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Sender unit test.

Discussion in 'The Garage' started by southernspeed, Aug 16, 2006.

  1. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    I've got 2 wires going to my fuel gauge sender unit (plus the earth).
    One is green, the other purple.
    I know what ohms I should be seeing but from which wire (I'm trying to see why my gauge is reading nearly full when nearly empty!)
    Do I measure between one of the wires and ground or between the 2 wires?
    I'm getting different readings from all 3 ways!
    Thanks guys. (oh, and girls of course!):D
     
  2. theperfectgarage

    theperfectgarage 1/2 ton status

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    Hey Mark, I think I have the spec. and procedure for that at home. If no one has your answer by then I'll post it up. In the meantime... you know what to do.:D
     
  3. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Yeah and I've got the hangover from it today!!:D
    Post up my man.:D
     
  4. spearchucker

    spearchucker 1/2 ton status

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    One wire is for the sender unit, the other wire is power to the fuel pump. I forget which color goes to which, but it's pretty easy to figure out. On the plug that's you disconnected, if you probe it on the frame side while the key is in the accessory position, one of them should measure +12V (keep the ground of your DVM connected to the frame). Obviously that's the one that goes to the pump, just match that wire with the one that goes to the tank. Now if you probe the sender prong it should read somewhere between 0 and 90 ohms depending on how much fuel is in the tank. If you're able to siphon the gas out of the tank you could watch your DVM to make sure it slowly changes and doesn't jump all over the place, which would mean that there's tarnish on the rheostat of the sender. When you measure between the 2 wires your actually measuring the resistance of the sending unit and the windings of the motor for the fuel pump, which of course is not what you want. Sorry for the run-on paragraph but my firefox is screwed up ... need to reinstall it.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Pump only gets 12V when the pump is commanded to run, which is only for a short period after key is turned to run and not started.

    Of course, you could hook the voltmeter up and just hit the key while watching the meter.
     
  6. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Yep, that's what I figured. Knowing my luck it's going to be the sender seeing as I've only just had it out to replace the pump!:rolleyes:
     
  7. theperfectgarage

    theperfectgarage 1/2 ton status

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    I'll add that if your gauge is going all the way to the "full " peg and staying there, It's generally a faulty earth. ( see, I can speak the Queens english. )
     
  8. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Spoken like a true Brit! No, if I fill up it goes WAY past where it used to for 'full' but even when nearly empty, it only goes to about 7/8 tank.
     

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