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Thread: 1985 K100RS cooling issue

  1. #1
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    1985 K100RS cooling issue

    I have a problem I would appreciate feedback on. After riding my 1985 K100RS two up to the top of Pikes Peak and shutting it off it proceeded to spew coolant from the overflow tank hose. I didn't think much of it because the temp light didn't come on. Topped off the overflow tank(half full) and rode on.
    That was two years ago and I have put on several thousand miles since with no issues. Then
    , about a month ago, again I had the same issue after riding up a gopher hill in NE New Mexico. I have changed the coolant yearly, and the fan seems to turn off and on OK while riding. However today I noticed my temp warning light doesn't come on when I turn on the ignition, so I have another issue in that I don"t know if the bike is getting hot. Where should I start?
    Also, has anyone adapted an auto type temp gauge to this model bike? The BMW gauge and wiring is rather pricey. Thanks for any help offered

  2. #2
    Registered User kioolt's Avatar
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    After 28 years it's possible that the bulb is blown. I believe the temperature switch is on the water pump. You could find it and short the wire to ground. That should make the lamp light.
    2004 R1150RT 151,500 miles , 1991 K100LT 128,000 miles, 1982 R100RT 106,900 miles
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  3. #3
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    The temp light not suppose to come on when you turn the key on (unless it is overheated).

    The reason the oil light comes on with the key is, until the engine starts, there is no oil pressure.
    The reason the alternator light comes on, until the engine starts, there is no charging.

    The oil is a normally closed switch.
    The temp sensor at the water pump is a variable resistance sensor.
    The temp sensing switch unit (in the relay box under the seat) are normally open switches for both the fan and the light.


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  4. #4
    Benchwrenching PGlaves's Avatar
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    Water boils at a nominal 100C (212F) at sea leavel with a nominal barometric pressure of 29.92 in. of Mercury. At the altitude of pikes peak the atmospheric pressure us enough lower that the boiling point is reduced to the point that it can boil even at a lower temperature. My K75 did it every time I went up there. As for what the elevation was in New Mexico I have no way of knowing.

    I would do two things. Thoroughly clean the openings/fins on the exterior of the radiator. Get a spray bottle and use it to presoak the radiator with drug store variety Hydrogen Peroxide. It really works to soften bug guts. Then rince with a garden hose. DO NOT USE a high pressure sprayer or car wash. I damaged a BMW radiator doing that and caused it to leak. I had to replace it. Cleaning the exterior is important because stuff clogs the little passages in the fins, and even worse, the coating of bug guts and other grime retards the transfer of heat from the aluminum to the air.

    Get a new radiator pressure cap. The spring may be feeble.
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  5. #5
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    Quote Originally Posted by littledog View Post
    it proceeded to spew coolant from the overflow tank hose.
    The coolant came out of THE HOSE??????

    When coolant expands in the radiator, excess coolant is supposed to go THROUGH the hose INTO the coolant reservoir (expansion tank).

    The only way for coolant to come out of a normal system is through the cap of the coolant reservoir.

    If the hose that runs between the radiator and the coolant reservoir is cracked, it will push coolant from the radiator to the reservoir when hot, but when it cools down, it will suck AIR into the radiator. Air in the radiator does not cool well. Just because the reservoir is full does not mean that the radiator is full.

    You should check the coolant level AT THE RADIATOR CAP (under the gas tank) and check the small hose that goes to the reservoir for cracks AT LEAST once a year and before any major extended trips.

    It is also possible that you have a bad radiator cap that is not holding pressure.



    LONG MAY YOUR BRICK FLY!

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    Lee Fulton Forum Moderator
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  6. #6
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    Thanks to all for the advice offered. I will clean the radiator and install a new cap and post at a later date as to the results.
    As far as the New Mexico elevation, it was a little over 6000 ft., so no comparison to Pikes Peak.
    Also as per my prior post, I would really like to add a temp gauge as all my bike has is the idiot light. Anybody who has done this?
    The interesting thing about the light is that I checked my 91 K100RS and the light did come on when I switched on the ignition and went out when I started the bike.

  7. #7
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    The 4valve bikes are wired differently.


    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

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

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