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Thread: 1990 K75RT Electrical problem

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  1. #1
    Nick Kennedy
    Join Date
    Nov 2011
    Telluride Colorado

    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
    Join Date
    Feb 2009
    Mississauga, Ontario
    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.

    "Every decent man is ashamed of the government he lives under."
    H. L. Mencken

  3. #3
    Registered User drneo66's Avatar
    Join Date
    Jun 2009
    Rochester, MN
    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, 2013 F800GS
    Former: 1995 BMW K75S, 2009 BMW G650GS
    MOA Member #:150400, IBA#: 37558

  4. #4
    Nick Kennedy
    Join Date
    Nov 2011
    Telluride Colorado
    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
    Join Date
    Nov 2011
    Telluride Colorado
    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
    Join Date
    Mar 2012
    Riggins, ID
    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.
    Gary Phillips - #6322
    Wildland Firefighter, Retired, Riggins, ID
    Heartland Moto Locos BMW Riders
    '77 R100/7 Dirt Hack, '83 R80ST, '85 K100RS w/EML, '93 K1100LT, '00 R1100RS

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