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Thread: 1990 K75RT Electrical problem

  1. #1
    Nick Kennedy
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    1990 K75RT Electrical problem

    Hi All
    My super reliable K75 had a funny glitch today. 41,000 miles.
    While heading out on a ride today about 1/2 mile down the road the battery light half way lit up. Only a dim glow, not fully lit up. I touched the brakes and the light went out. This happened 4 more times in the first 40 miles.
    I stopped and filled up. About 5 miles later the bike shut off for about 1/2 second, never done this before.
    I put in a bottle of Red Heet last week, to start the season and ran that tank of fuel out.
    I turned around and headed home via a friends house. Shut the bike off for about 45 minutes.
    It started right up and I rode home with no glitches.
    Battery is new, terminals clean.
    On my next ride I'm going to hook up a volt meter across the battery and put it where I can see it, riding in my tankbag and see what the voltage does- if and when the battery light comes on again.
    Anyone have any ideas what could cause this odd behavior?
    I'd like to solve the problem as I don't trust the bike to head out of town very far now.
    Thanks in advance

  2. #2
    Registered User WalterK75's Avatar
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    I had a similar problem a couple of times with a 91 K75. One solution was my mechanic replaced the thermostat as it was failing and that messed up things with the ECU. The other time the battery wasn't getting charged and the voltage regulator was replaced.
    Walter

    "Sometimes I wonder if the world is being run by smart people who are putting us on, or by imbeciles who really mean it." Mark Twain

  3. #3
    Registered User drneo66's Avatar
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    How old is the battery? Was it maintained well over the bike's winter slumber? Connections tight? Double check the ground on the transmission too - I've oddly come across a couple bikes that it has been loose on.

    As you said, a voltage meter will tell you a lot when you're out riding.
    Current: 2007 BMW R1200RT
    Former: 1995 BMW K75S, 2009 BMW G650GS
    MOA Member #:150400, IBA#: 37558

  4. #4
    Nick Kennedy
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    Thanks for the reply's
    I'm going to go ride the thing in a couple hours and report back the results

  5. #5
    Nick Kennedy
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    Yesterday afternoon I rode 7 miles into town with a voltmeter connected.
    When I started out the voltage held very steady at 13.9v. Then it started jumping around, always lower, as low as 11.2 v for a few seconds. But all over the place from 11.2 to 13.9v
    I parked it for 1/2 hour and rode home. Voltage stayed very steady at 13.7v and did not wavier.
    Battery light never lit up at all.

    I think I'm going to change the voltage regulator and see it that is causing this glitch.
    This bike is 29 years old now.

  6. #6
    Registered User 6322's Avatar
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    Hey Nick,
    I had an electrical issue with a K75 a number of years ago. The ground cable had corroded inside the insulation and was only revealed after I peeled the insulation back. Took a while to diagnose because it was so simple. Good luck on your diagnosis.
    gp
    Gary Phillips - #6322
    Wildland Firefighter, Retired, Riggins, ID
    Heartland Moto Locos BMW Riders
    '77 R100/7 Dirt Hack, '83 R80ST, '85 K100RS w/EML, '00 R1100RS

  7. #7
    Nick Kennedy
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    Gents
    The saga continues.
    Here's the latest:

    So I want to change my oils so I start up and ride down my hill. At 2 miles the Battery light comes on and he bike shuts off for about 5 seconds and the battery light is half lit. Before it stops rolling it refires and I ride 2 miles home.

    I had pulled out the Clymer book today [ like brand new I never have had to do anything to this bike] and saw it was easy to pull the brushes and rectifier [ voltage regulator?] while the alternator was in the bike. So I pull the ECU plug to pull the battery and notice that's it not clipped in tight. hmmmm

    So I pull the battery and the brushes out, they look good. I pull the alternator plug It looks good and I clean it with contact cleaner. I clean all the fuses with contact cleaner. I pull the ground wire and check it for continuity while flexing it, seems perfect. I assemble it all, up hook up my voltmeter and ride down my hill and back 4 miles. Works Perfect Voltage holds steady at 13.88v

    I think its the ECU plug was loose and I'd fixed it yahoo.

    I change my oils and start the bike to fill the oil filter for the final top off and it shuts off after 20 secs. and the battery light is half dim. After a few moments the light comes on bright and I restart the bike. hmmmm

    So over the next 20 minutes I turn on the key to start it and several times the battery light is half lit and it won't crank, but after several seconds light come on strong and it starts.


    So my question to all you electrical genius's 's out there is: Why is the battery light going to half Lit? What would cause this condition and be so intermittent and short lived? And what could cause it to go half lit and the bike shut off or not crank? But a few seconds later Like 6-8 the light comes on strong and it cranks and runs

    Got to fix this as the season is here.

    Thanks in advance for reading this.

  8. #8
    Registered User
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    https://ibmwr.org/index.php/2001/11/...ng-and-repair/

    Part of the electrical system grounds through the starter.
    Ron

    91 K75RT ABS

  9. #9
    3 Red Bricks
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    In addition to dirty starter commutator/brushes, many electrical problems can be traced to dirty ignition switch or kill switch contacts.

    https://forums.bmwmoa.org/showthread...rt-Kill-Switch

    https://forums.bmwmoa.org/showthread...witch-Cleaning








    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  10. #10
    Nick Kennedy
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    Hello Gents
    Update
    So I took out my starter and cleaned it, easy procedure 1 hr total time.
    Took her out for ride and voltage sat steady at 13.9v, no battery lights, ran great!
    Nothing like a little brick pulling through 6 grand at full throttle, love it.
    Thanks for all the advice and postings, Ron you nailed it.
    How the system grounds through the starter I'll never know.
    Best Regards
    Nick

  11. #11
    Quote Originally Posted by nickrides View Post
    Hello Gents
    Update
    So I took out my starter and cleaned it, easy procedure 1 hr total time.
    Took her out for ride and voltage sat steady at 13.9v, no battery lights, ran great!
    Nothing like a little brick pulling through 6 grand at full throttle, love it.
    Thanks for all the advice and postings, Ron you nailed it.
    How the system grounds through the starter I'll never know.
    Best Regards
    Nick
    The "load shed relay" which has the headlight and some other stuff connected grounds to the hot brush of the starter. If the starter is not energized this provides a path to ground for the relay coil, thus providing current to the headlight, etc.

    However, when the starter button is depressed 12v+ is applied to the hot brush. And the load shed relay coil now has 12v+ applied to both ends of the coil - thus no ground. So the relay turns the headlight and stuff off until the starter button is released. It is really rather ingenious a design.
    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/

  12. #12
    Nick Kennedy
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    Mr. Glaves
    Thanks for the simple yet detailed explanation.
    You would have made a good teacher!

    Those K75 are fun as hell to have around.

  13. #13
    3 Red Bricks
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    Quote Originally Posted by nickrides View Post
    Mr. Glaves
    Thanks for the simple yet detailed explanation.
    You would have made a good teacher!
    That probably came from his many many years of writing a column in the ON called Bench Wrenching that told you anything you ever wanted to know about wrenching on K75s.

    His detailed and consise explinations of maintenance and troubleshooting have helped thousands of us Kbike owners over the years. Even though he has backed off from writing his column for the ON, he has thankfully remained active on this forum to the benefit of all of us. If you go to any of the MOA National rallies, check to see if he is giving one of his seminars. They are not to be missed if you intend on wrenching on your own bike! Thanks Paul!





    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

  14. #14
    Registered User
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    85 K100RS w 199,700 miles: Just cleaned my starter and switches too.

    After a couple "shut offs" on the first hot days, I cleaned starter switch, kill switch and starter and now a few hundred miles later, problem gone. Used the old Kbike tech info. again. Had done this stuff around 60,000 miles as well. The starter brushes still seem longer than 1/4". I realize that no amount of armature cleaning can help if a brush is too short. Thanks for the ground lesson again Paul. Nashville next month from Phila.

  15. #15
    Nick Kennedy
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    So
    My K75 RT still has this problem Today:
    It shut off for a second and then about a minute later it Shut Off.

    I coasted to the side of the road. The Alt and oil pressure lights were half lit. After a couple of attempts to start it the Lights came on full bright and it started and I finished my 140 mile ride, no quirks.
    It will not crank with the warning lights half dim.
    Could it be a intermittent open in the battery between cells? Will the bike run with a battery voltage of say 10v?
    The Battery is a SLA one year old, when I check it, it looks good.
    I know my next procedure is cleaning the ignition switch and kill switch, but I'm leery of breaking something in there while taking them apart.

    Should I replace my battery?

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