gas gauge or sending unit? which is at fault?

Discussion in '40- & 55-Series Tech' started by wantatlc, Jul 30, 2005.

  1. wantatlc

    wantatlc

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    HELP: gas gauge or sending unit? which is at fault? SEE IMAGES

    so, I hooked up my gas gauge that was not working by running power from the back of the volt gauge power stud and jumped it over to the gas gauge power stud on the backof the gas gauge to get the gauge power. then, I grounded the ground stud on the back of the gas gauge to the frame. now, gas gauge has ground and power where it was disconnected before.

    So then, I now have power to the sending unit at the gas tank under the passenger seat and there is a good ground there as well.

    BUT, the issue is that when I have the power attached at the sending unit and it is grounded, I can hear the sending unit crackling in the gas tank! are you suppose to be able to hear that sending unit? seems very odd. it is very noticable crackling from the unit. I got nervous and disconnected the power at the sending unit to be safe.

    SO, now the gas guage registers beyond Full - no mater what! before it was not even hooked up, so it naturally registered Empty. But now, sending unit attached or not attached, the gauge reads Full. AND the sending unit crackles.

    so, can you tell which is bad from this?

    if the sending unit is bad (since it crackles with electricity) would replacing it make that gas gauge stop registering Full and give a correct reading? is the sending unit the reason why it is registering Full no matter what? or is the gauge broke or would it always read Full until it got a correct signal from the sending unit?
     
    Last edited: Aug 1, 2005
  2. nailbender

    nailbender

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    Sounds like you toasted the gauge but I am offten wrong. I would pull the sender from the tank, cover the hole in tank and ground the sender to the body with a wire and see what happends. A dead short in the sender might fry the gauge.
     
  3. cruzerman350

    cruzerman350

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    tyhe guage should not have a hot lead going to it. the fuel sysdtem runs offa negative lead to prevent sparks. i dont know how to explain it except you need one wire grounded and one from the tab on teh sending unit that goes to the back of the guage.
     
  4. badkarma

    badkarma Is is edible? SILVER Star

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    Crackling IN the gas tank?!?!?! YIKES!
     
  5. wantatlc

    wantatlc

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    cruzerman350

    1ST, there is a wire that comes up from under the rig near the tank that splits off and one end goes to a 'tab' (to supply power) and the other end is grounded under one of the 6 screws.

    2ND, on the back of the gas gauge in the dash board there are 2 studs (as there are for every gauge in the dash from volts to oil pressure, etc.). what is suppose to be hooked to those 2 studs? If you are looking at the gas gauge head on to read the amount of gas, what is attached to the right stud and what is attached to the left stud?????

    you see, before I did anything, the wire that comes up from under the rig near the tank that had one end on the tab (to supply power) and the other end grounded under one of the 6 screws. BUT, the gauge read Empty all the time. when I checked for power to the power tab at the sending unit there was none, but the ground was/is correctly grounded. ALSO, on the back of the gas gauge, there was NOTHING attached to those 2 studs. so, how could I expect the gas gauge to get a reading from a sending unit that has no power and to a gas gauge that has nothing attached to the studs (which I assumed were power and ground based on a friends advice). So, when I hooked up a power and ground to the gauge, the gauge shot to Full, the sending unit got power, and the sedning unit started to crackle. So, movement was made, but in what direction I do not know as the gauge moved, and the sending unit got power???

    So, if the sending unit is grounded, how does it get power and what do I hook to the back of those 2 studs on the gas gauge?

    BTW, the power and ground wires that come out of the sending unit work their way up to the dash and into what looks like a many wire PLUG with all the wires coming to it for various gauges. that plugs into the dash, BUT nothing connects onto the actual gas gauge studs.

    can anyone help simplfy this for me and tell me what the correct connectoins are?
     
  6. wantatlc

    wantatlc

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    yeah I know :doh: advice I got was jump power from another gauge to the gas gauge, ground the gas gauge, make sure the sending unit is gorunded and getting power. well, upon doing that, the gas gauge jumped to full, the sending unit got power, but made that scary-ass crackling noise!!!!!!!!!!

    seemed logical at the time? :eek:
     
  7. cruzerman350

    cruzerman350

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    one post is supposed to be grounded and the other is the "tab" wire. the only way i know of to find out which is which is to hook it up, see if it works, and switch if it doesnt.

    also if you have any problems take out the sending unit (unless it is new) and lube it and make sure it moves freely or it wont work at all. i was going to tell you to check and make sure your gge isnt stuck but it sounds like you figured that one out.

    that is all i know about the gas guage. i had it explained to me once but it was really confusing. a service manual might help also if you can get your hands on one. i think most auto parts stores have them too look at.

    hope this helps
     
  8. cardinal fang

    cardinal fang

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    The guage runs off resistance from the sending unit in the tank. As the float raises or lowers in the gas tank, the resistance changes. The stock sending unit in the tank has a round wire, like a spring, with a little metal tab that travels across the spring as the gas level changes. The guage in your dash measures the resistance and moves the needle as the reistance changes by the metal tap moving across the spring wire. So basically the guage is a ohm meter. The wires on the tank are on the tank so the proper resistance is possible. Keep those wires. Obviously the wire that attacheds by the tab to the tank, has to get to the gas guage in your dash so the guage can read/show the resistance.
     
  9. wantatlc

    wantatlc

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    I understand 100% how it works. very simple.

    I do not understand how to make it work. things do not make sense to be honest.

    As I said, the wires for the power tab and ground that come from the sending unit (under the rig and up to behind the dash) end at a giant plug in the back of the dash that has like 10 wires coming to it. If that is all that is hooked up, the sedning unit has no power and the gauge acts dead on Empty.

    Then, the gas gauge itself has 2 posts on the back. when you start adding power or ground to the posts, irrespective of the above wires I mention, power goes to the sending unit and the gauge shows Full. this seems to mean progress.

    how does it all work togther.
     
  10. wantatlc

    wantatlc

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    you know - I could be totally wrong in this the more I think of it.

    I assumed (based on what another told me) that I had to jump power and ground to the gas gauges posts in the back from the master power and ground studs on the far right side of the entire dash cluster. in reality, maybe those are just the posts to hold the gauge into place and the big plug where all the wires come and connect into is where the ground and power connect as that is where the power and ground from the gas tank sending unit connect into.

    if that is the case, the sending unit and gauge are dead in the water as neither have power or ground.
     
  11. cardinal fang

    cardinal fang

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    What year is your cruiser? I think your right. The big plug supplies the power the guage.
     
  12. wantatlc

    wantatlc

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    1974

    if you are correct, then what? where would the issue be?
     
  13. pbgbottle

    pbgbottle Forum Lifer

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    looking at your pic it looks like the circuit board might be fried under the fuel gauge bolts on the dash . power does come from the big plug in the pic and it is the blue with red wire and that post sends power to the oil sender and the fuel sender .does your oil gauge work yellow with black wire looks cut off . if it does you might have to make a jumper wire from the hot oil sender post to the fuel sender post .and from the other fuel gauge post in the big plug should be a yellow with red wire for the fuel gauge which should go to the sender on the tank center post . ground the sender using one of the screws and guage should work .

    if your multy wire plug is still functioning . .take those wires off the studs they do just hold the gauge in but also are the contact points to make the gauge work .but you do not need to conect anything to them . power goes in the gauge( blue with red wire ).comes out other side (yellow with red wire) goes to center post on sender . and sender is grounded with a screw. guage was reading full because you grounded directly to truck not through sender . power in power out reads full gauge .may have killed the dash gauge . if needle stuck full maybe just need to wack it .might just be stuck . i did this to my water temp guage .grounded it shot to full hot and stayed there .it was just stuck wacked the guage and it came unstuck .
    anyways i hope this little bit of info may help
    if not using multi plug .than you can connect to the studs but power in gauge on one stud .power out gauge other stud to sender center .ground sender done
     
  14. cruzerman350

    cruzerman350

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    ok im confused now. i know i was wrong but now i want to know why. so explain it simply.

    a power wire goes to ________and from there it goes to ___________

    like that so i know what is going on. sorry for the bad info on my part
     
  15. pbgbottle

    pbgbottle Forum Lifer

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    power wire goes to gauge . out of guage . to sender which is then grounded .
    fuel gauge (Small).JPG
     
    Last edited: Aug 3, 2005
  16. cruzerman350

    cruzerman350

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    thank you much simpler.
     
  17. wantatlc

    wantatlc

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    okay, several of the wires on the plug have been cut. this is becuase there are aftermarket Oil temp gauge and Engine temp gauge installed- so I suspect previous owner cut them. The Volt meter is still connected and the Gas gauge does not work. speedo and odometer still work.

    Anyway, coming into the plug from the gas tank sending unit are a yellow wire with red from the center power post and a white with black wire coming from the ground of the sending unit. so both wires (ground and power) come from the sending unit and end at the plug.

    I believe the gas gauge is NOT stuck as I unhooked everything and it now dropped back to Empty. so I should be good there.

    so I am back to square one as follows:
    -power wire comes to dash plug from sending unit
    -ground comes to dash plug from sending unit
    -gas gauge has 3 bolts/studs on the back that have nothing attached to them

    QUESTION: if I want to bypass the plug to just eliminate it from the issue and bypass it, what should I do to go direct to the back of the gas gauge and it's availble 3 bolts/studs? it seems you say this is possible to bypass the plug and use the bolts/studs? should something (those power and ground wires) be attached to any or all of these 3 bolts/studs? if so, what goes to what?

    thanks for the help.
     
    Last edited: Aug 3, 2005
  18. wantatlc

    wantatlc

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    I see the schematic, but it is not totally clear for the simple fact that it does not take into consideration the "plug" nor the 3 possible bolts/studs on the back ofthe gas gauge.

    so, in my case, are you saying that the yellow with red wire (power) that COMES from the sending unit should connect direct to the gas gauge? if so, which of the 3 bolts/studs as there are 3 possible to attach to? if this is true, I would need to cut the yellow/red wire off the plug and attach direct to "a" bolt on the back of the gas gauge. which bolt?

    would this also mean that the white/black ground wire that comes out of the sending unit and terminates at the plug should also be cut free and attached to "a" bolt on the back of the gas gauge? if so, which bolt?

    would this then also mean that a power wire needs to be jumped from some working gauge to the remaining bolt/stud to get power to the gas gauge itself? if so, which bolt?

    this would effectively bypass the plug, use all 3 bolts/studs, and attach the sending unit direct to the gas guage bolts/studs and the gas gauge should be drawing it's power direct from being attached to the dash unit itself's master power.

    see my attached schematic of the situation. I am not sure where the wires should go to the gas gauge assuming I bypass the plug. currently, the wires go to the plug. but if I cut them free of the plug and go direct to the back of the gas gauge, what wire goes to what bolt and what about the left over bolt?
     
    Last edited: Aug 3, 2005
  19. pbgbottle

    pbgbottle Forum Lifer

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    i'll try to help out this seemed to work try it out .white with blue positive to gauge ,black one out of guage .to center conector on fuel sender ,then yellow wire attached to any one of the screws .back to ground . guage should work

    the wires i used were just scrap pieces i had laying around . i cycled the fuel sender up and down and the gauge seemed to work

    make sure the connectors under the nuts aren't contacting any other part of the circuit board . it may cause wacky readings
    IMG_0058 (Custom).JPG
     
  20. wantatlc

    wantatlc

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    your the man. no better and easier diagram to follow than that!

    In my case, I will then do:

    - jump power from some place (maybe another gauge) to the bolt on the right (if looking at the gauge head on) - or try to first to rely on the main power that is already attached to the unit
    - cut the yellow/red wire at the dash "plug" that runs from center power tab on sending unit to the plug and now attach it to the left bolt bypassing the plug
    - cut the white/black wire at the dash "plug" that runs from the under the ground screw on the sending unit to the plug and now attach it to the bolt on the left bypassing the plug

    I will try this setup on my rig and see what happens. I will report back. thanks a ton!
     
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