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Thread: 91 K75S fuel fuse blowing

  1. #1
    JohnWC
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    91 K75S fuel fuse blowing

    This bike may just drive me out of BMWs. After replacing all the tank components a year ago, except the sender unit, but not riding it much since then, in recent weeks it keeps blowing the 7.5 amp fuel fuse. I tried using a 10 amp, thinking it seemed to be at startup after sitting, but it didn't help. Now the bike will stop running at any time as I ride it, just shuts off. Yes, it's a lot of fun when that happens. The fuse is always blown. Put in another fuse, it goes for a while then stops. It can't be ridden. I have tried completely disassembling and cleaning the sender unit, even re-wired it since the wires were falling apart. No luck.

    I thought about trying to bypass the low fuel system in the sender and just wiring it straight through to the pump, as I read someone had done. But I suppose I will have to bite the bullet and order a $280 replacement for the little sender unit. They are not available used or aftermarket. Of course other similar units for other brands are about $75 to $150, or less. Gotta love BMW parts prices. And there's no guarantee that will fix it, which is the best part. Maybe I need to take it to a dealer and pay about $500 for a diagnosis. Then I can add on the part and get it up to about $1000 to fix the fuel fuse. Everyone raves about the durability of the K75s. Maybe that's true if you get a good one. That's my mistake.

    Old or new, you need deep pockets to own one of these German machines.

  2. #2
    3 Red Bricks
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    Do you have a good scematic and an ohm meter? If so, you might think about troubleshooting it instead of throwing parts and money at it.

    You did not get a bad one, you got one that has maintenance issues or has issues probably caused by previous owners ham handed attempts at maintenance. Yes, they can get expensive if you can't do the work yourself (what vehicle isn't?).





    Last edited by 98lee; 05-29-2021 at 01:08 AM.
    LONG MAY YOUR BRICK FLY!

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    Lee Fulton Forum Moderator
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  3. #3
    3 Red Bricks
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    Is it fuse #6 (second from the bottom)?

    Do you have the optional alarm system installed in the tool tray?

    Is there any aftermarket accesory plugged into the connector (inside the relay box under the fuel tank) for the optional alarm system?

    https://forums.bmwmoa.org/showthread...l-Early-Kbikes





    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  4. #4
    JohnWC
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    Quote Originally Posted by 98lee View Post
    Is it fuse #6 (second from the bottom)?

    Do you have the optional alarm system installed in the tool tray?

    Is there any aftermarket accesory plugged into the connector (inside the relay box under the fuel tank) for the optional alarm system?
    lee,
    There doesn't seem to be any alarm system on the bike. As for the ohm meter, I actually used your detailed description of the sender plate to work through the problem a while back. It's somehow an intermittent dead short in the fuel system. The bike will start, run, then stop, and now I never know when. It is the 6th fuse, the fuel one. When I had this earlier, as soon as I hit the starter button, the fuse blew. If I remember, I bypassed the sender, (this was all done with the tank apart) and the fuse stayed okay. Hence the complete cleaning and rewiring of the sender unit, mainly using your data to check all the ohm readings. The sender unit all checked out okay. I can't imagine that a wire has worn through to the frame somewhere on the bike. People with 300,000 miles on these bikes don't seem to have that problem.

    I'm debating whether to take it all apart again (about the fourth time) and just try to wire it straight through, since I wouldn't sell this sender to anyone else. But I'll have to put it all together to test it in actually riding. Draining and taking that tank off, etc is getting a bit old. Plus I'll loose the low fuel warning light.

    You're right that not be able to work on these bikes does get expensive. But I've found that just buying the parts from BMW isn't exactly cheap either, let alone my time. Almost $300 for that sending unit? Why not $500? Can't run the bike without it. Price gouging comes to mind. I suppose I should do what most BMW people do: spend $20,000 on a new one, and trade it in every three years when the warranty is up. From what I can see on the mileage contest winners, that's the usual method. And from my experience with this brand, it's the smartest way to go.

  5. #5
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    It costs some pretty good money in inventory tax/warehousing to keep parts for a '91 around.
    OM
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  6. #6
    Registered User mlytle's Avatar
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    Quote Originally Posted by Omega Man View Post
    It costs some pretty good money in inventory tax/warehousing to keep parts for a '91 around.
    OM
    yup. not condoning price gouging, but the fact that the parts are available at all for a relatively low production 30yo bike is great. these ain't mass produced hondas or chebbies.

    perhaps OP should get rid of this troublesome, expensive annoyance and get........A HARLEY!
    Marshall
    92 K75s, 94 K75s, 09 K1300s

  7. #7
    JohnWC
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    Quote Originally Posted by mlytle View Post
    yup. not condoning price gouging, but the fact that the parts are available at all for a relatively low production 30yo bike is great. these ain't mass produced hondas or chebbies.

    perhaps OP should get rid of this troublesome, expensive annoyance and get........A HARLEY!
    I'm not even going to respond to that.


    For anyone who actually wants to help, and who has actually worked on these machines, I found a possible clue to the problem:
    I noticed that after just five minutes of running, the wires and connector of the fuel tank sender unit are getting warm. It should be pulling only 7.5 amps. Is it normal to have those wires heating up? What would that be indicating? The pump is new.
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  8. #8
    Registered User drneo66's Avatar
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    Quote Originally Posted by jconway607 View Post
    I'm not even going to respond to that.


    For anyone who actually wants to help, and who has actually worked on these machines, I found a possible clue to the problem:
    I noticed that after just five minutes of running, the wires and connector of the fuel tank sender unit are getting warm. It should be pulling only 7.5 amps. Is it normal to have those wires heating up? What would that be indicating? The pump is new.
    Warm would be understandable, hot could be caused by excessive current, poor connections, and/or insulation breakdown.
    Current: 2007 BMW R1200RT, 2013 F800GS
    Former: 1995 BMW K75S, 2009 BMW G650GS
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  9. #9
    Recalculating..... veefore's Avatar
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    The sending unit, pump etc. in my 93’ looked like remnants from the wreck of the Titanic. The pump was dead as a door nail and the sending unit was a rusted ball. I would’ve expected nothing less from 28 year old motorcycle. Let alone one that had been sitting for 12 years and built well before ethanol started corroding our lives. Installed a new sending unit (280.00), pump, refurbished injectors , cleaned the tank and the bike runs like a charm.
    I am by no means a mechanic but I bought a shop manual, a test light, read the forums and worked my way through it. When I got frustrated I walked out of the garage and left it until I was in a better state of mind. The fact that the parts were available......priceless.
    Last edited by veefore; 05-31-2021 at 03:54 PM.
    2015 R1200RT, 1993 K75s

  10. #10
    3 Red Bricks
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    Does your meter have a DC amps setting? Try reading how much amps the pump is pulling AT THE PUMP. Then check how many amps AT THE FUSE. If it is higher at the fuse than at the fuse, unplug the temp control relay and try again.

    The fuel tank connector plug is a known weak spot on older bikes. Either corrosion or loose pin fit or partialy to fully broken strands in the wires a couple of inches to either side of the plug develop with 30 years of mishandling.




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  11. #11
    Nick Kennedy
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    Maybe try a 75 amp fuse

    https://www.youtube.com/watch?v=i5ryowgcnGM

    Nick
    1990 K 75 RT

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