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Thread: 87 BMW K75S Speedo/Odometer issues

  1. #1
    Join Date
    Oct 2019

    Cool 87 BMW K75S Speedo/Odometer issues

    hello Brick throwers! I have a frustrating issue with my speedo/odo and I'm looking for help. The bike is a '86 purchase, and like many of you over the years I have had both speedo and tach issues. The latest issue is the Speedo and Odo are inop. Below is what I have RECENTLY done to try to restore operation. As a background, I am the original owner and have ALMOST ALL records save some at the very beginning of my ownership (youth!!??). I have worked on the bike over the years, but am only now starting to delve into some of the heavier maintenance items on the bike due to my last huge bill from the local dealer after a major overhaul. Therefore I consider myself a rookie in the bike tear down, build up processes, and I don't have much electrical troubleshooting experience. Below is the latest timeline:
    - Speedo/Odometer intermittent /fails and tach needle had been inop (hanging like a string) for a short time. Incentive to both bring bike in for major maintenance, and send out the instrument cluster to Overseas Speedometer in Austin.
    - Instrument cluster delivered back from Overseas and I reinstalled the instrument cluster prior to putting in shop, speedo/odo still inop, tach works great. Call Overseas speedometer and talk to the owner and he said off the bench all worked fine. I have no reason to doubt him.
    - Notified by techs that speedo and odo not working at the completion of their work.
    - Pulled the sensor off the back drive to clean and check for damage. The plastic casing, which I had never removed before, has a hairline crack on the aft housing but all else looked good. Not sure if that would affect voltage/signal to instrument cluster.
    - Pulled off tank and tried to trace the wiring as much as I could. Cleaned all contacts with contact cleaner and pipe cleaner (and smaller) sized wire brushes. Cleaned all grounds I could find on the bike.....but I tried to follow the wiring diagram for the bike and must admit I am sure I missed a number of potential contact/resistance areas.
    - Had my dog pee on the front least seemed to make him feel much better!
    So that's about it. I have a voltage meter now and after hopefully some good suggestions here, I am going to enlist the help of a very nice neighbor that happens to work on his pristine Model A he has a wealth of knowledge and use of tools that I don't.
    Thanks in advance for what I'm sure will be a wealth of Brick knowledge!
    "When I left, I joined the army, and when I took the service exam my psych profile fit a certain... moral flexibility would be the only way to describe it..."

  2. #2
    Registered User
    Join Date
    Sep 2005
    Commerce Township, MI

    I wonder if there's a way to test the pulse generator.

    Update, so I set my multi-meter to frequency and connected the pulse generator. Then I placed the generator next to my soldering iron and turned it on. The soldering iron has a transformer and will generate a 60 Hz field. The multi-meter then showed 60 Hz. Which means the pulse generator was sensing the field given off from the soldering iron.

    If your pulse generator is bad, I don't think it would sense the field from the soldering iron, but I don't have a faulty pulse generator to varify my theory.

    Last edited by richo2006; 06-28-2020 at 02:38 AM.

  3. #3
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.

    You can test your speedo system by doing similar to what Rich did.

    Remove your speed sensor from the final drive. Have a friend hold or make a bracket to hold a soldering iron or gun tip about 1/8 - 1/4" away from the sensor tip (turned on). Turn on key and kill switch. If every thing is working, your speedo should register a speed (somewhere around 30-40 mph IIRC).

    If you get no needle movement, start flexing the wire to the sensor. First on either side of the connector (one of the flat two pin connectors near frame member just in front of you coolant overflow bottle), then work back toward the sensor. If the needle moves during the wire movement on the sensor end of the connector, you have a bad sensor wire (not uncommon)-andneed to replace the sensor. Mave sure the contacts in the connector are not corroded and are clean.


    Ride Safe, Ride Far, Ride Often

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

  4. #4
    Join Date
    Oct 2019
    Thanks for the information. I will try these techniques and hope for the best. I will update this thread later with the results.
    "When I left, I joined the army, and when I took the service exam my psych profile fit a certain... moral flexibility would be the only way to describe it..."

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