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Fuel gauge stopped working on R1150GS

Discussion in 'GS Boxers' started by space, Jun 22, 2007.

  1. space

    space a.k.a. Jake

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    My fuel gauge is no longer showing any bars. It was working one moment, showing a full tank, and then the next moment its reading disappeared. (First check: am I sitting in a puddle of gas? No? Good.) The yellow low-fuel light hasn't come on, so I'm guessing the problem is at the LED side, not the fuel tank float side. I presume if the float arm got stuck down or its variable resistor grounded to the tank, the low-fuel light would be on as well. Incidentally, my tank is still nearly full.

    I'm going to try to fix it this weekend. Has anyone seen this happen before? How'd you solve it? Thanks.
    #1
  2. picard

    picard engage!

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    Mine shows empty after every fill-up until the tank is about 1/2 empty. Then it works. The yellow light only comes on when the tank is really going empty (~1 bar). I didn't bother fixing it so far because it works where it matters. have you run the bike down below 1/2 tank since the problem occured?
    Please keep us posted what you find. Thanks,
    #2
  3. rosmoe

    rosmoe Been here awhile

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    Got an 03 GS, fuel gauge never worked right since I bought it new. Now only goes to 9 bars when full. BMW isn't know for their great fuel gauges, Waiting for mine to quit working all together. Just ride it & hope nothing important fails.
    #3
  4. space

    space a.k.a. Jake

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    rosmoe -- I hear ya. But it worked perfectly and then stopped altogether, which tells me that it's probably a bad connection somewhere. Hopefully an easy fix, once I track it down with the voltmeter.

    fraflo -- no, I haven't gotten it down to below half a tank yet. I'll report back what I find. Thanks for the input.
    #4
  5. johnjen

    johnjen Now, even more NOW!…

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    The easiest 1st thing to do is check the electrical plug under the right side of the tank at the rear. Make sure the connectors are not corroded etc. Then re-connect and test to see if things are back. If not then the fun begins. It could either be a fault with the sender unit in the tank or the lcd panel on the dash or the wiring inbetween.

    JJ
    #5
  6. space

    space a.k.a. Jake

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    OK, so here's the story. Upon closer inspection of the cable for the tank electrics, I noticed a frayed wire. Here's what I saw after cutting back some of the heat-shrink:

    [​IMG]

    Maybe I caught the wire some time while putting the tank back on? But most of the stripped wire was covered by the heat-shrink tubing, which showed no signs of damage. And what's that black stuff in there? Weird.

    Unfortunately, replacing that cable is not an option: BMW sells it together with the fuel pump & filter mounting plate, which costs nearly $300. Forget that. Hacking time!

    After playing around with the voltmeter a little, I figured out how the fuel level potentiometer works. Here's a diagram:

    [​IMG]

    As you can see from the top photo, the broken wire is the ground line for the fuel level sensor. So both pin 3 (continuous fuel level) and pin 4 (low-fuel light) are isolated from ground, producing the confusing condition of the continuous level reading empty while the low-fuel light is off.

    Easy solution: some minor bypass surgery. On the inside of the tank, I unsoldered the brown ground line from its connector, put an eyelet connector on it, and stuck it on the fuel pump ground. Here's the result. The terminal it was previously soldered to is in the foreground.

    [​IMG]

    That did it. Put the tank back together, and now both the low-fuel light and the continuous gauge work.
    #6
  7. johnjen

    johnjen Now, even more NOW!…

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    Good stuff.
    That black stuff is the burnt insulation of the ground wire. And the wire itself also shows it has been over heated.

    Another interesting thing to note is that the yellow & white wires are part of a 3 wire cable that also contains the brown wire that overheated. The green and brown wires that feed the pump are individual wires.

    So what does this mean? Well, the sensing circuits (or at least the wire that feeds them) were pulling more current than they were designed to. The why and how are still unknown at this point. A WAG would be that the larger brown wire that feeds the pump may have limited current carrying capacity so some of the current HAD to be carried by the smaller brown wire.

    JJ
    #7
  8. Objectivist

    Objectivist Adventurer

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    The overheated ground wire must be fixed also.
    One possibility is that its connector had backed out of the plastic holder and not been making good contact with its mating connector, thus having much resistance and thus very high current flow resulting in high heat melting the insulation. Anyway, something caused the heat and should be identified and corrected if you don't want another meltdown or worse.
    #8
  9. space

    space a.k.a. Jake

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    That was my thought from the blackened stuff -- burnt wiring. But there are actually two ground lines running into the tank on my bike: the small gauge, burnt one, which runs to the level sensors; and the larger gauge one, which runs to the fuel pump. Past that molex-like connector they are totally separate. You can see the two wires running to the same pin in my first pic.

    I have no idea what could have sent much current down that line. According to the wiring diagrams, the continuous level sensor (pin 3) goes straight into the rider's information display, where I presume it's pulled up with a ~100 ohm resistor to get a voltage read -- no way to short there. And the low-fuel sensor (pin 4) goes to a box with a transistor symbol, where presumably it switches the higher current through the lamp. I'm scratching my head on this one.
    #9
  10. space

    space a.k.a. Jake

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    OK, just read your edit. Still, if the larger gauge wire wasn't connecting, it shouldn't send the fuel pump current down the smaller ground wire since they don't (originally) connect inside the tank.

    I'm going to start zip-tie'ing that connector together, following Objectivist's idea. Regarding the old ground wire, I just cut it out of the connector since it wasn't needed anymore.
    #10
  11. johnjen

    johnjen Now, even more NOW!…

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    Yeah that smaller brown wire probably doesn't have a direct path to the pump so that part of my WAG is probably wrong. But since the smaller brown wire did over heat it means that some where in either the cable or the sense circuits there is a problem. It could be a wire to wire short somewhere in the cable.

    Also the wiring diagram isn't entirely accurate, at least in one circuit. It shows a single wire connecting to the tank connector, where in fact there are 2. This isn't earth shattering but it does raise the question, are there any other differences?

    1 last thing. Since your larger brown wire is now doing double duty (the pump and the sense circuits) it may also over heat. We still don't know what caused the smaller brown wire to crispy critter and you may have just moved the problem to the large brown wire.



    JJ

    #11
  12. space

    space a.k.a. Jake

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    Very good point. I'm going take a thorough look at the wire harness to spot for any possible shorts. Thanks for the feedback.
    #12
  13. Quelish

    Quelish Mileage Grinder

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    #13
  14. Irishdude

    Irishdude Adventurer

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    I also had the same Symtoms today,
    Filled her up , started the bike, guage shows full bars, went for a drive and noticed that it was showing 3/4 full a lot sooner than what you'd expect, so I thought I must be driving alot hehe, next it showed no bars and this within an hour of filling the tank to full, turned the bike off started it again and it showed nearly full bars , but in no time it was showing no bars..
    Do ye reckon I too have to go searching in the cables to find a short ?
    #14
  15. GS Addict

    GS Addict Pepperfool Supporter

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    Start with cleaning the traces on the sender board. They get gunked up over time, a common problem. I do mine every fuel filter change.
    http://www.advrider.com/forums/showthread.php?t=248714&highlight=fuel+sender see post #6
    #15
  16. RockyBoy

    RockyBoy Been here awhile

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    What if the fuel gauge and the temp gauge gives out at the same time, does that mean a problem else where?
    #16
  17. GS Addict

    GS Addict Pepperfool Supporter

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    Is the clock working? If no check for a blown fuse.
    #17
  18. RockyBoy

    RockyBoy Been here awhile

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    huh, I never thought about that. Thank you ADV hive mind!
    #18
  19. RockyBoy

    RockyBoy Been here awhile

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    The clock is working, the fuel gauge isn't, and the oil temp doesn't rise as fast as it used to, it would straight to half way after fifteen minutes ride, now it will only go up a quarter of the way.
    #19
  20. GS Addict

    GS Addict Pepperfool Supporter

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    Start buy checking the connectors for corrosion.
    #1 is just below the tank (must be removed if you lift the tank)
    #2 is under the tank forward of the fill assy. (grey 8 pin connector) in the group it is underneath a smaller one. You can release it from the holder by pushing the little lever and sliding forward.

    #2 gets a lot of dirt and sometimes water
    #20