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  1. #1

    fan diagnostics

    Hi, I have two questions I hope someone can help me with. Has anyone followed the Chilton test of jumping the fan connector, hitting the starter button and gotten the fan to run? Second, I have followed all the tests from the threads I have been able to find with no luck getting the fan to run with that test. I tried a relay from another bike and even bought a new one with no luck. Any ideas? I believe Dan505 got to the point of trying a new relay but did not post his results.

  2. #2

    Fan Test

    I assumewe're talking about a K75 or Two-valve K100.

    If that's how Chiltons suggested testing the fan, they missed the mark slightly. To do this test, jump from the coolant sensor lead to ground, then touch the starter button long enough for the engine to crank. The fan should run briefly. The coolant sensor in L-Jet K-Bikes is a two function item-one side is for injection and the other for the fan and light. You can eliminate error by making your jumper lead in the relay box. You will need a schematic to determine which wire to jump.

    A simpler test might be to spin the fan with your fingers. If it feels "tight", it's on it's way out. The fan itself is the most common item to fail when the fan does not work.
    Last edited by lostboy; 01-23-2009 at 12:57 PM. Reason: Left out a sentence

  3. #3
    Thank you, thank you, thank you. I had to do the post twice(dumb) and I forgot to mention k75 piece. Now I can put everything back together and put the relay on the classifieds. thanks again

  4. #4
    Lostboy is correct on the fan test by grounding the temp probe line. The temp probe is a reducing resistance ground. You can take a resistance reading cold, I heated the temp probe with a hair dryer to see if the resistance reduced.

    Below is a past post on fan diagnostics I previously posted.

    The fan switch located in the relay box uses 2 fuses. The relay activating circuit is controlled by the fuel injection relay fuse. The fan itself is controlled by the horn relay fuse. I'm not looking at a wiring diagram right now but I believe these fuses are 5 and 6. But if your bike runs and the horn works fuses are good.

    When the fan switch goes bad the temp light will not light either, so I am assuming your fan switch is good. Also temp probe sounds good.

    Now what really is wrong. The fan motor has a habit of melting the plastic brush holders. This is the same motor Porsche uses for the condenser fan in older 911s.

    3 was to fix.
    1. Order a used working fan assembly around $70 BeemerBoneyard or other online vendors. I have done this for a K75.

    2. Order a new fan assembly around $150 from BMW or $130 BeemerBoneyard.

    3. Order a new fan motor and use you fan assembly around $70 BeemerBoneyard
    . I have done this on my K100. The fan presses off and on the motor shaft, 3 clips hold the motor in the assembly, and you will have to solder the connector to the motor terminals. The new motor is a lot quieter than the others were.

    Ralph Sims

  5. #5
    Registered User
    Join Date
    Mar 2008
    Belton Tx
    Slightly OT, but in the same area. I know my fan works, but I am not sure about the over temp light. How do I test that?

    91 K75RT ABS

  6. #6
    once I did the test correctly I verified the fan works fine. Reference testing the light, there are instructions in one of the other older posts that works fine once you get to the relay.

  7. #7
    Here is a great article on fan diagnostics. My 1st fan problem was the fan switch, which I call a relay. If the fan switch is bad the fan will not run, nor will the overheat light light. This article will tell you how to check the switch and light.

    On 2 Ks I had one bad switch and 2 fans with the brush holders melted.

    Ralph Sims

  8. #8
    Registered User Beemer01's Avatar
    Join Date
    Mar 2007
    Chicago, Ill

    Marvhil -

    Reach in and try turning the fan - if it won't turn easily the motor is shot. Happens all the time to the old Ks.

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