What's the trick to forcing the fuel pump to run?

Discussion in 'Engine/Drivetrain' started by Perkolator, Mar 16, 2023.

  1. Perkolator

    Perkolator Toyotaholic

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    I recall there's a trick to forcing the electric fuel pump to run on these tucks, but I'm blanking on how to do it. Anyone remember? I'd look but don't have time to search for it on my phone

    Tried starting up my truck the other day after like 4 months (I know, I know). Well the darn fuel pump decided it doesn't want to go anymore, whereas I used to hear it before starting the truck. It wanted to start with starter fluid so I guess my only issue is fuel delivery.

    Thanks
     
  2. fred heath

    fred heath Addict

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    Unhook the two wire plug that runs from the harness to the oil pressure sender. This fools the pump into thinking it’s grounded. The pump should now run with the ignition switch on the “on” position.

    Edit: I’m talking factory in tank pump.
     
  3. Perkolator

    Perkolator Toyotaholic

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    Sweet, thanks. OPS is somewhere near the oil filter, correct? I'll try it tomorrow morning and see if I can find it, not like there's much on the wiring harness in these old trucks, lol. Yes, factory in-tank pump
     
  4. fred heath

    fred heath Addict

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    Yep. It’s a simple two wire plug. Mine rests about 1/2 way down the passenger inner fender. Should be able to trace it directly to the OPS.
     
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  5. MrDinkleman

    MrDinkleman Addict

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    Hmmmm....

    I have a 1979 Celica that has an electric fuel pump. The OPS grounds a single wire. On my Celica, and I believe any carbureted car with factory electric fuel pumps, the pump will not run unless the engine is running (OPS switch) or the ignition is in the "START" position (OPS bypassed).

    I guess an "easy" way to test the pump is to pull the fuel line off the carb and try starting the engine. Fuel should flow from the hose.
     
  6. fred heath

    fred heath Addict

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    Mr. D is right. Modular plug, single wire, yellow with black or dark blue stripe.
    Can’t depend on memory any longer.
     
  7. Perkolator

    Perkolator Toyotaholic

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    Well "hmmmm" is the right word for my situation

    I'd guess the PO did some DIY wiring changes at some point, because even though it's not the right way, the fuel pump definitely came on with ignition in the ON position, before starting the engine - I would hear it come on behind me in the fuel tank area and then a second later hear liquid/fuel in passenger side firewall where there's a fuel pressure regulator and gauge before the Weber carb. Never really thought much about it being wrong until now that I've got issues.

    Couldn't locate any OPS off the block anywhere :shrug: Only thing I could find related to oil pressure is an oil pressure gauge line that the PO installed, which comes off the block left of the oil filter and feeds into the cab/gauge pod. Guessing this is where the OPS used to be located. Need to get a wiring schematic and open up the electrical tape on the wiring of passenger fender to see what's going on.

    Didn't investigate further because my truck's literally sitting in about 0.5-1" of standing water because of how much it's rained here recently (I live at the bottom of a big hill and it just comes out of the ground)....and then the battery started to die, so I need to charge it back up again. Also discovered the vacuum line between Weber and distributor was disconnected and mud wasps filled up the port hole on the carb - yaaay!

    Guess I've just been lucky because up until this point I haven't had to do any repairs to this truck, it's always just started up and ran for the last 5yrs :verdict_in:
     
  8. MrDinkleman

    MrDinkleman Addict

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    Oh, jeez.

    I have to apologize. I had it backwards, assuming the wiring for 1979 Celica is the same as OP's truck's. The oil pressure switch is normally closed (grounded without pressure). So simply unplugging the switch wire should allow the fuel pump to run with ignition in "ON" position as OP remembers. In fact, according to my 1979 Celica FSM, that is how you test the pump; unplug the switch. Sorry.

    The switch is located next to the filter. So it does sound like the PO swapped out the OPSwitch with an OPSender.

    Sorry for the confusion.
     
    Last edited: Mar 21, 2023
  9. fred heath

    fred heath Addict

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    Yes, you’re correct. When using an external oil pressure gage a “T” fitting is usually installed into the threaded ops hole on the block. The ops sender is then piggybacked to this fitting. The other smaller nipple goes to the gage line itself.
    Many times there’s a rubber sleeve covering the electrical sender itself.

    If the “oil” light in the dash cluster comes on when you first turn the key the sender is working. It sounds like the po either deleted the sender completely (very dangerous) or simply left the sender wire detached. The wire for the sender is yellow with a black stripe. Everything is as I described it before. Only with 1 wire rather than 2. It should have a spade type terminal on the end.
     
    Last edited: Mar 21, 2023
  10. fred heath

    fred heath Addict

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    Had to clean some crud off but this is what your ops should look like. Very similar in appearance to a water temperature sender.

    Other picture is where the plug is located on my inside fender. You can see where the wire exits the harness.
     

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    Last edited: Mar 22, 2023
  11. Perkolator

    Perkolator Toyotaholic

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  12. fred heath

    fred heath Addict

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    Left a comment under your #2 picture.
     
  13. jetas

    jetas Grand Toyotaholic

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    That looks like an actual hose coming off a barb fitting. Is there actually a wire there?
     
  14. Perkolator

    Perkolator Toyotaholic

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    Yep, just the barb fitting, no wiring nearby. When the ground finally dries a little and I don't sink in the grass I'll cut open the harness and see if I can find that yellow/black wire and what they've decided to connect it to. What I should do is what you did and rewire the whole truck
     
  15. fred heath

    fred heath Addict

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    Is the sender still there?
     
  16. fred heath

    fred heath Addict

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    Did a little more exploring on the wiring harness.

    The oil pressure sender wire comes out of the same wire loom as the water temperature sender (yellow w/green stripe). There are only maybe 3 or 4 wires total in the loom.

    If you trace the wire back from the water temperature sender, that’s the same loom where you’ll find the oil sender wire.
     
  17. Perkolator

    Perkolator Toyotaholic

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    Haha, looks like the POs wiring mods strike again! Water temp sender (yellow) isn't hooked up to anything and there's no nearby wiring not hooked up to anything. Above at the thermostat looks like a water sender fitting (blue) for the aftermarket gauge pod inside the cab.
    IMG_6194.jpg


    Got a better pic of the intake side of the block where I'd imagine the OPS would have been located - nothing there except the barb for the aftermarket oil pressure gauge
    IMG_6191.jpg
     
  18. fred heath

    fred heath Addict

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    That’s a really dangerous setup. You have no idiot light to tell you when you loose oil pressure.

    Gages are ok as long as you keep a constant eye on them. I have Auto Meter liquid filled gages, but depend on idiot lights to warn me if I’m not paying close attention.
    Check the large wiring harness coming out through the passenger side firewall. Mine has some factory yellow tape around it about 8” out. There should be a small branch feeder exiting near the bottom of the main harness. Open that loom and see if the yellow/black and yellow/green are inside it. Those are the wires you will need. See enlarged area on photo.
     

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    Last edited: Mar 23, 2023
  19. jetas

    jetas Grand Toyotaholic

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    As long as your gauge is accurate you dont even need the light.
     
  20. Perkolator

    Perkolator Toyotaholic

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    Well I think I did find the yellow/black wire underneath the electrical tape, as well as what looks like the right shape connector for that water temp sender too - both wrapped up under electrical tape. Also found an unknown wire disconnected, perhaps something smog related?
    IMG_6201.jpg

    Anyway, once I gave it a shot of starting fluid the truck fired right up for a second and when it ran dry I heard the fuel pump come back on - so I got it running again, yay.

    Guess I need to go find a new OPS and splitter for installing it next to the oil gauge line. Also maybe gear up for doing some replacement wiring on the truck to make it better. Old wiring harness looming was just falling apart under the tape - some sort of fabric looming

    btw - anyone know what that relay (?) and round thing is next to my charcoal canister/fuel filter? The round thing isn’t connected to anything
     
    Last edited: Mar 24, 2023

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