1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

What would cause the gas gauge to peg past full?

Discussion in 'Electrical & Audio' started by AuroraGirl, Sep 29, 2019.

  1. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    I recently replaced the gas tank and the sending unit. The old ground was completely shot(new one with the sending unit.) I cleaned up the surface of the old ground and installed the new one(it looked fine). However, my gas gauge still pegs past full. I even took the face off the cluster and pushed the peg to e and turned the key. It goes straight back to where it was. (its not full, its bout 1/3). Would this be a grounding issue or a hot wire issue?
     
  2. Dmack

    Dmack Full Access Member

    Posts:
    92
    Likes Received:
    92
    Joined:
    Mar 13, 2019
    Location:
    Central OR
    First Name:
    Dave
    Truck Year:
    1983
    Truck Model:
    K20
    Engine Size:
    350
    It usually means you have an open circuit. Could be a bad ground, broken wire, bad sender (I had a new one for my old 68 that was bad from the factory)...
     
    Kim Burke and C10MixMaster like this.
  3. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    Hmm. I will recheck my ground and see if I have power at my sender. Fortunately, the part of the bed that was rusted out and I cut out for safety and access gives me a straight shot to the post where the positive goes in without having to drop the tank.
     
  4. Swims350

    Swims350 Full Access Member

    Posts:
    4,008
    Likes Received:
    208
    Joined:
    Jul 30, 2010
    Location:
    Logan WV
    First Name:
    Chris
    Truck Year:
    1983
    Truck Model:
    K10
    Engine Size:
    none
    I've got an 84 c10 here that's done it several times it's the ground not getting connection on it. I know because I tried to tighten it up and then it pegged past full.
     
  5. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    So I need to clean the frame better and maybe the bolt? If I read what you wrote correctly. I would assume that if the gauge moves that means its getting power, just not grounding to get the right resistance?
     
  6. Dmack

    Dmack Full Access Member

    Posts:
    92
    Likes Received:
    92
    Joined:
    Mar 13, 2019
    Location:
    Central OR
    First Name:
    Dave
    Truck Year:
    1983
    Truck Model:
    K20
    Engine Size:
    350
    If you already hooked up the new sending unit ground wire, try a jumper wire from the tank\sending unit to a good clean ground. I have a 20 foot wire with alligator clips on it so that I can reach from the negative battery post to anywhere on my truck. That way I know for sure that I have a good ground for whatever I am testing.
     
    donnieray, ali_c20 and Blue Ox like this.
  7. Swims350

    Swims350 Full Access Member

    Posts:
    4,008
    Likes Received:
    208
    Joined:
    Jul 30, 2010
    Location:
    Logan WV
    First Name:
    Chris
    Truck Year:
    1983
    Truck Model:
    K10
    Engine Size:
    none
    ours doesn't move at all now it just stays pegged past full. I know for sure ours is the lil stud on top of the unit and not the frame side of the ground, we've always had troubles with it.


    On a side note I've got an 86 gmc here now and it's the opposite it stays on empty when I put over 6 gallons in it. I have to figure that one out.
     
  8. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    So I tried grounding the gauge to a good ground. Didn't work. Does anyone know how the wiring for the post on top works?
     
  9. Kim Burke

    Kim Burke Supporting Member Supporting Member

    Posts:
    356
    Likes Received:
    322
    Joined:
    Dec 9, 2017
    Location:
    Mesa, AZ
    First Name:
    Kim
    Truck Year:
    1979
    Truck Model:
    C1500
    Engine Size:
    454
    You may need to check the sending unit. Using a multimeter, jump across the + & - posts and move the float by either dumping in more fuel or removing unit from the tank. Being a new unit, the float e should reposition with more fuel.
    Good luck, I feel your pain. You will feel like a genius when you find and fix!
     
  10. Dmack

    Dmack Full Access Member

    Posts:
    92
    Likes Received:
    92
    Joined:
    Mar 13, 2019
    Location:
    Central OR
    First Name:
    Dave
    Truck Year:
    1983
    Truck Model:
    K20
    Engine Size:
    350
    Think of the single post on top as the positive and the wire coming off the housing is the negative.
     
  11. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    Yes. The ground is good, my guess is its the top. I need to follow it back through the cab and see if its broken or something.

    But how does the wire setup work? If the power goes to the sender on the wire, how does it return to the gauge? Since it grounds to the frame.
     
  12. chengny

    chengny Full Access Member

    Posts:
    3,989
    Likes Received:
    742
    Joined:
    Feb 22, 2012
    Location:
    NH
    First Name:
    Jerry
    Truck Year:
    1986
    Truck Model:
    K3500
    Engine Size:
    350/5.7

    Just as a general discussion on the gauge/sender/chassis relationship - and also to make sure I have the theory of operation straight in my mind;

    The components mentioned above - when connected by the pink (hot) and blue (ground) wires - form the "sensing leg" of the gas gauge located in the instrument panel.


    12 volts is supplied to the positive terminal of the sender (via the pink wire). It's source is the negative side of the gauge's measuring coil.

    The less resistance in this total combined circuit, the greater the electron flow through the coil wires - and consequently the stronger the force is that it (the measuring coil) can exert on the gauge's magnet. The magnet is attached to the lower end of the indicating needle. Blah, blah, blah…

    In addition to the sensing leg, there is another - opposing - circuit called the "control leg". It is identical to the sensing leg in every way except that the negative side of its coil is lead straight to ground (rather than through a variable resistance and then to ground like the sensing leg). Also, its coil is located on the opposite side of the magnet from the measuring coil's.

    So what you have is; two coils located on either side of an indicating needle - AKA the "ray". The ray is pivoted somewhere in the middle and has an integral magnet mounted at the bottom.

    These coils create their own magnetic fields - the strength of which is a function of the electron flow (i.e. current) passing through them. When these circuits are energized - by turning the ignition switch to the RUN position - the opposing magnetic fields begin to "fight it out". Each coil tries to pull the magnet - mounted on the needle - towards itself.

    Due to the fact that its negative leg is run straight to ground - resulting in nearly zero resistance, the current flow through the control coil is essentially constant (as is it's magnetic force).

    On the other hand, due to the variable resistance generated by the sender (as a result of changes in the fuel tank level), the current flow/magnetic strength of the measuring coil can be higher or lower than that of the control coil.

    The location of the two coils - in relation to the magnet - is such that:

    1. The control leg is always trying to peg the indicating ray to the FULL end.

    2. The sensing leg is always trying to peg the ray to the EMPTY end.

    So as extreme examples:

    1. If the pink wire to the sender is broken/disconnected the control leg exerts the greater magnetic force and pegs the needle Full.

    2. If the pink wire is allowed to contact ground (but only if that happens before passing through the sender), the sensing leg has a greater force and the ray pegs low.


    As GM explains it (better and way more succinctly than I can):

    VARIABLE VOLTAGE TYPE
    The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.

    When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil.

    When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.

    The diagram below uses the coolant temp gauge as an example. But be aware it is exactly wrong - that's because the resistance across the coolant sender are the inverse of the gas/oil pressure gauges. An open in the sensing leg will cause the gauge to peg low, while a short to round will peg it high.

    98524984_1181x853_2126x1535.jpg
     
  13. AuroraGirl

    AuroraGirl Full Access Member

    Posts:
    50
    Likes Received:
    12
    Joined:
    Sep 8, 2019
    Location:
    Northern Wisconsin
    First Name:
    Taylor
    Truck Year:
    1980
    Truck Model:
    K2500
    Engine Size:
    5.7
    Wow that was a good explanation. I found it better than the GM one. So that should tell me right quick its my power connection. Ill try grounding the power wire and see if it changes my fuel gauge. If nothing happens that probably tells me the wire is broke in the connection but not grounded anywhere.
     
    MrMarty51 likes this.
  14. Snoots

    Snoots Supporting Member Supporting Member

    Posts:
    2,940
    Likes Received:
    3,337
    Joined:
    Jan 23, 2013
    Location:
    Georgia
    First Name:
    Roger
    Truck Year:
    1973
    Truck Model:
    Jimmy Sierra
    Engine Size:
    350
    Copy of FuelGaugeCircuit-01.jpg
     

Share This Page