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Thread: '85 K100RS Bad fuel pump or fuel pump relay?

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  1. #1
    K-biker
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    '85 K100RS Bad fuel pump or fuel pump relay?

    Hi all,

    I have an 85 k100rs that has been ok till now. Riding along and felt something knock between my legs like a slight vibration. Bike coasted to the curb and would not start. Thought it was the loose plug on the COMPUTER ECU ECM CONTROL MODULE BRAIN BOX and unplugged and re plugged it in. Have had this happen in the past. Put a zip tie on it to hold in position. Tried again but no start. All seems fine, starter cranking, battery power ok, gas in tank. Could it be the fuel pump relay or the pump? And how do I figure out which one? http://www.bmwmoa.org/forum/images/smilies/dunno.gif

  2. #2
    3 Red Bricks
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    Open filler cap. Turn on key and kill switch. Tap starter button. You should hear the pump (in the tank) run for a couple of seconds.

    When you plug in the computer plug, you MUST remove BOTH battery covers and hold the rear of the computer with one hand as you are pushing the plug in the front with the other hand. You MUST hear a distinct CLICK when the plug locks into the computer.
    The computer is rubber mounted, so just pushing from one side without holding the back side allows too much flex to get the plug properly seated.

    IF you are sure the plug is properly seated, check your fuses especially #1 and #6. Check for voltage at #6 when the starter button is pushed. (only lasts a couple seconds, so have meter set up before pushing button. If you do have voltage, but pump doesn't run, you need to pull the pump and bench test it.



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  3. #3
    Rally Rat
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    Does your bike still have the small battery negative cable (maybe 18 gauge)? If so, replace it with a later cable. The original, especially with some age, can give you fits.

  4. #4
    K-biker
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    Thanks 98lee. Did all of the above and have to pull the pump. How do you bench test it?

  5. #5
    3 Red Bricks
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    Put 12v directly to the terminals. No fuel connected, just see if it runs (probably won't). The small terminal is positive. If the pump does not rum, try reversing the polarity. It might unjam it. But it usually is fubared.

    A very cost effective replacement (I use these in all my bikes):

    http://www.euromotoelectrics.com/BMW.../fp-231kit.htm


    With the following adapter:

    http://www.euromotoelectrics.com/BMW.../> 576foam.htm


    If your vibration damper is going bad:

    http://www.euromotoelectrics.com/BMW...-576damper.htm




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  6. #6
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    bench test

    Took out the pump and saw the err to my ways. I used a bead of gasket sealant (recommended by the garage I was in) that broke down in the tank and got into the pump. This raises several questions:
    1. What can I soak the pump in to try and loosen/dissolve and reverse pump out the bits?
    2. Did I even need the gasket sealant in the first place (to seal the gas cap gasket to the tank) and if so what is the correct one?
    Last edited by 117336; 08-29-2012 at 02:54 PM. Reason: needed more info

  7. #7
    BMW uber alles! Zagando's Avatar
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    Quote Originally Posted by 98lee View Post

    A very cost effective replacement (I use these in all my bikes):

    http://www.euromotoelectrics.com/BMW.../fp-231kit.htm


    With the following adapter:

    http://www.euromotoelectrics.com/BMW...> 576foam.htm


    If your vibration damper is going bad:

    http://www.euromotoelectrics.com/BMW...-576damper.htm





    Hey Lee; does your above statement mean that this fuel pump will fit/work in a K75S (with the foam adapter also mentioned)?

    Just gathering info in case I need to replace my screeching, chirping OEM 1994 fuel pump.

    Thanks!
    ---Jeff

    ex: K75S Berlina R100GS/PD , K100RS , R75/5 , R60/2

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