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Help FZR250 Temp gauge rises when lights are turned on?

Discussion in 'Yamaha 250cc In-Line 4's' started by Brandon Otte, Mar 12, 2020.

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  1. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    So where does the negative battery lead run to ? Is it the engine ?
    And is there an earth lead connecting the engine to the frame ?
    If not try fitting one
     
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    my67xr, May 6, 2020
    #101
  2. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Just found this



    So connecting directly to and from the battery shouldn't KABOOM the gauge, it should to hottest reading and as we've been assuming, he refers to the temperature sensor as the sending unit, it is essentially a variable resistance path to ground

    I reckon there is an alternative path to ground somewhere in parallel with that temperature sensor

    Try disconnecting that temperature sensor connection end and seeing if you get any needle deflection whatsoever on the temp gauge - it shouldn't budge
     
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    ruckusman, May 6, 2020
    #102
  3. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    The red and brown in the connector block near the fuse go nowhere. Don't worry about them as extra for different model with same wiring loom.
     
    • Informative Informative x 2
    Murdo, May 7, 2020
    #103
  4. Brandon Otte

    Brandon Otte Well-Known Member

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    Thanks that’s really helpful, am about to do some testing. Here’s a quick video off the engine cold and just turning ignition on and off then lights on with ignition on.
    The gauge rises abit when the ignition is turned on and doesn’t move when lights are turned on at this temp. I found the hotter it gets the more the gauge jumps.
     
    Brandon Otte, May 7, 2020
    #104
  5. Brandon Otte

    Brandon Otte Well-Known Member

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    Okay so some advice please guys - am I confused because...
    I just unplugged the temp sender wire and tried a continuity test to the battery negative and it’s beeping away like it’s just an earth wire. To be clearer I’m connecting the sender wire to my multi meter and the other end of the multi meter to the battery negative or any earth on the bike. Would this mean the temp sender wire isn’t actually the temp sender wire?? Maybe it’s been tampered with??
     
    Brandon Otte, May 7, 2020
    #105
  6. Brandon Otte

    Brandon Otte Well-Known Member

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    That’s with the ignition off. With ignition on there’s no continuity. Was that just ground through the gauge? Thought I had found the issue.
     
    Brandon Otte, May 7, 2020
    #106
  7. Brandon Otte

    Brandon Otte Well-Known Member

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    Checked gauge and it reads full hot when sender wire is grounded to battery so that checks out.
     
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    Brandon Otte, May 7, 2020
    #107
  8. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Diode mode will beep if there's significant continuity - check the actual resistance
    as I suspected there's a parallel path to ground through that wire it seems.
    Realistically when the end from the temperature sender is disconnected, there should be no connection to ground whatsoever, otherwise that bypasses the temperature sensor at least partially leading to inaccurate readings
     
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    ruckusman, May 7, 2020
    #108
  9. Brandon Otte

    Brandon Otte Well-Known Member

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    Yeah so
    Is that with the ignition on or off though? With it off there’s direct connectivity with it on there’s not.
    What do you think of wiring a new sender wire from the gauge to the sender unit?
     
    Brandon Otte, May 7, 2020
    #109
  10. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, so you responded whilst I was typing, I am going to recommend just slinging an isolated earth wire directly between the gauge and temperature sensor, don't get fussy, it's just to test

    Seems that with the key off, it connects that ground wire, OK, I don't see the point of that, unexpected, then with the key on, it disconnects that wire and switches the positive on - that's expected, at least the switch positive part is

    Try connecting the earth to the temp sensor, disconnect it from the gauge, and check continuity and resistance, key off then key on

    This is probably simple, but it's perplexing right now
     
    ruckusman, May 7, 2020
    #110
  11. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Can you measure the resistance through the sender unit? you'll need both cold and hot reading's
    If it's cold it should measure above 155 Ohm's,
    at normal operating temp it should be around 47-57 Ohm's,
    and when the temp gauge reaches red it should be between 16 and 26 Ohm's
     
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    my67xr, May 7, 2020
    #111
  12. Brandon Otte

    Brandon Otte Well-Known Member

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    Okay am about to try it. So from the sensor input on the gauge to the sensor right?
     
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    Brandon Otte, May 7, 2020
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  13. Brandon Otte

    Brandon Otte Well-Known Member

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    Okay will do that too thank you.
     
    Brandon Otte, May 7, 2020
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  14. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Yes - that should isolate the ground connection to just one path, unless the third mystery wire is playing a part in all of this
     
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    ruckusman, May 7, 2020
    #114
  15. Brandon Otte

    Brandon Otte Well-Known Member

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    Resistance is .740. Here’s a photo of what the multi meter is set to.
     
    Brandon Otte, May 7, 2020
    #115
  16. Brandon Otte

    Brandon Otte Well-Known Member

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    image.jpg
     
    Brandon Otte, May 7, 2020
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  17. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Resistance through the temp sensor to ground (correct) is ~740 Ohms - seems reasonable
     
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    ruckusman, May 7, 2020
    #117
  18. Brandon Otte

    Brandon Otte Well-Known Member

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    image.jpg Here’s confirmation that the gauge joins the sensor input to ground with it off. Full beep.
     
    Brandon Otte, May 7, 2020
    #118
  19. Brandon Otte

    Brandon Otte Well-Known Member

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    Yeah that’s correct. I’ll have to try resistance as it warms up once I’ve got it back together. Am trying the new sensor wire now.
     
    Brandon Otte, May 7, 2020
    #119
  20. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, so mystery green wire with red stripe switches to ground with key off

    I am assuming that brown is positive from battery or thereabouts, and should switch on with key
     
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    ruckusman, May 7, 2020
    #120
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