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Thread: 1987 chevy tbi - wiring switch to dual pumps

  1. #1
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    Question 1987 chevy tbi - wiring switch to dual pumps

    how do the fuel pumps recieve power.i have the wiring diagram which shows a tan and gray wires going to each fuel pump.Does the fuel pump power come from these wires? these wiries come form the dash switch.i know the gas gauge is controled by the selector valve but what about the fuel pumps?
    Last edited by crabtruck; 04-16-2011 at 10:23 PM. Reason: title

  2. #2
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    Welcome to the forum duners.
    You have the first year of the TBI's huh?
    Which model/eng/trans ?

    The 87 GMC Sierra 3500 that I took apart to trace the actual wires
    had tan from switch to selector and left pump.
    Had gry from switch to selector and right pump.
    Red/Wht from left and Red/Blk from right to selector were senders.
    Senders from selector to gauge was Red to plug-splice then Pink.
    Tanks each had ground for pump and sender of course.
    Switch has one of four connections as ground.
    So the trick here is that the hot is moved from pump to pump
    as the switch is telling selctor which valve to open.
    The pump that winds up with two grounds does nothing.
    This causes no problems with shorting, just nothing happens.

    This leads back to the nature of your question: how do they get power?
    The last wire on the switch gets power from first one place, then another.
    Follow that wire and after it goes through firewall you will see a plug-splice.
    Keep following and you will see it connect at firewall block. (Tan/Wht)

    At that point on firewall it splits two ways. Main one is right back at you,
    and it jumps into the large loom heading down. Eventually it is the second wire
    of the plug-splice that the senders were coming up on.
    From that plug it heads back to engine to make 3 important connections.
    One is through the fuel pump fuse to fuel pump relay and throught to hot,
    as the first way pumps are powered - at the command of ECM(computer).
    Second is to the oil pressure switch on engine as the second way pumps
    are powered for all but the first 10-20 seconds after starting.
    Third connection is a info wire to the ECM, so it knows pumps have hot.

    Above it was noted that the wire from switch to power split 2 ways at firewall.
    The unmentioned split goes thru firewall main connector there
    and on dash side is connected to a module that has a red wire to gauge fuse
    and a Blk/Wht ground wire. Has one IC chip, 3 transisters, 4 diodes,
    8 resistors, and 3 capacitors. You don't care, long as it works.

    There is one more way to get power to pumps, but it is for shop not driving.
    When the fuel pump relay is not "turned on", but it is installed,
    ALDL terminal socket "G" under the dash can be used to connect
    a fused hot wire which will directly power pumps.
    Used for trouble shooting certain situations.
    Normally when truck is running it's already "hot"

    Soo ...........why do you ask ?

    And where is your diagram from ?

  3. #3
    Senior Member gmctrucks's Avatar
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    Welcome to the forum and howdy from TEXAS. Crabs the man.
    GMC

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    THANKS CRABTRUCK. SO YOUR SAYING THAT THE POWER TO THE FUEL PUMP,IE(GRAY AND TAN WIRES) IS THE fUEL PUMP POWER? ONE OF THESE WIRIES SHOULD HAVE POWER TO THEM WHEN I SWITCH THE DASH SWITCH,AND SHOULD HAVE CONSTENT POWER TO ONE PUMP OR THE OTHER.i HAVE ONE TAN/WHITE WIRE,ONE GRAY,ONE TAN AND ONE I THINK IS PINK COMING FROM THE FIREWALL TO THE FUEL SELECTOR VALVE.MY PROBLEM IS THE TAN/WHITE WIRE IS THE FUEL PUMP FEED,BUT WHERE DOES IT GO TOO OR HOW DO I SPLICE IT INTO THE FUEL PUMPS/SELECTOR VALVE?

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    crabtruck,

    i have all the chevy manuals for the truck.I just can`t figure out the wiring "flow".it shows the tan /white wire coming from the dash switch to the firewall.But i have the tan/white wire also near the left tank/fuelpump that i`m not sure were this goes.i know its a fuel pump power because i can hook it up directly on the left side fuel pump and the truck will run. Someone hacked it up before i got the truck and this is going to be my daily driver so i need to have the fuel gauges and dual tanks working if i can.

  6. #6
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    If you have the factory chev manuals please post you wire diagram.

    Just for clarity I say: selector is getting hot and ground swapped by switch,
    pumps are not - switch just makes hot available or not (by providing gnd not hot)

    I'm including several links that may be good reading.
    Also my personal drawing I made while stuck on side of road many years ago.
    One other note for future readers: some setups used the "momentary" switch
    instead of the "click" type switch for switching tanks, believe that was mech pumps.
    1978: http://www.truckforum.org/forums/attachments/chevy-truck-forum/700d1186402775-wiring-harness-83-dual-fuel-wiring.jpg
    1983: http://www.truckforum.org/forums/che...g-harness.html
    1986: 1986 Chevy Truck Fuel Gauge problem.
    1989: Fuel pump Wiring color codes on 89 GMC 1500

    See if this picture is more clear than my chatter.
    .
    Attached Images Attached Images   

  7. #7
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    crabtruck,

    One problem was the dash switch,the wrong one was in the truck.thanks for your input.I got the fuel pumps working right now i`ll try and get the dash gauge to work. thanks for your help.your second diagram was a big help.the first one is for a truck without fi i think.the fuel injection wiring is a little different color wiring.

  8. #8
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    Glad one of the drawings worked for you, was the free hand one that one ?
    Not sure if others see it the way I do when I make certain "shortcuts" in pix.
    Many of the wires are standardized colors regardless of engine or year,
    as long as the same design is being carried over.
    However, if you find differences that are not repairs, I'd like to hear about them.

  9. #9
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    this is a great post helped me a lot with my post about dual fuel tanks, thanks
    2005 Chevy 3500 6x6 ( it is a dually)
    crew cab, long bed, Duramax with the Allison. almost enough power to pull this old man
    off the p!ss pot.

  10. #10
    Senior Member gmctrucks's Avatar
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    Like I wrote Crab is the man he has helps a lot of us so much on this forum were honored/lucky to have him here.
    GMC

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