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Dual tank switch kills the engine

Discussion in 'Tech Discussion' started by shinkle, Apr 10, 2021.

  1. Dan Brown

    Dan Brown Member

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    I believe you said your truck DOES NOT have EFI, so the fuel pumps probably aren't in the tanks. I'd check the solenoid valve that switches the tanks to be sure it's shifting the flow to the passenger side tank. However, if that's the problem the truck probably wouldn't die "INSTANTLY" and should run for a bit off the fuel in the carburetor's float bowl.
     
  2. SquareRoot

    SquareRoot Full Access Member

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    Actually, he did in post #7. He said the truck is stock and in 87 TBI was stock. Why would you have electric in-tank pumps on a carb'd setup? Moving on.

    I would have swapped leads on the switch and see if the problem switched sides. That would rule out the switch and the only thing left is what's downstream from there.
     
  3. Yapay

    Yapay Junior Member

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    I pretty much have your same truck. 87 tbi 350 with saddle tanks. Replace the passenger fuel pump, relay under the hood and the switch over that’s under the truck. While you’re there might as well drop both tanks and replace the driver fuel pump too. Also check your fuses under the driver dash.
     
  4. Grit dog

    Grit dog Full Access Member

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    Based on OPs further description, I'm going with bad fuel pump as most likely. Then relay and/or wiring if pump is good.
     
  5. shinkle

    shinkle Junior Member

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    You guys are great. Once fixed I will give an update on what I found.
     
  6. shinkle

    shinkle Junior Member

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    Update. Let me start by saying I barely know how to use a multimeter, but I thought I would poke around the wiring on the dual tanks and see what I could find. I started with the bad passenger tank. I found a wiring harness that goes from one tank to another following the frame crossmember. On the passenger side of the harness there is a connector with 2 wires (appears to be pink and gray) . These wires go to the top of the tank. I assume this is wiring to the sending unit/pump. I unplugged the connector and checked continuity to the sending unit and the multimeter read 045. I then checked the wiring harness side of the connector and read nothing. So I plugged it back together

    I scooted over to the good driver side and found the same connector set up. I think the wiring harness splits here and goes to the driver side with a split to the passenger side. I unplugged it and found the gray wire is broken/missing from one end of the connector. Its the end going to the crossmember wiring harness to the passenger side. I think this is my problem Do you think if I cut open the wiring harness, find the gray wire, reconnect it to the connector plug that the passenger tank will get power and the sending unit and pump will work? I assume I should get a new connector. See pic.

    plug.jpg

    passneger continuity.jpg

    passenger continuity reading.jpg


    THE GRAY WIRE ON THE "A" SWITCH TO AUX TANK IS THE WIRE MISSING

    tank wiring.PNG
     
    Last edited: Apr 15, 2021
  7. shinkle

    shinkle Junior Member

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  8. 75gmck25

    75gmck25 Full Access Member

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    It’s very likely that you have found your problem, and repairing the gray wire is the fix.

    I think they sell those wire connectors in the parts store, and it would be easier to splice on a new one than it is to work with that short broken wire in the old one. I don’t know the name for that connector, but GM used them a lot.

    If you want a quick fix for troubleshooting, use two scotch-lock connectors and a short piece of wire to connect to the gray wires on either side of the connector. If the truck then runs, you definitely found the problem is the broken wire.

    Bruce
     

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