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Thread: Fuel Tank / Pump / Level Sensor Wiring - All Early Kbikes

  1. #1
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    Fuel Tank / Pump / Level Sensor Wiring - All Early Kbikes

    One weak point on the early Ks is the fuel sender plate on the bottom of the gas tank. The plate holds the fuel sender float and rheostat and has the pass through for the 4 wires.

    The four wires come from a 4 pin plug that is usually clipped to the right front of the tool tray or hanging near the right rear of the tank.

    The wires on the plug that run to the sender plate are:

    Fuel Pump Positive: Green

    Chassis Ground: Brown

    Low Fuel Light: White

    Signal for Fuel Gauge: Yellow




    IMPORTANT!!! As they pass through the plate the wires CHANGE COLORS!!! (Don't ask why! )


    Inside the tank:

    Fuel Pump Positive: Yellow

    Chassis Ground: Black

    Low Fuel Light: Blue

    Signal for Fuel Gauge: Blue


    Here is what the outside of the plate looks like:
    The metal tab that is spot welded to the outside of the plastic housing is where the ground is passed through. It is one of the most common failures. The spot weld fails and the tab does not make good contact to the plate or the solder conection from the wire to the tab fails (no ground, no pump). Sometimes you can attach a wire to the broken off tab or its wire and put a ring connector on it and put it on one of the four mounting studs instead of trying to resolder the tab to the plate.

    sender closed outside.jpg






    This is what it looks like inside the plastic (The previous owner had broken off the plastic and resoldered the wires and had put a huge gob of RTV over the wires to seal them. Don't know about the conductivity of RTV. Other than that. it should work).

    sender open outside.JPG






    This is what the inside of the plate looks like:
    Can you tell why this one does not work? (Black Ground wire) Sometimes the welded on tab breaks off the plate on the inside also.

    Green wire from outside becomes the Yellow wire seen here (Pump +)
    Brown wire from outside becomes Black wire seen here (Chassis Ground)
    White wire outside becomes Blue wire on right (Low Fuel Light)
    Yellow wire outside becomes Blue wire on left (signal for Fuel Gauge)

    sender inside.jpg






    The Ground is used by all three circuits. It runs to the main Chassis Ground bundle on the left side of the upper frame frame tube underneath the fuel tank.
    It is the ground for the Fuel Pump.
    When the low fuel light switch connects the White (outside) wire to the ground, the light comes on.
    The Fuel Gauge circuit puts a rheostat, controlled by the float, between the Yellow (outside wire) and the ground, thereby giving a variable input for the gauge.


    If there is anything that I have left out or anything about this that you would like explained more, please post the question and I will update/elaborate further.



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
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  2. #2
    Instigator cele0001's Avatar
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    Maybe unrelated but what is needed to retrofit fuel gauge to k75rt 92 that has only light for the fuel warning
    I want to be unique, just like everybody else.

  3. #3
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    Not positive, but I believe this extra harness is required:

    61 12 1 459 488 WIRING ADDITIONAL INSTRUMENTS $90.75

    Not positive if all early bikes had the appropriate MAIN wiring harness. Maybe?
    I'm pretty sure that all the RTs had the correct MAIN harness for the gauges.


    Last edited by 98lee; 04-27-2014 at 10:02 PM. Reason: forgot link. oops!
    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
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  4. #4
    Registered User dieselyoda's Avatar
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    Thanks for that.

    Haynes, Clymer and the BMW manual were giving me color codes that didn't match to anything.

    I'm pretty sure I'm hunting for a new fuel pump now as the only way it will start and run is if you smack the tank.
    1997 R1100RT, 1981 KZ 440 LTD, R80RT, R90/6 sidecar, K1100RS,1983 K100RS (Cafe now)

    “The major civilizing force in the world is not religion, it is sex.”

  5. #5
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    You have a multimeter.

    1) Test the wiring between the plug and the pump. Green outside the tank to Yellow inside and Brown outside to black inside.

    2) Try running a jumper from the black wire side of the pump to the ground lead bolt on the side of the transmission (by the shifter).

    3) Remove 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
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  6. #6
    Still plays with trains. tinytrains's Avatar
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    At the age of 20, the pass through on my K75 would open up when hot, shutting off the fuel pump. The solder connections go bad from heat cycles. To track it down, I added a pair of wires to the fuel pump and ran them out the filler to a volt meter, then rode it around until it quit.

    You may want to do something like this, but make very good connections to the pump with lugs. You do not want any sparking!
    2009 R1200R Roadster

  7. #7
    Registered User dieselyoda's Avatar
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    I must have sticking brushes in the pump.

    I checked continuity, nada, zip, zilch, banged the tank, got about 15 ohms, rather a tad high. Banged the tank again and got her running, as soon as I shut it off, I could see the resistance increase steady until dead again.

    At this point, I'm going to go with loose connections and hopefully a nice healthy spark will end my misery.
    1997 R1100RT, 1981 KZ 440 LTD, R80RT, R90/6 sidecar, K1100RS,1983 K100RS (Cafe now)

    “The major civilizing force in the world is not religion, it is sex.”

  8. #8
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    Quote Originally Posted by dieselyoda View Post
    I checked continuity, nada, zip, zilch, banged the tank, got about 15 ohms, rather a tad high.
    On which wire?

    Test each wire, not through the pump.




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  9. #9
    Registered User dieselyoda's Avatar
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    Put my hands in the fresh fuel I just put in and right on the two pins.

    I put my meter right on the two pins at that pump. Actually, I got my daughter to do it because she has tiny hands. The bike side seems to be pretty good and working. Bang the tank, I got good stuff happening.

    I'm sure you know that anytime you have a EFI vehicle that doesn't start when cranking, you smack something hard on the vehicle to see if the pump goes?

    This last time, I was tempted to use a match to check inside the tank.
    1997 R1100RT, 1981 KZ 440 LTD, R80RT, R90/6 sidecar, K1100RS,1983 K100RS (Cafe now)

    “The major civilizing force in the world is not religion, it is sex.”

  10. #10
    MOA #24991 Pauls1150's Avatar
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    Just a WAG on the "Why" - Outside the tank looks like it conforms to the Bosch wiring color standards, so the inside wiring was done by a subcontractor...?

  11. #11
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    Quote Originally Posted by Pauls1150 View Post
    Just a WAG on the "Why" - Outside the tank looks like it conforms to the Bosch wiring color standards, so the inside wiring was done by a subcontractor...?
    Quite possible as the float/sender assembly was made by VDO.



    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

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