not hot
    either side, relay or wire relay-to-fuse is bad. If fuse hot one side only, well... fuse.

    4) Remove relay and jump pins "A"(org) and "E"(gray) on relay plug with key off.
    Org is always hot, will be tiny spark if pump working, should hear pump.
    Before jumping, confirm right pins for your truck by looking at the relay
    pins that mate them. Don't want the two that go to ECM. Fifth wire is
    to ALDL. This would say relay is problem.

    5) Apply 12v to the ALDL/DLC pin "G" with relay installed and see if pump runs.
    See if can be started that way. This bypasses the oil sw, relay and ECM control.
    Hot goes through fuse only. This would say relay, oil sw, or connections is problem.

    The common faults for this are the relay, loose plug on oil sw.

    Hope I'm making sense, I'm out of coffee. Good luck.

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  10. #10
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    Quote Originally Posted by leolkfrm View Post
    hey crab, in 93 did they tie the ecm temp switch to the fuel pump or was that 95
    Maybe you were thinking of the hot fuel module ? That was there in much earlier years for R/V....
    Attached Images Attached Images  

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