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  • HELP!! Fuel gauge reads backwards???

    Background info : Everything worked fine until the guy who was helping me install the roll cage in my XJ "accidentally" cut a hole in the top of my gas tank when I wasn't there. So, in my absence he dropped the tank and put another one in from his XJ that he had laying around using my original pump and sending unit assembly. Now the gas gauge reads empty with a full tank and the gas light is on. After driving the XJ a few times I noticed that the more I drove it the higher the gas guage went as if it was working backwards! Eventually the gas light went out and the gauge was showing 1/4 tank until I filled it up again and the gauge immediately went to empty and the gas light came back on. I checked the plug for the wiring going into the tank (unplugging the harness makes the gauge go to full), checked the ground (appeared to be OK and I also ran a separate ground wire from the tank to the frame with no change just to make sure). I do have an extra fuel pump and sending unit that I could try if I felt like dropping the now full tank (this is not a daily driver and is not driven much in the Winter unless it snows a lot and I hate to siphon gas), but what would cause the gauge to work backwards?

    BTW: The Jeep runs perfect, I just can't stand not having a working fuel gauge when I'm out on the trail! Anyone have any ideas??

  • #2
    Sounds like the main head (Large Plastic Nut) holding the components for your gas tank was not sealed properly. A new gasket is called for "usually" and it is supposed to be torqued on. Also check the ground connection!!. He may have pulled it off. Where you fill the gas tank up; take off the outside screws, than remove the inner screws which will allow you to see if the ground is connected. All - all - ; I think it is not properly pressurizing. Good Luck.
    PS: Another thought on the "ground connection" is about 24 inches from the tank where it connects to the frame. See if it is "clipped" back to the frame?
    Last edited by Olt Man; 12-29-2005, 01:59 PM.

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    • #3
      Originally posted by Olt Man
      Sounds like the main head (Large Plastic Nut) holding the components for your gas tank was not sealed properly. A new gasket is called for "usually" and it is supposed to be torqued on. Also check the ground connection!!. He may have pulled it off. Where you fill the gas tank up; take off the outside screws, than remove the inner screws which will allow you to see if the ground is connected. All - all - ; I think it is not properly pressurizing. Good Luck.
      PS: Another thought on the "ground connection" is about 24 inches from the tank where it connects to the frame. See if it is "clipped" back to the frame?
      First off, thanks for the reply!

      I'm not sure that my tank is as you describe it should be. The main head (if I'm right on what that is) is a metal ring that the pump and sending unit are attached too and it sits in the hole in the front part of the tank and is held in place by turning the locking ring until it locks into place by small tabs (no bolts to torque down anywhere that I can see). Secondly, the ground wire that is attached directly to the main head runs into the wiring harness along with the power for the pump and the sending unit wire where it has a plug with 3 or 4 connectors on one end. The other end of the harness is going through a hole in the body and then goes somewhere under the carpet to who knows where. The ground doesn't run to the frame back at the tank that I can see. Is there supposed to be another ground for the tank that I don't know about? Wouldn't me attaching a new ground wire directly to the main head on the tank to the frame make the gauge work if it was a ground problem? Let me know if I'm all wet about something as I am starting to get confused

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      • #4
        Had to wait till I got home to check my FSM and some other doc's I had.

        Anyway, since you described the needle movement of pegged with ground off and zero with ground on (as I understood it), and the needle moves towards full as gas is consumed, it sounds like your connections are OK.

        Are your REALLY sure that you used the origional sender? Because in 1991 DC changed the polarity of the sending unit and if you used the sender from a later model (say the one from the new tank), what you are experiencing is EXACTLY normal.
        Simply placing the origional sender in a different tank shouldn't make any difference, and that's the head scratcher.
        Last edited by rstarch345; 12-29-2005, 06:32 PM.

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        • #5
          is the float in upside down? haha, wierd...

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          • #6
            inside the tank, on the pump/sender assy there are wires that hook up to a small chip like unit that varys resistance based on float position, id bet he screwed those wires up and put them on backwards. That o-ring seal on the tank that seals the pump will be fine to re-use, ive seen some 20 years old that look just like new, just dont bust the locking tabs on the lock ring when you take it off.

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            • #7
              i thought ther wer 2 wires for the pump and 1 for the sending unit. if u switch those wires around, no pump, no go, easy answer to the problem. no backwards reading gauge.

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              • #8
                It sounds like the wrong sending unit. I know that is always a problem when YJ guys swap in CJ guages in their dash. They are totally backwards, if you use the YJ sending unit with a CJ guage, it's exactly how you describe it.

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                • #9
                  Either the unit is not your original unit or it's been tinkered with. The wires on the unit itself should not have been touched, it should have been removed and installed as a whole assembly.

                  Originally posted by joshs1ofakindxj
                  i thought ther wer 2 wires for the pump and 1 for the sending unit. if u switch those wires around, no pump, no go, easy answer to the problem. no backwards reading gauge.
                  The fuel pump and sending unit share a hot wire and a ground. There is one wire going to the fuel gauge.

                  Originally posted by Brian78z28
                  inside the tank, on the pump/sender assy there are wires that hook up to a small chip like unit that varys resistance based on float position, id bet he screwed those wires up and put them on backwards.
                  Possible. This would be equivalent to installing the float assembly upside down.

                  Time to interrogate the installer.

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                  • #10
                    thats what i dont get when he says it was his original pump and nothing was touched, something is different, like you said though i think its more likely a different sender assy.

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                    • #11
                      All very good reply's, but since I was not present during the swapping of the tank I can't say for sure what sending unit/pump assembly he put back in there other than to say he SAID he used my original unit. He must be mistaken from what I've determined as well as what everyone is saying here. The XJ that he had the extra parts from was a 91+ so I'll bet he just got them mixed up and didn't realize the newer one wouldn't work right with my 90XJ. I guess I'll have to get the siphon hose out and start sucking!! Luckily he gave me the pump and sending unit that he said came from that extra tank (must actually be my original one) so I'll just need to drop the tank and switch them out....

                      Thanks for all the help guys! rstarch345 I would have never guessed that DC switched the polarity of the sender from 90 to 91 so thanks alot for the info. That HAS to be what's going on here.

                      Anyone guess what I'll be doing this weekend? Other than and of course!

                      Everyone have a safe and Happy New year

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                      • #12
                        Originally posted by PegLeg
                        I guess I'll have to get the siphon hose out and start sucking!! Luckily he gave me the pump and sending unit that he said came from that extra tank (must actually be my original one) so I'll just need to drop the tank and switch them out....
                        Or you could get a 91+ gauge and put it in the cluster. Just a thought.

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                        • #13
                          Originally posted by Amphibious
                          Or you could get a 91+ gauge and put it in the cluster. Just a thought.
                          Now that's an idea! Although tearing apart the dash might prove to be more difficult with the roll cage installed....I'll have to take another look at it when I get home to see if that will be easier than sucking gas and dropping the tank. Thanks!

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                          • #14
                            Am I missing something?

                            Guys, this is surely a polarity issue. I'm not at all familiar with the vehicle and its 'electronics' but wouldn't the simple reversal of the wires at the gauge fix this? I know my suggestion wouldn't work on a CJ because the temp and fuel are tied together but how about on the Cherokees? (I'm a stranger in a strange land here...) Just trying to save what will likely be a pretty good bit of work.

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                            • #15
                              Originally posted by blackwater
                              Guys, this is surely a polarity issue. I'm not at all familiar with the vehicle and its 'electronics' but wouldn't the simple reversal of the wires at the gauge fix this? I know my suggestion wouldn't work on a CJ because the temp and fuel are tied together but how about on the Cherokees? (I'm a stranger in a strange land here...) Just trying to save what will likely be a pretty good bit of work.
                              I'm not absolutely sure about a '90 XJ, but it's probably mounted on a PC board, which means you can't physically reverse it. There are no wires except in the harness. Hacking into the harness is as problematic as the other alternatives.

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