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Thread: 1987 K75C 58,381k Karibic Bleu requesting fuel supply troubleshoot help

  1. #1
    PeteG
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    Woodacre, California
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    1987 K75C 58,381k Karibic Bleu requesting fuel supply troubleshoot help

    Symptom: Abrupt stall at freeway speed. Will crank, but fuel pump not activating. No pump sound after tapping start button.

    Everything independently tests OK. What am I missing?

    1) Testing for a broken sending unit ground wire. Made a ground jumper from base of sending unit to frame, tapped start button. and fuel pump does not start.

    2) Tested voltage at sending unit harness supply side (green wire female) by tapping on start button. Reads 12.5 volts for a 1.5 seconds.

    3) Removed fuel pump and did a bench test. Works fine. Sounded a little growly when coming to a stop.

    4) Removed sending unit and bench tested for continuity. Green wire at harness male post has continuity to dirty yellow hot wire to pump. Brown wire at harness has continuity to black ground wire to pump. I tried wiggling the wires and was able to maintain continuity. Both A-OK.

    5) Trying to isolate sending unit harness by reconnecting sending unit to harness, tapping start button and reading voltage at pump connection wires. Runs up to 12.5 v and back down. Harness connection seems to be working.

    6) Removed fuel filter and will replace. I have yet to give it a blow test. It was replaced about 40k ago.

    Full disclosure, I cleaned ignition and kill switches prior to this failure. Had about 10 successful starts and runs prior to failure. Iím thinking this is not related.

    What am I missing? Can someone explain exactly how you get 1 - 2 seconds of voltage to the pump by tapping the start key? In other words what supplies voltage for that short period of time when the starter has stopped and K75 not running? Iím just curious.

    Without feedback Iím inclined to replace filter and pump and keep the current pump as a spare. Feels strange to not find anything obviously wrong.

    Due to fire danger home power shut off I won't be able to check for possible responses until tomorrow afternoon at the earliest.

    Thank you in advance,
    Pete


  2. #2
    3 Red Bricks
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    Pete,

    When you hit the start button, it sends a signal to the Ignition Control Box (under the fuel tank up by the steering head).
    The Ignition Control Box sends a couple second pulse to the Fuel Injection Relay (under the back of the tank in the relay/fuse box).
    The Fuel Injection Relay sends the power to the pump through the fuel level sensor (fuse 1 & 6 must be good).

    If the bike starts, the Hall Effects Sensor sends pulses to the Ignition Control Box that are above 700 rpm (I think that is the number) and the Ignition Control Box sends a constant signal to the Fuel Injection Relay.



    The pump wires are long enough that you could hook up the entire electrical circuit with the pump outside the tank not connected to the fuel line. Everything would be connected as normal, but no fuel involved. See if it works.

    If no, disconnect the pump and check the voltage at the leads (although I think you said you already did that).

    If yes connect the fuel line and see if it still works. If yes, put the pump back in the tank and see if it still works.



    Hope the fire stays well north. Power outages are inconvenient, but nothing like the hell the people up there are going through (again).




    Last edited by 98lee; 10-28-2019 at 10:47 PM.
    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  3. #3
    PeteG
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    1987 K75C fuel supply troubleshoot

    98lee,

    Thanks so much for the response and class on Ignition control. I may not get back to this until the weekend, but will update as things progress. I'll follow your hook up sequence. I did try to blow through the fuel filter and there was heavy resistance. I was happy to see that it may be the source of the problem. I'm not sure if the clogged fuel filter would cause the pump to stop when hitting the start button which is still a concern. I did check all of the fuses and all were OK. I didn't know about fuse #1 and had initially checked only #6.

    Pete

  4. #4
    3 Red Bricks
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    Pete,

    The fuel pump relay pull in circuit gets power through fuse #1.

    It's good practice when tracing electrical issues to check ALL the fuses, as many times multiple fuses effect a given circuit.






    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  5. #5
    3 Red Bricks
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    Pete,

    If the pump works on the bench and the wires to the pump show 12v, but the pump does not work in the circuit, it is possible that not enough current (amps) is getting to the pump due to a partly broken wire,or a corroded connector, or relay contacts or connections, or fuse or fuse contacts.





    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  6. #6

    Plus 1 on checking connectors and wires

    I had a very similar issue with a K 1100LT a while back, it left me stranded once, finally traced the problem to the connector between the sending unit and main harness. I replaced it with a GM water proof connector and never had another problem. The connection is aluminum and is easily compromised to the point of stalling and no starting.

  7. #7
    Could the side stand or its switch be an issue?

  8. #8
    3 Red Bricks
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    Quote Originally Posted by richw View Post
    Could the side stand or its switch be an issue?
    Nope.

    The K75 does not have a sidestand switch.

    It has a mechanical sidestand retract mechanism that retracts the sidestand when the clutch is pulled in. Can't crank the bike unless it is in neutral or the clutch is pulled in. Can't get it in gear to drive away unless you pull in the clutch. Can't pull in the clutch without retracting the sidestand. (Unless the mechanism is out of adjustment.) That was BMWs thinking.

    Once they found in the real world that people don't always keep things properly adjusted, they went to the sidestand switch on the later 4valve K bikes.






    Last edited by 98lee; 10-31-2019 at 01:19 AM.
    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  9. #9

    Smile

    Quote Originally Posted by 98lee View Post
    Nope.
    Just a thought. Thanks for the education. ,

  10. #10
    PeteG
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    Update + observation + question

    Havenít had time to make 98leeís suggested tests. May not happen until next week. New fuel filter arrived and tested. Blow level difficulty from 0 to 10, zero being the easiest, 10 being most difficult. New filter = .5 Old filter = 9.5 (could barely get air through). I consider this to be a good sign of potential problem.

    I noticed the fuel pump was the wrong size of 43 mm instead of 52 mm for a 1987 K7C. Looks like a previous owner installed a from 1/93 pump and vibration damper into this K75C. My understanding is that the smaller diameter pump was introduced when the width of the tank forward mount increased in size from 90mm to 130mm.

    The from 1/93 pump has no vent hose. The correct pump and filter screen does have a vent hose provision. How important is it to have then vent hose? Iíve already gone 40k with this pump. My understanding is without the vent hose if there were boiling fuel it would come out on top of the tank rather than through the vent distribution line. Is that correct? What I am getting at is should I be replacing this pump with the correct pump, damper, and fuel vent attachment?

    Pics of current pump below.

    Thanks,
    Pete
    Attached Images Attached Images

  11. #11
    3 Red Bricks
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    Walter,


    No problem using that pump. Many K75s have had their pumps changed from the original 52mm pump to the newer 43mm pump or an aftermarket (37mm ?) pump with no issues (including several of mine). The change is usualy brought about due to the high cost of the original.

    Never had a heat problem, and I ride in some pretty hot (100+) temps.






    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  12. #12
    PeteG
    Join Date
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    Quote Originally Posted by 98lee View Post
    Walter,


    No problem using that pump. Many K75s have had their pumps changed from the original 52mm pump to the newer 43mm pump or an aftermarket (37mm ?) pump with no issues (including several of mine). The change is usualy brought about due to the high cost of the original.

    Never had a heat problem, and I ride in some pretty hot (100+) temps.






    Excellent, thanks much Lee.

    Best,
    Walt

  13. #13
    PeteG
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    Mixed result repair update

    Successfully tested pump, vacuumed black flakes from tank. There was no tank gunk and there were few (10 to 15) black flakes of unknown origin. replaced fuel filter and reassembled. Fuel hoses looked good and were not deteriorating. There was no loose play in the pump so Iím assuming the vibration damper is OK.

    Bleu fired right up. Filled tank to full and noticed gas leaking from sending unit. NOTE TO SELF: Always check gaskets that may be decades old before reassembly. Replaced gasket with new. A flex test on the old gasket snaped it in two.

    32 mile commute home, 9 mile freeway and 23 mile back country was successful. Ran smooth, good power, like a finely-tuned sewing machine.

    Stalled on freeway portion of return trip the next morning. No sputtering, just quit. There is a section of freeway under construction for a 3rd lane addition with no shoulder. Luckily I had been detouring that section because of lack of shoulder. After coming to a stop I tried a restart. Restarted and idled fine. Cautiously made my way in slow lane to final destination (7 miles) with no additional stall.

    Because of an almost instant restart Iím ruling out a failing hall sensor.

    It is difficult to tell whether this is related to my prior starter relay issue. Iíve cleaned starter ground contacts, ignition switch, & kill switch. The starter relay is an inspected, and cleaned prior stuck relay. I have not checked the starter brushes or cleaned the starter armature. Current alternator was transferred from my other K75. I notice a warbling/growl upon acceleration that was not present before.

    Intermittent stalls are no fun. Suggestions welcome.

    My research has taken me to bad ICU ground or connection, intermittent fuel pump seize & sending unit ground wire failure and bad four-pin sending unit connection. The sending unit wires on the tank side looked in very good shape. When testing sending unit wire continuity I did try wiggling the wires and they maintained continuity. The fuel pump seize kind of fits with my not hearing the pump upon start button push on the prior stall and tow. Of course, same could be said of a bad ground in the circuit.

    My apology for the lengthy clues and rambling.


  14. #14
    PeteG
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    12.28.19 Update and HAPPY NEW YEAR wish to ALL BMW MOA members . . .

    and a thank you for all the great help, education, and part resource advice made available on this forum.

    It has been 550 miles from my initial stall and fuel filter replacement. My last post reported a freeway stall with immediate restart at rest on freeway shoulder. Approximately 140 miles ago there was another stall going 40 mph on a back country road. It lasted several seconds and essentially push restarted before I came to a complete stop.

    Yesterday was my first freeway ride of 25+ miles. It went without incident and my confidence was building that the stall problem was solved. Prior to that my max freeway ride distance was 8 miles. Today was my 3rd stall on surface street at 35 mph. No restart. Engine cranked fine. Battery indicated full charge. Did light testing while waiting for tow arrival. Removed fuel filler cap and pushed starter button. NO SOUND. I tried this 5 times. NO SOUND, nor start. Checked fuses, all looked good, no voltage meter available. Checked and wiggled sending unit connection and tried start. No fuel pump sound or start.

    After tow and safely in shop. Pushed starter button and could hear pump sound. Pushed again and started and ran fine! Clearly there is either an intermittent electrical or pump problem. I'm leaning toward pump replacement and keeping what I have as a spare if the problem persists. Tough problem to troubleshoot when everything is working fine. Marty, the tow guy, and I are becoming friends!

    Will update when resolved and am adding multi-meter to my tool kit.

  15. #15
    There was a pattern of problems with the electrical connections for the fuel pump. Sometimes it was the connection where the pigtail on the tank plugs in, but often the problem was where the wiring passes through the bulkhead on the bottom of the tank. I would thoroughly bench test the fuel pump out of the tank and also test the wiring through the bulkhead for continuity.

    Best guess: tank bulkhead wiring.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

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