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Thread: Coolant Temperature Sensor

  1. #1
    Registered User breyfogle's Avatar
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    Question Coolant Temperature Sensor

    Does anyone know the failure symptoms of a K-bike coolant temperature sensor ? I've got an 89 K75S that has begun to act "funny" during cold starts (Cold being engine at ambient temperatures of 50 to 60 F or so, not having been run). Basically, when cold it will not idle fast enough to run and if it stalls it is hard to restart. As the engine gets a bit of heat into it, it improves. When warm it seems to start and run normally.

    First thought was vacuum leak but everything seems to OK WRT to the common sources of vacuum leaks (e.g. the Z-hose). Second thought was maybe the computer thinks the engine is hot when it is really cold. Does anyone know if the coolent temerature sensor is normally open or closed when cold and whether that tells the computer the engine is hot or cold ?
    '89 K75S Original Owner
    '94 (Beta) R11RS, ( RIP 12-5-2010 courtesy of blind left turning cage driver ) ....

  2. #2
    Lifetime Member Ridealot's Avatar
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    <"Basically, when cold it will not idle fast enough to run and if it stalls it is hard to restart. As the engine gets a bit of heat into it, it improves. When warm it seems to start and run normally">

    Those are the exact symptoms my bike displays when it is time to change the spark plugs. How many miles since you threw in some plugs?
    Tom
    Salem Or.
    '93 K1100LT w/Bushtec
    '09 F650GS '15 Ducati Multistrada

  3. #3
    NWKRider
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    when I needed to replace the coolant sensor on by bike It stalled after warming up. Also hard to start warm. Have you ran for codes? KCamper

  4. #4
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by breyfogle
    Does anyone know the failure symptoms of a K-bike coolant temperature sensor ? I've got an 89 K75S that has begun to act "funny" during cold starts (Cold being engine at ambient temperatures of 50 to 60 F or so, not having been run). Basically, when cold it will not idle fast enough to run and if it stalls it is hard to restart. As the engine gets a bit of heat into it, it improves. When warm it seems to start and run normally.

    First thought was vacuum leak but everything seems to OK WRT to the common sources of vacuum leaks (e.g. the Z-hose). Second thought was maybe the computer thinks the engine is hot when it is really cold. Does anyone know if the coolent temerature sensor is normally open or closed when cold and whether that tells the computer the engine is hot or cold ?
    The sensor is a resistive element, it is not an on/off device. I have
    seen very few fail. They can be checked at the plug for the FI
    computer.

    Go to the IBMWR website http://www.ibmwr.org - look under
    the K-tech section. I believe there is a writeup on testing the
    sensor. You will need a Volt-Ohmmeter and know how to use one
    to do the test.

    Best,
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

  5. #5
    Registered User breyfogle's Avatar
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    Quote Originally Posted by deilenberger
    The sensor is a resistive element, it is not an on/off device. I have
    seen very few fail. They can be checked at the plug for the FI
    computer.

    Go to the IBMWR website http://www.ibmwr.org - look under
    the K-tech section. I believe there is a writeup on testing the
    sensor. You will need a Volt-Ohmmeter and know how to use one
    to do the test.

    Best,

    Just the info I needed, thanks ....

    I don't expect to find a faulty sensor either. I'm just trying to troubleshoot my problem beginning with the lowest cost / easiest to do possibilites and hoping it's not the EFI computer !!
    '89 K75S Original Owner
    '94 (Beta) R11RS, ( RIP 12-5-2010 courtesy of blind left turning cage driver ) ....

  6. #6
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    when the temp sensor on my K11RS (believe it to be the same basic system, but could be wrong) blew, it caused the cooling fan to kick in at all kinds of odd times- like riding down the interstate at 60mph on a 20 deg day, when the engine temp gauge was barely to the middle (normally only comes on when bumping into the obttom of the red zone). and to sometimes not come on until a bit too far into the red. however, the bike continued to run like it's usual clockwork self. HTH.
    Ride Safe, Ride Lots

  7. #7
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by bikerfish1100
    when the temp sensor on my K11RS (believe it to be the same basic system, but could be wrong) blew, it caused the cooling fan to kick in at all kinds of odd times- like riding down the interstate at 60mph on a 20 deg day, when the engine temp gauge was barely to the middle (normally only comes on when bumping into the obttom of the red zone). and to sometimes not come on until a bit too far into the red. however, the bike continued to run like it's usual clockwork self. HTH.
    That's because the coolant temp sensor is a dual-element device. Half of the sensor tells the FI system how warm the engine is - the other half tells the fan/light relay how warm the engine is.

    It would be unusual for both halves to fail at the same time.

    Best,
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

  8. #8
    A bozo on the bus deilenberger's Avatar
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    Quote Originally Posted by breyfogle
    Just the info I needed, thanks ....

    I don't expect to find a faulty sensor either. I'm just trying to troubleshoot my problem beginning with the lowest cost / easiest to do possibilites and hoping it's not the EFI computer !!
    I wouldn't worry about the computer. Unless the bike was struck by
    lightening, I have never heard of a K L-Jetronic computer failing. What most people think may be a failure is fixed by removing and reconnecting the big connector to the computer.. cleaning the contacts.

    It is MUCH more likely (about 100% more likely) to be a failed temperature sensor or dirty connection somewhere. And I'd bet 1 beer on that (which I never do lightly..)

    Best,
    Don Eilenberger http://www.eilenberger.net
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

  9. #9
    Blocking the slow lane
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    Are the valve clearances correct? If one exhaust valve is tight, the bike won't start well when cold and lose about 20% MPG.

    Did the fast idle cable stretch? Its in a manual somewhere, but there should be a 1.5mm gap on the fuel rail at the first position, and 3mm at the second position.
    Last edited by jdiaz; 12-06-2004 at 11:24 PM.
    Jon Diaz
    BMW K75
    BMWMOA Ambassador

  10. #10
    Registered User breyfogle's Avatar
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    Quote Originally Posted by deilenberger
    I wouldn't worry about the computer. Unless the bike was struck by
    lightening, I have never heard of a K L-Jetronic computer failing.
    I agree completely that it is easy for novice mechanics to blame "the computer" when the real problem is almost always with one of the old fashioned sensors or their wiring. BUT:... Several years ago and totally unrelated to the current problem, I actually did have the ignition computer(under the tank) fail while touring. The dealership spent almost a week trying to find the problem until, as a last resort, they swapped the computer for a known good one. Problem solved (except for the expense!!)
    '89 K75S Original Owner
    '94 (Beta) R11RS, ( RIP 12-5-2010 courtesy of blind left turning cage driver ) ....

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